General · technician tips

General HVAC service tips

702 practical tips that apply to any brand: safety, refrigerant handling, tubing and brazing, electrical, airflow and combustion habits that keep jobs clean and safe.

Jump to: Safety (29) · Refrigerant handling (226) · Tubing & brazing (40) · Electrical (149) · Airflow & ducts (63) · Combustion & gas (32) · Other (163)

Safety

Hazards and habits that keep you out of trouble on the job.

Discharge line burns

Air conditioner

The discharge line and head stay very hot during and after a run — don't grab them.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Flame detector lockout test

Boiler

Close the fuel valve with detector running; control must lock out.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Discharge every capacitor

Electrical

Never assume a cap is dead; can hold up to 600 V. Put it in a box, wear insulated gloves, and bridge the terminals with a 20–30 kΩ resistor for a few seconds. Never short with a screwdriver — it can rupture the foil. A built-in bleed resistor does not count.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Don't run without overload protection

Electrical

Never start a compressor under service without functioning overloads; replace pitted contacts. Don't hold a contactor in with a screwdriver — you'll burn a good compressor.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Duct heater minimum airflow

Electrical

The sail/pressure switch may close below the heater's rated minimum cfm; verify airflow with instruments or elements burn open and can energize the duct ("a live duct can lead to a dead technician").

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Lockout relay reset

Electrical

High-impedance lockout coil, in series with the safety string, starves the compressor relay once a safety opens; reset only by interrupting coil power (manual reset button).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Match plug/receptacle to equipment

Electrical

A 120 V appliance on 240 V burns out quickly; a 15 A-protected appliance on a 30 A circuit can burn out or damage the breaker. NEMA configurations exist to prevent this.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Overloaded motor = burnt motor

Electrical

Slowing the rotor (heavy load, worn fan bearings, high head pressure) cuts counter-EMF and current climbs sharply; locked rotor burns a motor fast.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Snap action keeps contacts alive

Electrical

Slow-opening contacts arc and burn; toggle-spring or magnet snap mechanisms open fast. Magnet closer to bar = longer run time; more toggle-spring pressure = longer run time.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Why capacitors and relays burn out

Electrical

Start caps: >20 starts/hr, long start-winding time (low voltage, bad relay — add crankcase pressure regulator), sticking relay, wrong µF/voltage, wet terminals. Run caps: line voltage >10% over, low voltage rating. Relays: voltage >±10%, wrong run cap, short cycling, loose vibrating mount, wrong relay.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Hard lockout reset

Furnace

Requires service call / power interrupt; soft lockout retries on its own after inter-purge.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Check combustion chamber for pooled oil before pressing reset

General

Explosion risk; pump out and vent if oil found.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Compressed-air cleaning PPE

General

Goggles and dust mask when blowing out condensers.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Duct cleaning cautions

General

Contain debris, cover furniture, PPE; fiberglass-lined duct, flex and ductboard damage easily; gas compressor stays outside; indoor-exhausting wet vac must be HEPA. Three steps: agitate, high-velocity vacuum, biocide. Avoid dead-end ducts or add a cleanout door.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Explosion-proof storage

General

Never store flammables in an ordinary refrigerator — thermostats/relays spark; use safety/explosion-proof units and keep them out of areas with explosive vapors.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

LiBr systems

General

Keep air out (solution turns corrosive); test specific gravity, pH/alkalinity, copper content, suspended solids, inhibitor; N2/helium leak test with soap; gloves/face/eye protection; octyl alcohol as wetting agent; lithium chromate inhibitor.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Low humidity symptoms

General

Static shocks, dry skin/nose, loose furniture joints, cracking woodwork → add humidification.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

No copper anywhere in an ammonia absorption circuit

General

Corrosion/explosion risk; steel or aluminum gauges, hoses, fittings only; copper only in chilled-water piping.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Oil: cold oil pumps and burns poorly

General

Viscosity doubles at 0 °F; outdoor tanks need additive / consider pour point when siting the tank.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Removing a pressure-operated water valve

General

Pump down, warm the valve and its pressure tube with a heat lamp 3–4 min to drive liquid back, isolate, bypass pressure to low side, open joints, then warm again — oil-bound liquid can blow out explosively minutes later; wrap openings in heavy towels, point away from people.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Rooftop safety

General

Ladder 1:4 and 3' above edge; hoist tools by rope; no ladders in gusty wind; secure hinged panels open and loose panels down; windbreak/safety belt in strong wind; shield tools from sun; hard hat/goggles, rubber gloves on electrical.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Scale on water-cooled condensers

General

Hot liquid line + high head with correct charge = scaled tubes. Soft scale: power wire brush in place. Hard scale: acid wash (fast, goggles/gloves, best left to certified cleaners) or replace. Clean at least yearly; well water needs treatment. New gaskets every time.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Burnout clean-up

Refrigeration

Goggles, rubber gloves, ventilation (acidic oil/fumes); nitrogen flush, triple-evacuate to 500 µ, suction + liquid burnout driers, run 30 min, recheck oil, replace driers until clean, then REMOVE burnout driers. Never solder leads to compressor terminals (glass cracks → leak).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Burnout diagnosis by smell/oil

Refrigeration

Pungent odor when cracking the piercing valve = burnout. Black acidic oil = severe; clear but pungent = mild; clean odorless oil = no burnout, trouble is mechanical. Use an acid test kit; don't touch acidic oil (gloves/goggles). Flush with nitrogen or recovery machine in flush mode.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Cryogenic fluids: full PPE, correct containers only

Refrigeration

Instant frostbite on contact.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Internal overload reset time

Refrigeration

After trip, internal overload can take 1–2 h to reset. Speed cooling with forced air, dry ice or CO2 spray on the dome. Don't tap the control — vibrating contacts arc and burn.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Never braze with refrigerant in the system

Refrigeration

Heat breaks it down into toxic fumes. Verify 0 psig after recovery, purge debris with nitrogen, flow 1–2 psi nitrogen while brazing with a cracked valve so pressure can't build. Keep the nitrogen tank away from the torch.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Never jumper the internal overload

Refrigeration

Running even briefly without the internal protector can burn out the compressor.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Service valve positions

Refrigeration

Back seated (CCW fully) = normal run, port closed. Front seated (CW fully) = line blocked, port open to component side. Mid-position = back seat then ~2 turns CW; used for evacuation/standing pressure tests. Cracked = 1/16–1/8 turn off back seat for a pressure reading. Crack before opening fully to avoid pressure shock.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Refrigerant handling

Gauges, recovery, evacuation, leak checks, charging and oil.

Best coil cleaning = pull the coil

Air conditioner

Recover/pump down, plug lines, steam or hot water + detergent.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Condenser above evaporator

Air conditioner

Add a U-trap/inverted loop in the suction line for oil return; slope suction line toward condenser; insulate it (sweating and efficiency loss otherwise).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Expansion valve leak-through (Pro Tip)

Air conditioner

A leaking valve/seat lets liquid into the suction line on the off cycle; frost on the suction line right at start-up and compressor knock points to it.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Filter-drier and sight glass in the liquid line

Air conditioner

; seal wall/plenum penetrations with non-hardening sealant.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Filter-drier location (Pro Tip)

Air conditioner

Install at the coolest spot outside the refrigerated space, upstream of the metering device — outside the condensing unit on splits, or just ahead of the TXV, sometimes both.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Gas-charged TXV head must stay warmer than the bulb

Air conditioner

If the valve body/capillary is colder than the bulb the charge condenses in the head and the valve throttles shut even though the coil needs feed — classic mis-diagnosis of a "bad TXV".

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

High head pressure causes

Air conditioner

No condenser airflow (dead fan, stuck relay, blockage), no water flow (water-cooled), dirty coil, increased load. High head → hot discharge → oil/refrigerant breakdown, acids, sludge.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Maintenance schedule (console/commercial AC)

Air conditioner

Weekly: belts, fans, water leaks, controls, oil level; monthly: leaks/oil spots, filters, humidifier scale, safety valves, dampers, piping insulation/vibration; 6-mo: clean fans, cabinet, registers; yearly: compressor efficiency, dampers, water circuits, exercise hand valves; 2-yr: condenser wet surfaces.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Moisture indicator turned tan (Pro Tip)

Air conditioner

Oil tints the element tan; if it keeps returning after flushing with clean refrigerant the system has too much oil. Colour codes vary by maker — read the label (some print "wet/dry").

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Never reuse a filter-drier (Caution)

Air conditioner

A used drier releases moisture into the refrigerant.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Refrigerant theft

Air conditioner

Locking caps on service ports; keys only to owner/techs; caps colour-coded (pink = R-410A).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Replace all driers with a compressor (Pro Tip)

Air conditioner

After burnout or compressor replacement change both suction and liquid line filter-driers; a temporary suction drier can be removed after clean-up.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Sight glass bubbles aren't always low charge (Pro Tip)

Air conditioner

Bubbles on start/stop are normal equalizing; bubbles also appear with an unloaded compressor or a restriction ahead of the glass (plugged drier).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Skin-temperature shortcuts

Air conditioner

Evaporator refrigerant ≈ 8–12°F below coil metal while running; condenser refrigerant ≈ ambient + 30–35°F (air-cooled) or water outlet + 20°F.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Small top-ups mean a leak

Air conditioner

Systems that need periodic small charges have a leak — find it instead of topping off. A loose Schrader core is a common small-leak cause that can be fixed without recovery.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

TXV bulb mounting

Air conditioner

Horizontal line: on top of small lines (book), bulb horizontal; vertical line: capillary enters bulb from the top. Clean line and bulb with steel wool, copper strap + non-rusting screws, insulate the bulb so only pipe temperature affects it. Oil coating inside the pipe means gas is a few degrees colder than the pipe surface — contact quality matters.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

When to replace a liquid line drier

Air conditioner

Moisture indicator reads wet; pressure/temperature drop across the drier (bubbles after it, temperature difference across it); compressor replaced; circuit opened (e.g., TXV change). Leave driers in permanently — oil and motor insulation give up moisture slowly.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Pressure-test the boiler before cleaning

Boiler

Isolate at operating pressure; if pressure drops look at valves first (most common small leaks).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Sight glass can lie

Boiler

Clogged gauge-glass fittings trap water and show a false high level; use petcocks to confirm.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

"Four gauge" vs "four aught"

Electrical

4 AWG ≈ 85 A feeder wire; 4/0 AWG ≈ 230 A service wire. Ask if unsure which is meant.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Bearing noise diagnosis

Electrical

Dry bearing noise is brief before seizure; knock = excessive endplay (measure with dial indicator vs spec); heavy rumble at start-up = badly worn; rotor hitting stator = replace. Overheated bearing causes: oil too heavy/thin, dirty oil, belt too tight, pulley hub rubbing, misalignment. Bearings are usually not repaired in the field — swap the motor.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Capacitor safety & testing

Electrical

Bleed start capacitors before touching (some have no bleed resistor); test with a capacitance meter, never the old short-after-charge trick.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Hermetic compressor "won't start" shortcuts

Electrical

Hums but won't start: wiring, low voltage, start capacitor, relay contacts, high head (discharge/receiver valves closed?), tight compressor (oil level), weak capacitor. Starts but stays on start winding: low voltage, relay, shorted run cap. Runs but trips: added fan/pump on wrong side of protector, high suction, high discharge, weak protector, unbalanced 3-phase, broken discharge valve.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Lost bellows charge

Electrical

Above-atmospheric element (closes on rise, refrigeration) that leaks can't push the switch → unit won't start. Below-atmospheric element (opens on rise, electric heat/defrost) that leaks lets atmosphere in → stays open. Charged bellows ≈ 75 psig or more — finger pressure won't compress it; if it squashes easily, replace.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Setback must last several hours

Electrical

Or recovery energy wipes out savings; pre-cool before peak hours because of heat lag.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Start caps are short-duty

Electrical

Start capacitors (dry electrolytic) are only meant to be in the circuit a fraction of a second; a stuck centrifugal switch or relay keeping them in will damage windings. Run caps are oil-filled for continuous duty. Never interchange them.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Verify a control by its cut-in/cut-out

Electrical

Watch when a device starts and stops against the gauge/thermometer to confirm it's operating as set.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Wipe excess oil off fan motors

Electrical

Oil residue on blades collects lint and kills airflow; confirm blades tight on shaft and seated in shroud; replace bent blades (vibration eats bearings).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Belt tension & alignment

Furnace

Snug not tight; tightest belt that doesn't slip under full load, measured with a tension gauge; shafts parallel AND grooves in line (laser tools exist). Misalignment overloads the motor.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Concentric vent kit

Furnace

One wall penetration for intake and exhaust on condensing appliances; keep PVC joints leak-tight, use a termination kit, protect condensate from freezing.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Water leak → electrical shorts

Furnace

Leaking evap coil or humidifier above the furnace is a common cause of board failures.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Absorption terminology

General

"strong"/"weak" solution means ratio of refrigerant to absorbant in this book; other publications flip it.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Air band adjustment only with instruments

General

A flame that "looks" air-starved may be a dirty nozzle or after-drip; don't eyeball.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Air-in-fuel test

General

Gauge on pump port: fluttering needle = air; for small leaks, bleed tube into a bottle of oil and watch for bubbles.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Ammonia leak checks

General

Smell, or wet litmus turns red; soap bubbles don't work (ammonia dissolves into the water).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Analyzer probe near hottest part of flue; let it cool 5 min before touching.

General

Analyzer probe near hottest part of flue; let it cool 5 min before touching.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Bleeding a one-pipe oil burner

General

Loosen bleed/gauge plug, 1/4" tube to container, open fully, run burner until solid oil, stop, tighten, restart.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Blow out oil lines with nitrogen only

General

Never compressed air or oxygen; use regulator and relief.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Booster pump placement

General

Near the tank (push, not pull); use a vacuum safety valve on the pump inlet, not a check valve (check valves blow pump seals).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Break the vacuum pump's vacuum before storing

General

Otherwise oil floods the cylinder and the pump oil-locks. Change pump oil before every evacuation; white/foamy oil = water. Test the pump alone to 500 microns via a tee.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Cad cell must have line of sight

General

Mount per burner mfr.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Centrifugal rotation direction is critical

General

Wrong rotation damages the compressor; run at reduced load after start until oil foaming subsides; sump heater limits migration.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Charging absorption solution

General

Purge vapor from solution cylinder into a pail of water to let liquid NH3 flow in; discard solution if white precipitate forms.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Check a cylinder float switch by inverting it with the cord connected

General

Machine should alarm/shut off; if not, the overfill protection is dead.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Check all three oil filters

General

Line filter, pump strainer, nozzle strainer.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Cold fan motors

General

Outdoor fan motors idle in cold weather thicken their oil and may not start/trip overload; use oil that stays fluid at the lowest idle temperature.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Combustion head has many names

General

End cone, burner head, fire ring, retention ring. Flame-retention heads are hotter/cleaner; check chamber refractory each visit when retrofitted.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Compound gauge scales are per-refrigerant and colour-coded

General

Read the right ring; digital manifolds avoid this.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Cross-break big panels

General

Prevents oil-canning when the blower starts/stops. Fabric joints at furnace absorb expansion and noise (check fire code).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Damp fluctuating pressure with a small-bore connector or liquid-filled gauge

General

Pulsations ruin gauge accuracy quickly.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Date-tag service

General

On the furnace/condenser; log charge, suction and head pressure whenever the system was opened.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Draft test: overfire first, then flue (between draft regulator and furnace)

General

; zero the gauge on a level surface, read after 30 s; low = move barometric weight to increase draft. Sealed-combustion oil units may have no regulator.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Duct leaks are not equal

General

A leak into unconditioned space hurts far more than one inside the envelope; seal ducts in attics/crawlspaces first.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Electrodes must not touch the spray

General

They carbon up and cause delayed ignition/blowback.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Find leaks under depressurization

General

With house at −50 Pa, leaks are exaggerated — use smoke sticks or thermal imaging.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Heavy extension cords, as short as possible, for recovery machines

General

Plus: drier per schedule, fittings leak-checked every 3 months, verify overfill shutoff, check gauge calibration, tank recert dates (5 or 10 yr; >300 psig cylinders every 5).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Humidifier commissioning/maintenance

General

Level in all directions; check leaks; grounded receptacle; monthly check for bacteria/scale; treat reservoir water.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

IAQ interview clues

General

Complaints that fade when the occupant leaves the house, RH >60% or <40%, musty odor, stuffy rooms, condensation on windows/pipes. Walk-through checklist in Fig. 28-8 (dust, pests, leaks, stains, soot near furnace/water heater, gas/oil odor).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Inspection checklist habit

General

Written periodic inspection (connections, safeties, noise, charge, moisture indicator, oil level, water flow, coils, supports, cleanliness; belts/coupling on open drive; overload/relay/capacitors on hermetics) — give the owner a copy.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Insufficient draft causes

General

Too many appliances on one flue, soot/nest/bricks in chimney, tight house (add dedicated combustion air pipe).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Interrupted vs intermittent ignition

General

Interrupted sparks only at start (then ignition carryover); intermittent sparks all cycle; field slang mixes these up.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Magnetic-bearing centrifugals

General

Need no oil; 60–300 tons; indoor non-freezing only.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Maintenance check sheet items

General

Leak test, head & suction pressures, oil level, water valve/drain, motor lube, belts, clean both coils, straighten fins, V & A readings, fans, filters, tighten electrical connections, lube moving parts; note wear parts (belts, filters) to bring next visit.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Manifold valve positions

General

Both closed = read pressures; both open = evacuate (pump on center); one open = charge that side.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Mercury spill

General

Evacuate 15 min, ventilate, shut HVAC, never touch/vacuum/wash down drain; send old mercury stats to an authorized recycler.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Never just reset a tripped protector (Pro Tip)

General

A tripped overload, pressure switch or oil safety is a symptom of a system problem; find the root cause rather than replacing the device.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Never put system pressure into a vacuum (micron) gauge

General

Water/ice/pressure ruins it. Don't leave compound gauge on the high side continuously either.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Nozzle handling rules

General

Never clean, never swap internals, hold by hex flats, keep boxed, use nozzle tool, exact replacement (gph/angle/pattern), replace yearly.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Oil deaerator install

General

One-pipe needs bypass plug installed in the pump; two-pipe already has it; filter goes between tank and deaerator, never between deaerator and burner (clog → pressure → blown seals). Cap the old return line.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Oil nozzle cleaning on boilers

General

Cloth and solvent only, never a wire brush (scratches orifice).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Oil nozzle size errors

General

Too small = weak flame; too big = smoke/soot and short cycling. Replacing with higher-capacity nozzle = overfiring (red flue pipe, high stack temp). Never raise pump pressure to "fix" a problem.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Only a high-side gauge on a nitrogen test

General

Compound gauge gets wrecked.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Re-zero gauges for elevation; never adjust psia gauges

General

Open gauge to air, set needle to 0 with calibration screw.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Replace burner motor with same rotation, frame and rpm

General

; pump speed = motor speed (3450 or 1725).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Required safety devices (most codes)

General

High-pressure control, spring relief valve or rupture disc, fusible plug where fire exposure; outdoor access ports need locking caps (IMC 1101.10); refrigerant lines across walkways ≥ 7'3" above floor unless at ceiling; no refrigerant piping in elevator shafts, public stairways or exits (IMC 1107.2).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

RTU curb gasket

General

Resilient gasket between base rail and curb is the leak barrier; acoustic lining and 2" fiberglass in the curb for noise.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

RTU oil tank

General

Never more than 10' below the burner.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Smoke test

General

Run 5 min until flue temp stabilizes, pull sample through filter paper, compare to scale.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Sooty electrode insulators short the spark

General

Carbon conducts; clean porcelains.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Stack relay must see flue gas

General

No damper between it and the gas; trips if no ignition in ~15 s. Stack-relay furnace = probably time to replace.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Sulfuric acid in cold flues

General

Noncondensing oil flue must stay warm; low-sulfur oil or additives reduce yellow-red crust; biocide additive for sludge; dual filters (bypass at tank, no-bypass at burner) on wet-tank jobs.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Thermistor vacuum gauge tube cleaning

General

Fill with solvent, rock, dump, repeat 2–3×, alcohol rinse.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Traditional CO2 fluid test needs temperature correction

General

Cooling sample reads CO2 high.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Troubleshooting discipline

General

Get the complaint and recent repair history, pick a probable cause, verify with pinpoint tests, repeat until found, then pick the remedy; fixing the symptom (e.g. just adding gas without finding the leak) guarantees a callback. Write down the exact model number and use the maker's chart for that model.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Unattended evacuations: solenoid on the pump inlet wired parallel to the motor

General

Power loss closes it, so system vacuum can't suck oil out of the pump.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Underground oil tanks

General

Check for water with water-finding paste before each fill/annually; seal plugs with temperature-resistant compound, never Teflon tape. Pitch tank to fill end; silver paint outdoor tanks; keep full in summer to limit condensation.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Upgrade transformer to 14 kV

General

When replacing a 10/12 kV unit — helps with cold oil/cold air/low voltage. Solid-state igniters (20 kHz) can't be tested with a 60 Hz transformer tester; don't short their terminals.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Use a pipe nipple on heavily-used gauges

General

Saves the gauge threads from repeated on/off.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Use gauge boots

General

Cheap protection against drops.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Vacuum/barometric draft regulator

General

Counterweight adjusts how much room air bleeds into the flue; thermostatic and electric vent dampers also exist.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Check charge at the right outdoor temp

Heat pump

>70 °F cooling, <40 °F heating; follow mfr.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Crankcase heater prevents flooded starts

Heat pump

On off cycle refrigerant migrates to the coldest point — often the compressor. Oil foams on start, pumps out (oil slugging) and breaks valves/gaskets. Heater keeps crankcase ~30°F warmer. Heat pumps and outdoor units need one even when small.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Gas/oil furnace aux heat must never run with the compressor

Heat pump

Hot furnace air across the indoor coil spikes head pressure and can burst the coil; verify interlock on every seasonal check.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

HP TXVs are MOP/cross-charged

Heat pump

Heat pump TXVs have limited vapor charge because the idle bulb sits on hot gas; don't swap in a standard AC TXV.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

MOP TXVs stay shut during pull-down

Heat pump

A gas-charged or pressure-limited TXV won't open until suction drops below its MOP — don't condemn it for staying closed on a warm start. Used on AC and heat pump coils.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Split-system charge is for a given line length

Heat pump

Nameplate charge assumes a specific liquid line length; add per the tag for longer runs.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Acid test the oil

Refrigeration

Pull a sample from the sump/drain (hermetics: out of the suction stub after removal) with an acid kit; acid = windings breaking down, replace compressor before a full burnout contaminates the system. Keep samples sealed until tested.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Adding oil

Refrigeration

Only from a fresh unopened container (opened oil absorbs moisture); lowest-wax oil for the temperature range (wax plugs orifices); meter slowly, listen; too much oil = liquid pumping damage. Fast method: tube on manifold centre port, evacuate tube, immerse in oil, front-seat SSV, let crankcase vacuum draw it — keep the tube submerged or you pull air.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

After back-seating, back off slightly so the seat doesn't "freeze"

Refrigeration

Thermal contraction can jam the valve; a hair of clearance prevents a snapped stem later.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Air in the system breaks the P/T chart

Refrigeration

Dalton's law: non-condensables add their own pressure, so measured pressure is higher than the refrigerant's true saturation pressure.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Ammonia

Refrigeration

Attacks copper/bronze with moisture (steel piping); stand to one side of valves; respirator; sulfur candle/stick shows white smoke at leaks; needs its own gauges, as do HC (isobutane) systems.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Atmospheric balancing before opening

Refrigeration

Never open a section in vacuum — bleed a whiff of vapor from high side (or recovery cylinder) until compound gauge reads 0–1 psig so air and dirt don't rush in. Plug every opening immediately; clean and dry joints first.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

AXV bleed systems need an accumulator

Refrigeration

A bleeder AXV allows a low-torque compressor but the system must have exact charge and an accumulator or it will frost back and slug.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Breaker strips

Refrigeration

Brittle plastic trim hiding refrigerant lines; let warm to room temp before removing.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Bubble solution: wipe it off afterwards

Refrigeration

It oxidizes copper. Specialized solutions outperform soap; big high-pressure leaks can blow through without bubbling. Bubble test is the only reliable one near urethane insulation (halide & electronic detectors false-alarm on the foam blowing agent).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Bubbles don't always mean low charge

Refrigeration

Uninsulated liquid lines through hot attics/roofs, long risers or an upstream restriction all make bubbles with a full charge. Confirm with head/suction pressures and superheat/subcooling.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Cap tube = charge-critical, equalizing system

Refrigeration

No receiver and pressures equalize on off cycle, so overcharge fills the evaporator; heavy suction-line frost at start-up = overcharged. Replace the liquid line filter-drier whenever a cap tube is replaced, and use the same length and ID.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Cap tube failure causes

Refrigeration

Most common: bent/crimped during condenser cleaning, or plugged by wax from overheated oil / compressor burnout. Newer designs use larger ID and longer tubes to resist plugging.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Cap tube sizing effects

Refrigeration

Too long/too small ID → liquid backs into condenser, starved evap, high head, low suction, high subcooling. Undercharge → runs continuously; overcharge → frosted suction line, oil slugging.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Change recovery machine oil after a burnout or before switching refrigerant

Refrigeration

Also swap the drier and evacuate the machine and hoses between refrigerants.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Charging direction and state

Refrigeration

Vapor: cylinder upright, into the low side (compressor running for top-off; high & low side with compressor off for initial fill, then close high-side valve before starting). Liquid: cylinder inverted, into the high side with compressor off; finish into the low side only through a metering/"quick charge" device with the compressor running. End by bleeding the high-side hose liquid through the manifold into the low side.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Check suction filter-drier by pressure drop

Refrigeration

Use the access ports on inlet and outlet; any measurable drop while running = replace the core. Undersized suction driers hurt oil return and efficiency.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Clean and oil valve stems before turning; oil port threads each time

Refrigeration

Rusty/scored stems destroy packing. Coat with the system's refrigerant oil before capping.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Clean terminals before ohming

Refrigeration

Dirt/oil on terminals gives false high/OL readings; move the probes around before declaring an open. Tag wires before pulling them.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Cold filter-drier = clogged

Refrigeration

Compare inlet and outlet temps of the liquid-line drier; a drop (or sweating/frost on the drier) means it's acting like a metering device and is restricted. A working drier runs slightly warm as it absorbs water. Install an oversized temporary drier to clean a wet/dirty system.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Compressor removal cautions

Refrigeration

Start/stop a few times during pump-out to avoid oil pumping; never rest an open-drive compressor on its flywheel (ruins seal); turn idle compressors by hand and add an ounce of detergent oil before start; use blank flanges; drain oil at once; keep gauges connected until the system is open.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Condensing unit location

Refrigeration

Close to the load, roof/basement, ventilated room with fresh-air intake (refrigerant leak can displace oxygen), screened openings, never beside stoves/steam/sun, not in the same room as the box (heat/noise); outdoor conversions need shroud, crankcase heater and insulation in cold climates; monitor room temp in summer. Cages/locks per code; label every line with refrigerant.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Counterflow matters

Refrigeration

Tube-in-tube condensers and SLHX are piped counterflow (water in where refrigerant exits) to keep a temperature difference along the whole length.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Crankshaft seal leak test

Refrigeration

Front-seat SSV, pull crankcase vacuum, then front-seat DSV with compressor running: head pressure creeping up = air entering through the seal. Oil on the seal/floor confirms. Crankcase must be >0 psig to use a leak detector.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Discarding a fridge

Refrigeration

Remove the door (child suffocation law), recover refrigerant, label as recovered.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Don't use the system compressor as a vacuum pump

Refrigeration

Motor windings need vapor flow to cool; it will overheat.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Dual compound gauges in multi-case systems

Refrigeration

Put one at the remote evaporator and one at the compressor; also read across each EPR separately (its drop is not line drop).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Dye: don't overdose; match dye to oil type; may need up to 24 h to show

Refrigeration

Excess dye cuts capacity and viscosity. UV dye has no false positives but sun/wind hide small leaks.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Electronic detector technique

Refrigeration

Kill fans/drafts, probe BELOW the joint (refrigerant sinks), move ~1 in/s, keep tip off surfaces (use plastic guard). No need to re-set for refrigerant type. Never in flammable atmospheres.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Electronic/halide/dye detectors can't see nitrogen

Refrigeration

For a nitrogen-only test use ears, bubbles, or ultrasonic.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Epoxy repairs

Refrigeration

Two-part works at room temp, cures in ~5 min pot life, hours to harden; holes ≤1/16" seal directly, larger need a patch of same material; pull 5 in. Hg vacuum first so epoxy is drawn in; test 5 psig then 100 psig. Buy refrigerant-compatible epoxy from an HVAC wholesaler; 6-month shelf life. One-part: heat beside the leak, not on the epoxy; clean with stainless brush, no emery.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

EPR "two-temperature valve" problems

Refrigeration

Leaking EPR → that evaporator runs too cold (freeze risk); stuck shut/clogged screen → no cooling in that case; frost on bellows → relocate valve out of the freezer. Adjust with gauge on inlet port, ½ turn, wait 15 min. Install a shutoff valve with access port after each EPR.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

EPR access port (Pro Tip)

Refrigeration

EPRs have a gauge port on the evaporator side for checking/adjusting the setting. EPR aliases: holdback valve, ORI valve, two-temperature valve, constant pressure valve.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Faster recovery and evacuation

Refrigeration

Short, fat hoses; remove cores; vacuum gauge close to the system via tee; metal or heavy-duty hoses (standard hoses collapse and are permeable). Inline filter-drier ahead of the recovery machine inlet, always.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Feel the lines first

Refrigeration

Suction line should be cool, not cold (cold = liquid coming back); liquid line warm, not hot (hot = uncondensed vapor, usually low charge or bad condenser).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Filter-drier placement and sizing

Refrigeration

Size by compressor hp, run hours and refrigerant quantity (go one size up when between sizes, and account for extra line volume); keep the liquid drier near the TXV away from the hot condenser — a hot drier gives moisture back; keep sealed until installed; replace drier whenever indicator goes wet, sometimes several times.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Flooded head-pressure systems need big receivers and traps

Refrigeration

Charge ≈ 2× normal; receiver must hold it all in summer; discharge line trap stops off-cycle liquid draining back into the compressor; inverted trap or (better) a check valve between LAC and receiver stops liquid flowing back to a cold condenser.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Flush a dirty TXV needle

Refrigeration

With the system running, repeatedly open and close the king valve/liquid hand valve so liquid surges scrub the needle; if it fails, clean/replace inlet screen or valve. A clogged inlet screen means the drier is disintegrating — replace it too.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Flush after burnout or when reusing line set on a retrofit

Refrigeration

R-11 is gone; use modern flush agents.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Four musts for outdoor commercial condensers

Refrigeration

Head pressure control for low ambient, short-cycle prevention, oil dilution protection (crankcase heater/pump-down), weatherproofing.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Front seat the SSV to swap a compressor without full recovery

Refrigeration

Isolates the suction line at the compressor.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Frost line tells the charge

Refrigeration

Sweating/frosted suction line = liquid floodback (overcharge or stuck stat). Partially frosted evaporator = starved (low charge or partial restriction).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Frosting drier = restriction

Refrigeration

A frosted/cold liquid-line drier means a pressure drop inside it — partially clogged. A warm evaporator with correct low pressure means no liquid is reaching it.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Hand pump can push oil in against 300 psig

Refrigeration

Purge charging lines first.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Hard start kit install

Refrigeration

Power off; remove original start relay; 3-wire kit: red+white to run cap, black to C; 2-wire kit: across S and R. Cycle several times; leave the kit on or go back to a new cap + original relay. Choose kit by voltage and hp. Stuck causes: long idle, plating, liquid refrigerant in compressor.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

HFO A2L / HC A3 are flammable

Refrigeration

Treat HFO-1234yf/ze as mildly flammable; propane/isobutane highly flammable; halide torches & electronic detectors are banned in flammable atmospheres.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

High-side liquid charging

Refrigeration

Compressor OFF, cylinder inverted on scale into the receiver valve; stops when pressures equalize — finish with low-side vapor, or isolate the receiver and feed liquid into the liquid line with compressor running (liquid only or TXV damage). If the receiver can't be isolated the compressor will pump into the cylinder — rupture risk. Queen (inlet) and king (outlet) valves give options.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Hissing at the metering device = starved

Refrigeration

A hiss at the TXV/cap tube always means vapor is passing — undercharge or liquid-line restriction; bubbles in sight glass confirm.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Ice in the TXV can also leak

Refrigeration

Frozen moisture may hold the valve off its seat during the off cycle → flooded evaporator, sweating suction line, low-pressure control short cycles. Fluctuating suction pressure is a clue.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Ice machine head pressure check without gauges

Refrigeration

Water-cooled: outlet water 95–105°F = normal. Lower water temp with normal head = light scale; high head beyond range = heavy scale (water valve can't compensate) → descale or replace condenser.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Ice machines: keep the gauges off

Refrigeration

Most are critically charged; every hose connection loses charge. Diagnose with temps, amps, trouble codes, ice shape/quality. If you must connect, use the shortest possible path and a quick-connect coupler, no standard hose.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Identify compressor lines

Refrigeration

Line going straight to the condenser is the discharge; follow others back; check the manufacturer diagram. Pinch then cut oil-cooler lines; catch oil in glass jars.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Identify refrigerant from the tag, never by smell or colour

Refrigeration

Sniffing can be lethal; use a refrigerant analyzer if unknown.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Inefficient compressor fingerprint

Refrigeration

High suction, low head, high superheat, NORMAL subcooling (refrigerant still has time to subcool) — same for cap tube or TXV. Leaking valves click sharply; worn piston/cylinder gives a duller click; rods/bearings loud at low suction.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Install a Schrader after recovery

Refrigeration

Whenever a system is opened, add a permanent access fitting for next time.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Inverted-cylinder oil charging

Refrigeration

Oil + a little refrigerant in a service cylinder, invert, connect; cylinder pressure above system pressure pushes oil in.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Isolate the smallest section

Refrigeration

Close the valves that strand the least refrigerant — liquid-line manifold valves or the liquid solenoid may beat the king valve. Jumper safety controls (LP/HP/temp) that would stop a pump-down, and watch gauges while they're jumpered.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Judging drained oil

Refrigeration

Should be translucent; dark/black & pungent = compressor failure; metal chips = replace compressor. Contaminated hermetic oil can be acidic — no skin contact. Fit new driers/filters afterwards.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Keep oil sealed; buy small containers

Refrigeration

POE/PAG pull moisture from air. Reseal immediately; some oils need plastic, some metal containers.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Leaky discharge valve test

Refrigeration

Front-seat SSV and DSV, run briefly, stop: high gauge falling and suction gauge creeping up = exhaust valve leaking; if suction climbs to ~0 psi the intake valve leaks too. Fluctuating head pressure while hand-turning = leaky exhaust valve.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Liquid / push-pull recovery restrictions

Refrigeration

No liquid recovery on heat pumps or <10 lb systems; no push-pull on heat pumps/reversing valves, systems with accumulators, charges under 5–10 lb, or where a solid liquid column can't form. Push-pull needs <1 lb refrigerant already in the cylinder to start; stop when bubbles appear in the sight glass, then finish with vapor recovery. Not all machines can take liquid.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Liquid line admission after service

Refrigeration

Open the receiver valve slowly — a slug of liquid can rupture the TXV screen or pack the drier desiccant into a clog.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Loosen service-valve packing nut one turn before turning the stem, retighten after

Refrigeration

Skipping it causes stem leaks.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Low head pressure can stop refrigerant flow

Refrigeration

In cold weather the high side cools, pressure falls, and the TXV loses the pressure differential it needs; evaporator starves, superheat and subcooling read high, compressor may short-cycle.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Low-side charging rules

Refrigeration

Vapor only through the low side (liquid = slugging); warm the cylinder with hot water/heat lamp/blanket, never a torch; never leave a cylinder connected unattended; only pure refrigerants (R-134a etc.) can be vapor charged — zeotropic blends (R-404A…) must go in as liquid or they fractionate. Hermetics need vapor through the low side when topping off to cool windings. Run 10–15 min then verify superheat/subcooling/pressures.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Low-side restriction signature

Refrigeration

Evaporator pressure high but suction pressure at the compressor low/vacuum; suction-line drier is the usual culprit. Gauge at the evaporator, not just the SSV.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Megger after any overload

Refrigeration

Overheated windings lose insulation; test after overloads and ask the owner how often it has tripped. Don't megger under vacuum (arcs, false readings). Only experienced techs — 500–1000 V can further damage weak insulation.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Moisture indicator tricks

Refrigeration

Three-colour indicators give early warning — schedule the drier change. Warm cold internal parts to room temperature with a heat lamp before opening so moisture doesn't condense inside.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Moisture-freeze signature

Refrigeration

Suction pressure slowly falls over hours (even to vacuum), then suddenly normal, repeating; full defrost fixes it temporarily. Warm the metering device with a hot pad during off-cycle — if it then works, there's moisture. Fix = new drier (plus recover/filter charge, evacuate).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Monitor cylinder pressure on the unused valve during recovery

Refrigeration

Must stay below the cylinder's service pressure.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Motor overheat without high amps

Refrigeration

Low refrigerant flow (vapor cools hermetic motors) and short-cycling restarts overheat motors even at normal current; external bimetal overloads only see current, so a motor can still cook.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Move cylinders >35 lb on a chained hand truck

Refrigeration

Move cylinders >35 lb on a chained hand truck

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Mufflers mount vertical near the compressor

Refrigeration

Suction/discharge mufflers are installed close to the compressor, usually vertically, so oil moves through and condensed refrigerant drains.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Never mix your own blend; zeotropes must be charged as liquid

Refrigeration

400-series blends fractionate if vapor-charged; always liquid charge (into high side, or metered into low side). Pure refrigerants vapor charge.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Never overfill a recovery cylinder; never heat any cylinder with a torch

Refrigeration

Fixed volume + temperature rise = pressure rise = rupture. Don't assume a float switch exists; monitor on a scale and stop manually — the float switch is a backup, not the shutoff.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Never pressurize with oxygen or acetylene

Refrigeration

Oxygen explodes with oil; acetylene unstable above 15 psig. Nitrogen, CO₂ or argon only, always through a regulator with a relief valve (180 psig).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Never tighten a cold port plug into a hot valve

Refrigeration

It shrinks on and won't come out. Stuck plug: warm valve body gently with torch (no glow) and turn.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Never use disposable cylinders for recovery / transport of recovered refrigerant

Refrigeration

Illegal and unsafe; evacuate empties to atmospheric before disposal. Store disposables under 125°F (51°C).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Never use liquid "antifreeze" driers

Refrigeration

Methanol-type agents keep moisture circulating, attack motor insulation, and dump moisture already held by a solid desiccant.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Nitrogen test for drier vs cap tube

Refrigeration

Separate cap tube from drier, feed regulated nitrogen through piercing valves; full flow from drier end and small flow from cap tube end = both open; no flow = that part is clogged. Replace drier any time the circuit is opened.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Noisy compressor after charging = oil went with the refrigerant

Refrigeration

Add oil only when there's evidence of oil loss; replace oil after a low-side leak let in air/moisture (measure out, replace same amount). Lost refrigerant always carries some oil.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Noisy unit checklist

Refrigeration

Compressor knock = oil (check sump level/sight glass; dark brown oil = replace; test for acid); shrill squeal = dry shaft seal; fan roar/rumble = bent blades, bearing slop, blades hitting shroud, overheating from blocked airflow; belt squeal = dress belt / align pulleys (never oil); whole unit rumble = brittle rubber mounts or broken springs; high head pressure makes the compressor vibrate more.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Noncondensables quick test

Refrigeration

Jumper the condenser fan to run, shut compressor off, watch head pressure: steady gradual drop = normal; stays flat for several minutes = air in system. Precise test: let condenser reach ambient, read head pressure, compare P/T temp to ambient — any difference = air. Fix = recover through driers, evacuate, weigh in charge.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Oil binding on flooded evaporators

Refrigeration

If a float-type evaporator isn't level the oil-return hole doesn't work; oil floats on the refrigerant and kills boiling (oil binding). Level the unit.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Oil safety control time delay

Refrigeration

Low net oil pressure at start-up is normal; the control's delay prevents nuisance trips. Many require manual reset on purpose so someone investigates.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Oil separator liquid migration

Refrigeration

On long off cycles refrigerant condenses in an uninsulated oil separator and sits under the oil; it can be pushed back to the crankcase through the float. Fixes: insulate the separator, check valve on separator outlet, or a thermostat-controlled solenoid in the oil return that only opens when the shell is 100–130°F.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Oil-pressure unloader cycling

Refrigeration

Oil-pressure unloaders on large recips are driven by low-side pressure switches; a timer bypass runs full capacity about a minute every hour or two.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

P/T chart temp is the boiling temp, not the pipe temp

Refrigeration

Chart gives saturation at that pressure only; actual vapor can be warmer (superheat).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Parallel compressors must be level

Refrigeration

Oil level regulators on racks only balance oil if all compressors sit on the same horizontal plane.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Passive recovery boosters

Refrigeration

Chill the recovery bag/cylinder in ice water, warm the compressor (crankcase heater, blanket, heat gun — never torch), tap the compressor with a soft mallet to free refrigerant from the oil. Passive is for ≤5 lb systems.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Piercing (tap) valves for hermetic systems without ports

Refrigeration

Clamp-on piercing valves give gauge access on small sealed systems (refrigerators, window units).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Post-compressor-swap sequence

Refrigeration

Nitrogen+trace leak check (~25 psig), deep vacuum held ≥1 h, break vacuum with refrigerant to ~atmospheric, reconnect electrics, run briefly, weigh in charge, leak check again, run several cycles, check superheat, data logger 24 h.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Post-condenser (Yoder) loop leaks

Refrigeration

Usually a screw driven into the loop; isolate the loop, pressurize with nitrogen and watch a gauge — drop = leak. Loop can't be replaced.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

psi vs psig vs psia

Refrigeration

Value followed by psi/psig is gauge; psia is absolute (gauge + 14.7). Use absolute for gas-law math.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Pull Schrader cores for evacuation and recovery

Refrigeration

Use a core-remover tool; cores restrict flow badly. Piercing valves are too small for deep vacuum — use real ports.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Pump-down check

Refrigeration

Keep running until compound gauge holds vacuum AND liquid line/evaporator feel warm — cold = liquid still there. Warm the evaporator gently with a heat gun on low to finish.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Pump-down steps

Refrigeration

System off, front seat king valve, compound gauge on low side cracked, run compressor, at 0 psig stop and quickly front seat the DSV (and queen valve if fitted). Saves recovery/evacuation/recharge when opening the low side.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Purge gauge hoses with system refrigerant, loosely connected, before opening service valves

Refrigeration

Small release during purge/recovery/charging is allowed by EPA. Quick-connect fittings cut losses.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

R-407C/R-410A run higher pressures than R-22

Refrigeration

Only in systems designed for them; R-134a is a possible R-22 retrofit in chillers.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Read cylinder label, not just colour

Refrigeration

Colour codes aren't mandatory and several are near-identical. Recovery cylinder valve colours (blue vapor, red liquid) also aren't guaranteed — read the handle.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Receiver systems tolerate charge variance; cap-tube and high-side-float systems don't

Refrigeration

Cap-tube systems must be charged exactly by weight; cap-tube systems have no receiver.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Record oil removed

Refrigeration

Replace with the same amount; replace oil entirely after a low-side leak (air/moisture in). Add oil only if there's evidence of oil loss.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Recover from the liquid line into an ice bath

Refrigeration

Pulling liquid from a liquid-line valve (not the DSV) is far faster; sit the recovery cylinder in ice water and run the compressor intermittently to hold head pressure down. Stop at the first chugging sound (oil pumping). Warm the low side with a heat lamp/warm water to speed up — never a torch (fusible plugs, joints).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Recovery "rest test"

Refrigeration

After reaching vacuum, close the low-side valve, note pressure, wait 5 min; a rise ≥10 psi means cold refrigerant pockets remain — recover again.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Recycled refrigerant can't change owners

Refrigeration

Only reclaimed (AHRI 700-tested) refrigerant can be sold; recycled gas goes back into the same owner's equipment.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Rotary compressors need check valves

Refrigeration

Rotary compressors rely on a discharge check valve (and often one in the oil/suction line) to stop backflow on the off cycle — a leaking check valve shows as pressure equalization problems or reverse oil flow.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Run 3 cycles with gauges on

Refrigeration

After installing piercing valves, run at least three full cycles and log LP, HP, evap and condenser temps.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Run the compressor a few minutes before recovery

Refrigeration

Boils refrigerant out of the oil so you get more out.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Rupture disc gauge

Refrigeration

Pressure showing on the rupture-disc gauge means the disc has burst and the system may have lost charge.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Screw compressors

Refrigeration

Need an oil separator (they pump lots of oil).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Scroll compressors

Refrigeration

Tolerate some liquid, can't be repaired, may lose oil at low temp, have a discharge check valve to stop off-cycle equalization.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Second drier on suction

Refrigeration

After a burnout, add a suction-line drier between evaporator and compressor to prevent repeat burnout; drier at cap-tube inlet on high side. Install arrow in flow direction, just before the metering device, caps off only at install.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Short-cycling: find the control first

Refrigeration

Thermostat differential too tight; low-pressure control cycling because TXV/compressor valves leak during off cycle (suction line sweats); overload (check voltage/amps, wrong protector); high-pressure switch (overcharge, dirty condenser, air, no condenser water). Anti-short-cycle timers protect the compressor but don't fix the cause.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Sight glass bubbles on cap-tube systems don't mean low charge

Refrigeration

Liquid can flash in the liquid line of cap-tube systems; only trust sight glass on TXV/other metering devices, and only as a confirmation.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Sight glass goes after the drier

Refrigeration

Mount sight glass/moisture indicator between drier and TXV, close to the TXV; otherwise it reads moisture the drier would have caught, and can't reveal a drier pressure drop. Start-up bubbles are normal; persistent bubbles = low charge or restriction.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Sight glass on big liquid lines

Refrigeration

Above ~2-1/2" lines, tee a small parallel 1/4" loop with a sight glass; bubbles divert through it.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Split condenser (summer / summer-winter)

Refrigeration

Three-way split condenser valve isolates half the condenser in cold weather; check valve on summer condenser outlet; bleeder line to suction reclaims the stranded charge. Uses far less refrigerant than valve flooding.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Standing vacuum test for leaks

Refrigeration

Many recovery machines hold vacuum on a timer and alarm if it rises — tells you there IS a leak, not where.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Start-up checklist

Refrigeration

Clamp meter on compressor, gauges on, water on for water-cooled, shut down at first sign of trouble; frosting suction line = TXV open too far, sweating = not open enough; record temps/pressures/amps for the first week and 24 h minimum; keep the record with the unit for future troubleshooting.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Starved vs flooded evaporator from the TXV

Refrigeration

Starved (poor cooling, frost/sweat only at the valve, no flow sound): clogged screen, lost bulb charge, frozen moisture, wax from wrong oil, undersized orifice. Flooded (frost/sweat past the bulb): loose/uninsulated bulb in warm air, needle stuck open, oversized valve/orifice, wrong bulb charge, undersized coil, too much coil pressure drop.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Suction filter-drier causing pressure drop should come out

Refrigeration

Low-side driers must have minimal resistance; a noticeable drop across one means remove it.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Thaw a frozen TXV without opening the system

Refrigeration

Pump down (or shut off), lock out, warm the valve above 32°F with a heat gun or warm rag — never a torch — then restart and watch pressures. Recurrence = replace drier; worst case recover and dehydrate.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Tiny leaks matter

Refrigeration

Domestic systems hold 1–2 lb; a 1 oz/yr leak must be found and fixed.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

TXV frost pattern shows charge

Refrigeration

On a below-freezing coil a healthy TXV frosts evenly back to the liquid-line fitting; if the body next to the liquid line stays clear, the system is short of refrigerant. Doesn't work on above-freezing coils.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Use a fixed wrench, not a ratchet, to crack a stuck valve

Refrigeration

So you can close it instantly if it leaks or breaks.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Vacuum stalls at ~5000 microns = water or ice in the system

Refrigeration

Look for a cold/frosty/sweating spot; isolate pump, stop, let it thaw or warm with heat lamp/blanket/heat gun (never torch).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Valve adapter use

Refrigeration

Tighten packing around stem before opening needle; leak-test at 15–20 psig.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Warm the whole system evenly during evacuation (heat lamps / room heat, never torch)

Refrigeration

Pressure drop freezes moisture; even warming prevents re-condensing in cold spots.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Wax vs ice in cap tube

Refrigeration

Ice clears with a new drier; wax needs cap tube AND drier replaced plus fresh low-wax oil. Check by warming cap-tube/evap joint with rag and warm water (no flame): hissing as it melts = ice. No hiss after running = clogged.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Wax-free synthetics have no floc point

Refrigeration

Mineral oil wax can plug orifices at low temp; floc test = 10% refrigerant / 90% oil cooled until cloudy.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Wet insulation signs

Refrigeration

Unit runs more than normal and cabinet exterior feels cold → air leak in outer shell letting moisture into insulation; find and seal the leak, dry it out. Freezers can buckle from ice in insulation.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Tubing & brazing

Cutting, flaring, swaging and brazing copper and keeping joints clean.

Annual AC inspection checklist (10 steps)

Air conditioner

Clear vegetation/erosion around condenser; clean both coils (self-rinsing cleaner indoors, water outdoors); disconnect drain, wet-vac, replace clogged tubing with clear; vacuum pan + algaecide tablets; inspect contactor points for burning; amp each strip-heat element vs spec; cycle time-delays and verify run time; measure evaporator split 16–20°F; inspect/mastic ducts; restore original settings.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Carry by the base pan

Air conditioner

Never use coils/tubing as handles when moving a room AC chassis.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Never torch a drier out (Caution)

Air conditioner

Heat drives moisture out of the desiccant back into the system — cut driers out with a tubing cutter. Driers work best cool.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Plastic blades, never metal, for fin cleaning

Air conditioner

Metal pokes through tubing; pressure wash/detergent outdoors; straighten bent fins.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Vibration absorber installation

Air conditioner

Install horizontal, perpendicular to the vibration direction, as close to the compressor as possible on both suction and discharge; anchor the far end; two absorbers at 90° if vibration is in two axes. Never stretch, compress or twist; don't overheat the braid when brazing.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Elbows

General

Long-radius or vaned elbows; air crowds the outside of a bend and gets turbulent.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Fan noise fixes

General

Rattle = tighten setscrew; press-fit hub loose = replace blade; bent/vibrating blades = replace, don't balance in the field; blades hitting shroud = move fan on shaft or move housing, never bend blades.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

One-pipe vs two-pipe oil

General

One-pipe: half the tubing, less copper-sludge, but must be bled; two-pipe: self-priming, but pumps ~15 gal per gal burned and a pressurized return leak goes unnoticed.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Round duct orientation

General

Crimped end points in direction of airflow; screw through the overlap; mastic the joint. Starting collar tabs bend inside plenum.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Insulate the accumulator and suction line (Caution)

Heat pump

Condensation on an uninsulated accumulator rusts seams, braze joints and bends; inspect those spots and replace a rusted accumulator.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

R-22 → R-410A is not a drop-in

Heat pump

Needs smaller metering device (10–15%), thicker-wall compressor & tubing, 600 psi drier, 235 psi evaporator, R-410A-rated condenser, POE oil with 2–3 oil changes to flush mineral oil, 800 psi gauges/hoses, 400 psi recovery cylinders. Contact OEM before any retrofit; usually metering device and oil change plus flush.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Replacing a reversing valve

Heat pump

Filing old braze risks filings in the circuit; many techs cut in all new stubs. Wrap body in wet rag / heat paste; a tuning-fork tip heats three stubs evenly.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Aluminum evaporator epoxy

Refrigeration

Sand oxide, file sharp edges, clean, start immediately (oxide reforms fast); plug big holes with metal; system must be open to atmosphere (no positive pressure) while epoxy cures ≥1 h; foil reinforces. Avoid brazing aluminum (anneals).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Bolt-on piercing valves are temporary

Refrigeration

Vibration loosens them; replace with brazed-on valve or a tee with Schrader. Install on clean, straight, round tubing with a dab of system oil on the seal; back the needle out first; back it off after piercing to open; leak-test.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Brazed-on piercing valve: remove core and gasket before brazing, system evacuated, no soldered joints or flammables nearby; metal-seal caps 1/8 turn past finger tight

Refrigeration

Brazed-on piercing valve: remove core and gasket before brazing, system evacuated, no soldered joints or flammables nearby; metal-seal caps 1/8 turn past finger tight

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Cap tube on suction line = free heat exchanger

Refrigeration

Soldering part of the cap tube to the suction line subcools liquid and superheats suction, improving efficiency and protecting the compressor.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Charging small cap-tube systems

Refrigeration

Charge by weight from nameplate; cap-tube pressures equalize fast so evacuation/charging is quick. Leave suction piercing valve for future service; pinch process tube behind valve, remove, crimp, braze, leak test.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Cutting capillary tube

Refrigeration

File a notch and snap it; a tubing cutter deforms the ID. Replacement must match ID and length exactly.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Handling precharged line sets

Refrigeration

Unwind carefully; kinks crack and leak later.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Hard-drawn ACR tubing practices

Refrigeration

Finish all connections the same day; cap open tubing; support hard tube every 6–8 ft, soft every 4–6 ft; wrap tube with plastic tape under galvanized clamps (galvanic corrosion); sleeve through walls/floors with conduit and seal; plastic-coated tube for burial/harsh spots; keep lines away from steam/hot-water pipes and furnaces (flash gas, suction sweating). Vibration loops only at the condensing unit.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Identify the metal with a scrape and a magnet

Refrigeration

Steel gray-white & magnetic; copper reddish; aluminum white, soft. Aluminum: solder/braze/epoxy; steel/copper: braze. Clean the lengthwise seam on steel tube before brazing. Use a heat shield next to flammables/sheet metal.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Leak-test the job properly

Refrigeration

Nitrogen through regulator + relief valve, rap each joint with a rubber mallet (paint/flux can mask leaks), bubble or ultrasonic, 24 h standing pressure; if no leak found, evacuate, add ~10 psig refrigerant, top with nitrogen and use an electronic detector; rebraze leaking joints from scratch. Venting that trace mix is allowed by EPA 608.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Never heat a filter-drier

Refrigeration

Drives moisture back into the system → freeze-ups and acid.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Noise hunting

Refrigeration

Ultrasonic detector finds vibration/leak noises. Rattling tubing: bend gently away or clamp rubber blocks / wrap tape at contact point and mid-span. Secure loose baffles with self-tappers or sealant.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Oil-pressure control trip causes

Refrigeration

Wrong oil level, wrong tubing size, unbalanced system, low charge, refrigerant migration, mis-set control, electrical faults, compressor faults.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Phosgene when brazing a pinch-off

Refrigeration

Equalize high/low sides and ventilate before brazing a pinched process tube; heating leaking refrigerant makes phosgene.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Pinch-off tool

Refrigeration

Only soft copper ≤3/8" OD; pinch only when necessary; don't over-tighten; leave tool on until end is brazed.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Plumbing vs ACR copper

Refrigeration

Plumbing tube is sized by ID and may carry processing oils incompatible with refrigerant oil; ACR tube is OD-sized, nitrogen-sealed. All elbows/bends long radius.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Prep before opening a sealed system

Refrigeration

Unplug, clean every component surface, install access valve, recover, cut tubing, plug ends if not replacing immediately, braze with flowing nitrogen.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Process tube adapter

Refrigeration

Cut sealed end, slide adapter on, tighten — no brazing/flaring. Finish by pinching behind adapter, removing it, brazing the end.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Protect valves and sight glasses when brazing

Refrigeration

Pull Schrader cores and removable moisture-indicator cores; otherwise wrap in a wet rag (no water inside) or cold pack — 275°F+ ruins the colour element; keep flux out of the sight glass. Shield neighbouring parts with sheet metal. Flux the male part only. Remove flux with warm water afterward.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Pull Schrader cores and wet-rag service valves before brazing

Refrigeration

Heat destroys core seals.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Quick-connect couplings

Refrigeration

One-shot diaphragm type must be recovered before disconnecting; oil the gasket with clean refrigerant oil; use two wrenches (back-up) and don't over-torque.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Replacement compressor checks

Refrigeration

Same capacity and same temperature range (low/med/high). Stub tubes smaller than the lines before swaging → compressor may be undersized. Clean 2" of each stub to bright copper; shield other joints with wet rags/heat paste.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Seal flare nuts on cold lines

Refrigeration

Moisture freezing between flare nut and tube can crush/split the tube at the TXV or coil; seal the nut or use short-shank nuts where frost occurs; brazed joints preferred. Oil film under a fitting = leak.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Sensing bulb mounting

Refrigeration

Clean the tube with emery, use a metal (never plastic) strap, full bulb contact, clock position by tube size, never on bottom or near a bend (oil pools), wrap in foam tape so ambient air/frost doesn't fool it. Vertical coils: outlet below inlet.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Slope the suction line toward the compressor

Refrigeration

Otherwise oil pockets collect.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Temperature survey finds restrictions

Refrigeration

Walk the liquid line (and suction line) with an IR/contact thermometer across every drier, valve, sight glass and bend; a temperature drop = pressure drop = restriction. Liquid line should be near room temp end-to-end.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Water line access for ice maker/humidifier

Refrigeration

Local code may require a T-fitting with permanent shut-off instead of a saddle valve, and may restrict compression fittings. Saddle valves leak more. Leave several large loops of 1/4" tubing behind the fridge so it can be pulled out for service.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Where leaks hide

Refrigeration

Brazed joints, high-vibration tubing near the compressor, oily/dusty spots, bent or crimped tubing, tubing rubbing on fan/moving parts, bad flares (re-flare and new nut rather than tighten). Run the system first to build pressure for bubble/dye tests. Block wind and bright sun.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Electrical

Meters, capacitors, motors, relays, wiring and control circuits.

Condenser fan wiring

Air conditioner

Condenser fan leads usually connect across C and R of the compressor so it runs in parallel with the compressor under thermostat control.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Contactors are rebuildable

Air conditioner

Contactors and motor starters can have coils and contacts replaced; coils come in 24/120/240 V. Three-phase contactor connects L1–T1, L2–T2, L3–T3. Starters add overload heaters.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Discharge line thermostat logic

Air conditioner

Because head pressure and discharge temperature rise together (Gay-Lussac), a discharge line thermostat opening on high temperature is an alternative/backup to a high-pressure switch.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Fans may share a motor

Air conditioner

Room AC evaporator and condenser fans can be on one shaft or separate motors; check the literature before diagnosing a "dead" fan.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Thermostat quick test on room AC

Air conditioner

Cover inlet and outlet with a cloth so air recirculates and drops fast to cut-out; watch with a thermometer.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Thermostatic mixing valve on tower return

Air conditioner

Holds condenser head pressure constant in cold weather.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Boiler wiring polarity

Boiler

Black lead should be hot, white neutral at the service switch; wrong polarity upsets flame rectification controls.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Outdoor reset max must be below boiler high limit

Boiler

Otherwise the limit interferes; never wire any reset control to bypass the high limit.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Baseboard voltage not interchangeable

Electrical

240 V on a 120 V unit destroys it; use 240 V to halve amps; dedicated circuit, no outlets/lights on it; built-in outlets must be on a separate circuit (NEC 424.9). Prefer double-pole thermostats. If the integral high limit fails, replace the baseboard.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Bulged capacitor = discard

Electrical

The dimple on top expands when the internal protector blew; always discard.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Capacitor ohm test

Electrical

After discharge, ohmmeter on high range (×100) across terminals: needle swings toward zero then returns slowly to infinity = good; stays at zero or stops part-way = bad. Then measure µF with the meter's capacitance function (disconnect cap, select a range above its rating; 0 µF or unstable = shorted).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Check relay terminal diagrams

Electrical

Some relays carry extra terminals for a second motor or other components; read the diagram.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Check voltage at panel AND at unit

Electrical

Compare nameplate voltage with voltage at the motor terminals; 120 V motor must see 108–132 V (±10%); never below 90%. Long runs: upsize wire.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Corroded contacts

Electrical

Detect with an ohmmeter; filing (mill/single-cut file, even a nail file) is temporary and guarantees a callback — replace the contacts or control.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

DDC wiring

Electrical

Sensor/actuator wiring is light twisted-pair carrying 0–10 VDC or 4–20 mA; match relay coil voltage (24/120/240 V) to controller output.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Dirty stat contacts

Electrical

Power off, rub clean paper between points; fine emery if pitted.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Don't breathe on or handle a stat while setting it

Electrical

Warms the sensor.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Don't condemn the motor

Electrical

Many returned "faulty" compressors test good. Record winding continuity, ground test (megohmmeter) and current draw before replacing. Check voltage while running (−10/+20%). Check three-phase balance.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Don't wrap stranded wire around a screw

Electrical

Loose strands cause ground faults or shorts; use a crimped terminal. Clean the terminal with steel wool, strip to barrel length, insert to maker's depth, crimp until the tool bottoms out, pull-test.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Electric furnace orientation

Electrical

Elements don't care, but mercury contactors and some relays/limits are position-sensitive; check before downflow/horizontal install.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Electric heat shutdown wiring

Electrical

Thermostat in series with all sequencers = all elements drop at once; thermostat in series with only the first sequencer = staged shutdown and needs a blower off-delay heat switch.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Estimating internal temp

Electrical

IR-thermometer the frame and add 20°F; keep hottest spot ≤ 72°F above ambient. IR guns can't read air temperature.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Exact replacement relays only

Electrical

Starting relays are matched to the motor; use the same spec or a cross-reference chart.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Find the breaker fast

Electrical

Plug a lamp into the outlet, flip breakers until it goes out; then compare breaker amps to nameplate. Dedicated circuit, no extension cord.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Fuse keeps blowing

Electrical

Confirm the fuse rating is right for the load; if it is, look for a short elsewhere. Same for a breaker that keeps tripping.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Greasing procedure

Electrical

Purge the gun tip, clean the zerk and drain plug, leave the drain out, grease gently until it emerges, run 10–30 min, refit plug. Over-pressure forces grease into windings.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Grounding vocabulary

Electrical

Ground = earth; grounded conductor = neutral (white/grey); ungrounded conductor = hot (black/other); grounding electrode conductor = wire to the rod. Equipment ground is always green.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Hard-start kit for PSC compressors on low voltage

Electrical

Must include a relay that drops the cap out; size per table A1 or manufacturer.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Heat/cool mode selector

Electrical

Mode switch stops the stat flipping between heating and cooling; if heat won't come on in cool mode, that's by design.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Hermetic "no cooling, won't run" sequence

Electrical

Verify power, then with power locked out ohm windings to shell (0 Ω = grounded), check open/short against spec; a HOT compressor can show a false open because the internal overload is open — let it cool. Compare start/run amps to nameplate afterward.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Hums but won't start (external motor)

Electrical

Current is flowing but no torque: test/replace start cap first; next the centrifugal switch (replace contacts, don't file); then the start relay. Deformed switch spring = replace switch.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Keep motors clean

Electrical

Dust is an insulator; blow out motors with compressed air; warm or blocked ambient air overheats motors.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Label the old stat wires before removing

Electrical

Colour codes aren't standard across manufacturers.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Lights flicker on start = weak circuit

Electrical

A flicker when the compressor kicks in, or >10 V dip at start, means wiring is undersized.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Listen while measuring

Electrical

Chattering contactors/relays = bad load; run every mode (heat, cool, fan on/auto, defrost on heat pumps).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Locked-rotor code matters

Electrical

High LRA can dip voltage enough to upset voltage-sensitive equipment on the same circuit; pick the correct code letter.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Measure both LRA and FLA

Electrical

Record locked-rotor and full-load amps; either above nameplate = worn motor.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Mercury stat level

Electrical

Check with a level on every call; a tilted mercury switch shifts calibration. Pull-in/hold-in contact pairs on 3-wire stats.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Millivolt stats

Electrical

Only for standing-pilot systems, no cooling; many electronic stats can replace them.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Motor terminology

Electrical

Rotor = armature; field poles/windings = stator windings. Confirm what the other person means.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Never clean electric heaters with flammable fluids

Electrical

; check for high-resistance connections with an ohmmeter.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Never replace a line-voltage stat with a low-voltage one

Electrical

; open the breaker and verify dead before swapping.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Open-circuit signature

Electrical

Across an open component you read full applied voltage; a clamp meter shows zero amps.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Outdoor-rated wire

Electrical

Check insulation rating for sun and rain; consult the utility on service capacity.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Outlet ground check

Electrical

Meter from hot to ground terminal: full line voltage = outlet is grounded.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Power-stealing stats

Electrical

Can false-trigger relays/control boards — add a bleed resistor (e.g. 270 Ω 3 W) between the stat leg and common; some hydronic zone valves also need one.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Relay + low-voltage stat on electric heat

Electrical

Reduces overshoot vs. line-voltage stats.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Relay operation

Electrical

Small coil current, large contact current; coil on low-voltage control side, contacts on load side. Energized: NO closes, NC opens; spring returns on de-energize.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Relay substitution rule

Electrical

Potential relay substitute may be lower voltage rating (90% of original), never higher (motor won't make enough cemf to open it). Keep covers on; never tap a relay to test it. Label every wire when swapping.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Replace elements kW-for-kW

Electrical

Same part number, ohms, amps, volts; higher kW = unsafe, lower = short heat.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Reversing rotation

Electrical

Single-phase: swap the start-winding connections. Three-phase: power off, verify with voltmeter, swap any two line leads.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Set up the homeowner's app/online account

Electrical

After installing Wi-Fi stats.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Shipping blocks and free rotation

Electrical

Remove shipping blocks, spin fans/motors by hand before power-up; ohm circuits for continuity before energizing; turn open-drive motors by hand after coupling alignment (dial indicator, equal shaft height; install belts before tightening motor bolts). Never join aluminum and copper conductors directly.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Start winding is the thin one

Electrical

Start winding = smaller wire, more turns → higher resistance and inductance than the run winding. Left in the circuit it overheats.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Stat/sensor location

Electrical

Inner wall ~5' up, away from drafts, dead corners, supply air, sun/appliances, hidden pipes/chimneys, uninsulated outside walls; mount on a firm wall (vibration = short cycling).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Switch on the hot leg

Electrical

Thermostat/switch must break the hot (ungrounded) conductor, never the neutral — NEC rule, and keeps the compressor shell from being live.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Test cord as continuity tester

Electrical

Add a lightbulb in series via jumpers to check continuity/ground. Same cord tests fan motors directly (clip 1 fan, clip 2 power, clip 4 ground).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Thermostat swap test

Electrical

No codes, wiring/continuity OK, equipment OK → substitute a known-good thermostat; if it fixes it, replace the stat. Stats are rarely repaired.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Three-phase dual voltage

Electrical

9-lead terminal box: jumper windings in series for 480 V, parallel for 240 V. Higher voltage = less current, more efficient. Lock out — three-phase voltages are deadly.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Transformer health

Electrical

27–28 V normal; ≤ 24 V means check loads/transformer; fuse the secondary; never land a low-voltage stat on line voltage.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Use the stat's "test outputs" function

Electrical

To check wiring before programming — bypasses delays. Set DIP switches for system type (heat pump vs gas/electric, O vs B).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

VFD basics

Electrical

VFD rectifies 60 Hz AC to DC, then inverts it to AC at the wanted frequency; motor speed follows frequency. Allows near-continuous speed range vs 2-speed winding taps.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Voltage drops that matter

Electrical

Clean contacts, correct wire and good switches drop almost nothing; a measurable drop across a switch, relay contact or connection means a problem. Meaningful drops are across motors, coils and real loads.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Which fuse for which motor

Electrical

Fast-acting for blowers (low start current); time-delay (≈10 s) for compressors; multipurpose blows instantly above 500%.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Wire stripper setting

Electrical

Too large a setting won't remove insulation; too small nicks the conductor or cuts strands, lowering ampacity and causing overheating.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

"Check the obvious" list

Furnace

Breaker, disconnect, blower door, pilot lit, filter, belt, dirt on blower, then run 10 min and observe burner/blower start, shutdown, post-purge, CO.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Belt inspection

Furnace

Check belts for fraying/cracking; misaligned pulleys wear belts fast. Tight belt = bearing wear; slipping belt = motor runs continuously.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Bent blower blades kill motors

Furnace

Out-of-balance wheel vibrates and shortens motor life; check blade condition when cleaning.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Bigger motor pulley = faster blower

Furnace

On belt drive, enlarging motor pulley or shrinking fan pulley raises blower rpm; variable-pitch pulleys let you tune airflow.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Don't vary voltage to control fan speed

Furnace

Fan speed falls with voltage, but varying voltage on induction motors isn't safe or accepted; use taps or a VFD.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

DSI EMI

Furnace

Keep high-voltage spark lead short and away from the control board; spark noise can scramble the module.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

ECM identification & noise

Furnace

ECMs have a control module with a multi-wire plug; they run quietly and soft-start. Loud noise other than at start-up signals a problem.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

ECM troubleshooting steps

Furnace

Verify 120 V power; verify low-voltage control communication; look for burnt terminals/loose or frayed wires; read trouble codes on the system board (LEDs/display) and follow the maker's chart; replace the module if faulty — modules aren't repairable.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Every service call: inspect flame + CO meter

Furnace

Book calls this mandatory.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Fan switch vs limit switch

Furnace

Many adjustable snap-disc controls can be used as either; don't assume function from looks, check set point and wiring.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

HSI igniters are fragile

Furnace

Don't touch the silicon carbide element; never touch energized; ohm-test with power off (cracked = open).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Service-call tool kit

Furnace

Digital manometer (static + gas), multimeter with µA for flame rectification, thermometer, drain snake, inspection mirror, combustion analyzer.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Thermocouple / thermopile

Furnace

Two dissimiler metals welded at the hot junction produce millivolts; thermopile (several in series) up to 750 mV to hold a gas valve open on a standing pilot. Flame out → voltage gone → valve closes.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

"Thermostat" means any temperature switch

General

Parts counters list bimetal discs as thermostats; be clear which device you mean.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

"Thermostat" means different things

General

Suppliers list bimetal safeties (roll-out switches) as thermostats; clarify with customers/parts counter.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Always put ground wires back

General

If a service step requires removing a component ground wire, reconnect every one before leaving. Never cut the ground prong off a cord; a grounding adapter is only a temporary accommodation — customer needs an electrician to install a grounded receptacle.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Check the symbol against the part

General

Manufacturers use their own wiring-diagram symbols; confirm each symbol by matching the diagram to what's physically in the cabinet.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Check utility supply before big motors

General

Confirm available voltage/phase/frequency with the utility before installing sizable-hp equipment; North America is 60 Hz, much of the world 50 Hz.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Clamp one conductor only

General

Clamping hot and neutral together cancels to zero. Loop the wire through the jaws 10× for small currents and divide by 10 (not for 10 AWG or larger).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Confirm 60 Hz

General

Use the meter's Hz function on VFD-fed motors and incoming line; wrong frequency makes motors overheat.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Cooling-tower monthly list

General

Fan/motor bearings, belt tension & alignment, strainer, pump flush, float level, spray nozzles, bleed line working, inlet screens, algae/leaves. Yearly: fix corrosion. Legionella risk — treat/disinfect standing water.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Damper types by wire count

General

2 wires = normally open or normally closed (spring return); 3–5 wires = power-open/power-close with end switch. Fire dampers are held open by power.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

DC polarity with analog meters

General

Reversed leads on an analog meter drive the needle backwards and can damage it at high DC voltage. Black lead to the negative side of the load.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Diac protects start winding from low voltage

General

Diac passes current only above its breakover voltage; stays on until current drops below holding current.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Disconnect within sight

General

NEC 440.14: disconnect must be within sight, ≤ 50 ft, reachable without ladder, tools or moving obstacles. Outdoor disconnects often hold 30–60 A fuses.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Drip leg at furnace and at meter outlet

General

Accessible, not where it can freeze, downstream of the shutoff.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

EAC cell washing

General

Power off EAC, furnace, blower; hot soapy water; dry completely before reinstalling (wet cells arc); dishwasher OK but skip the dry cycle; wet-filter "dry" switch or run blower. Need a high-voltage probe to read ~9 kV; special training required. Inspect for bent plates, dirt bridging wires to plates, broken insulators.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

EAC service triggers & cheesecloth test

General

Service when trouble light stays on, strong ozone smell, dusty rooms, constant arcing. Ask when last cleaned; check doors closed, switch on, fuses. New carpet, leaky ducts, untreated concrete, nearby construction overload the cells. Cheesecloth over a supply grille discolored after 2–3 days = EAC not working. Black wash water = it's working.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Fan motor fault list

General

Loose connections, dry/worn bearings, burnt motor, loose fan, out-of-balance fan, blades hitting housing. Loose connection → slow, loud hum, overheating; find it by ohming the circuit while wiggling wires (resistance that changes = loose terminal).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Flooded equipment

General

Replace all external electrical parts, caps, relays, overloads, limits; replace or rebuild open motors; clean compressor terminals, spray insulation inhibitor, ohm everything for grounds; detergent + bacteria cleanser on the cabinet.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

In-line ammeter in series only

General

Connecting an in-line ammeter in parallel blows its fuse. Procedure: power off, verify no voltage, open the circuit in series with the load, clip meter in, power on, read, power off, restore.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Intermittent-duty motors are cheaper

General

Condensate pumps and damper actuators can use intermittent-duty motors.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Meter pre-checks

General

Before measuring: correct function (A/Ω/V), AC vs DC, and leads in the correct jacks (red lead often moves for amps). Red to +, black to common.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Noise diagnosis

General

High pitch = velocity too high or sharp edges; low rumble = fan/motor travelling the duct; popping on start/stop = expansion. Pull the grille: noise stops = grille edges; continues = velocity (measure) or sharp edge in duct. Fix with lower velocity, lined/felt-wrapped ducts, acoustic chamber. Speeding up a blower to cover undersized equipment/ducts causes noise.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Ohmmeter only on dead circuits

General

Kill and lock out power, voltmeter-check for zero volts across the device, then ohm it. Disconnect at least one leg of the device so you don't read parallel paths.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

PCB-filled capacitors

General

Old run caps may contain PCB fluid — don't open; don't touch or breathe fumes if broken; dispose as hazardous waste.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Pre-1970 buildings: test before touching two metal parts

General

Older wiring is prone to ground faults. Voltmeter-check between e.g. furnace casing and ductwork before bridging them with your body; if voltage is present, lock out and find the cause.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

SCR stays latched

General

Once gated on, an SCR keeps conducting even with the gate open; it turns off only when current drops below holding current or voltage reverses (every half-cycle in AC). Triacs conduct both directions and are used for AC motor speed control.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Set range above expected voltage

General

Checking a 120 V circuit? Set 240 V range first to survive surges.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Single phasing

General

Losing one phase makes the other two carry 1.5× FLA; phase-loss monitors shut the motor down. Test: voltage on all three legs at the motor; if good, ohm each winding — all three must match.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Solenoid water valve

General

Wired in parallel with compressor — opens when compressor starts, closes on stop; 120 V most common; on/off only. Pressure water valve modulates with head pressure (bellows at compressor head), can act while unit is off; adjustable spring; packing gland needs occasional adjustment. Thermostatic water valve modulates on condenser water leaving temperature via bulb.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Solid-state switching

General

Semiconductors switch with no moving parts; conductivity can be triggered by voltage, light, pressure or temperature.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Testing a diode with an ohmmeter

General

Lowest ohm range; black lead to cathode (striped end), red to anode = low resistance; reversed = very high. Low both ways or high both ways = bad. With a DMM diode-check, a voltage reading in only one direction = good; disconnect at least one lead first.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Thermistor types

General

NTC: resistance falls as temp rises. PTC: resistance rises with temp. PTC in series with a motor cuts current when it overheats.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Thermostat height and stratification

General

Stat mounted high reads warmer/more humid than occupants feel; too low reads cooler. Room with no air movement stratifies (e.g. 70°F floor → 78°F ceiling).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Unloaders vs cap-tube systems

General

Systems with capillary tubes equalize pressure on the off cycle and don't need an unloader; TXV/AXV systems without bleed ports start against load and need a high-torque motor or hard-start.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Wattmeter vs VA rating

General

Manufacturers list VA; a wattmeter reads true watts and self-corrects for power factor; a motor drawing more than rated watts is costing money.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Bad run cap = high amps

Heat pump

A failed run capacitor makes the motor draw excess current, tripping the overload or damaging the motor. Check run caps on every call.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Control circuit vs power circuit

Heat pump

24 VAC thermostat circuit closes the contactor coil; contactor contacts switch the 230 VAC compressor/fan circuit. Trace problems by separating the two circuits.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Defrosting a frozen coil

Heat pump

Compressor off, fan on; on a furnace system run heat with all zones calling; on a heat pump run heating but disable electric backup (pull W1 or open the strip-heat breaker) because restricted airflow overheats the elements; clear condensate lines first.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Label thermostat wires by terminal letter, not colour

Heat pump

Colours aren't standard across brands.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Thermostat must be heat-pump compatible

Heat pump

Needs O/B, E/W2 etc.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Cap-tube restart stall is normal

Refrigeration

A cap-tube system stopped and restarted immediately may not start — low-torque motor can't overcome unequalized head. Unplug a few minutes, retry.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Chattering stat contacts

Refrigeration

Chatter = pitted/burned points; replace whole thermostat.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Check cheap parts before the compressor

Refrigeration

Power-in connections, thermostat, terminals, relay, capacitor, defrost timer. Isolate each or swap in a known-good test part.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Compressor watt check

Refrigeration

Best pump test is VA/watts at normal pressures; reading well below motor rating = worn pump.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

CPR set to compressor RLA

Refrigeration

Set the crankcase pressure regulator's calibration screw so running amps stay at or under the nameplate RLA during pull-down/after defrost. Inlet pressure cancels on the valve; only spring vs crankcase pressure act. Also called suction regulator / holdback / compressor low-side pressure control.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Current relay orientation

Refrigeration

Weight-type current relays must be mounted level and upright; rotary-solenoid types mount in any position. Terminals: L (line), M (main/run winding), S (start).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Defrost termination sensor placement

Refrigeration

Put the termination thermostat where frost leaves the coil last. Fan delay after defrost until coil re-cools.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Dispensing freezers

Refrigeration

Viscosity (dasher amps) control, not temperature, decides "servable"; heat-treatment models need full teardown only every 14–28 days; rate by Btu/hr.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Hermetic terminals

Refrigeration

Single-phase hermetic terminals are C, S, R. Common joins one end of both windings; S and R are the other ends. Terminals are glass-fused or bolt-on "built-up" (gasketed, field-replaceable).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Hot pull-down (Caution)

Refrigeration

Starting a box at ambient temperature puts high suction pressure/high amps on the compressor; a CPR throttles suction to protect the motor until it pulls down.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Hot-gas defrost not working

Refrigeration

Check timer runs, force defrost, confirm compressor runs; feel the hot-gas line. Solenoid with one side hot/other cold = still closed — tap body, check coil voltage, jump control; still stuck → replace. Solenoid coils must be mounted on top with valve level or they chatter/stick.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Ice maker checklist

Refrigeration

Confirm water + power, mold <15°F, several cycles completed, thermostat in series with mold heater, ejector makes 2 revolutions/cycle with no ice left on blades, water solenoid in series with mold heater.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Identify unmarked terminals by ohms

Refrigeration

Three readings: highest pair = R–S (odd terminal out is C); middle = C–S (odd one out is R); lowest = C–R (odd one out is S). Power off first.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Interior light = quick power test

Refrigeration

If lights work the supply is fine and the fault is in control/compressor; if not, start at breaker/cord.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Low-pressure control doubles as safety

Refrigeration

Low suction pressure means little vapor cooling the hermetic motor → overheat. Cut-out must stay above atmospheric pressure.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Manual start test cord

Refrigeration

Clips: common, run, start, ground to dome; close start then run, open start after a second or two. Runs = problem is external. For CSIR/CSCR use new capacitors of same µF/V and extra jumpers; discharge test capacitor through 20 kΩ 2 W resistor. Use insulated clips.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Most hermetic faults are external

Refrigeration

Check wiring and controls before condemning the compressor; fix external faults fast or they kill the motor.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Mullion/door heater failure

Refrigeration

Gasket frozen to cabinet → check mullion heater continuity with both leads disconnected. Most cabinets have a spare heater in the insulation — test and connect it. Pull new wires through foam with stiff steel wire; drill ≤1/2" hole in back if needed, seal with silicone.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Off-cycle defrost needs wide differential

Refrigeration

Thermostat cut-in high / cut-out low to give the coil time to clear; only works if box air is above freezing.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Potential relay advantage

Refrigeration

Contacts are NC and close at rest, so no arcing when the thermostat makes — less contact wear than current relays. Coil (terminals 2–5) is high resistance, in parallel with the start winding; contacts 1–2.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

PTC relay has no contacts

Refrigeration

The thermistor is the "contact": low ohms cold, high ohms hot. Allow 2–5 min cool-down before restart or the overload trips. One SSR size covers 1/12–1/3 hp motors.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Pump-down defrost non-recycling relay

Refrigeration

The extra relay keeps the compressor off until the thermostat closes, even if suction pressure rises — don't bypass it.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Relay test order

Refrigeration

Hard to see a relay energize on a compressor; test motor, capacitor, overload and thermostat first — if all pass, the relay is the suspect.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Relief valve inspection checklist

Refrigeration

Missing/broken seal wire, corrosion, leakage, unreadable nameplate, stacked valves, dirt/rainwater in the purge line → replace or correct. Mount on receiver above the liquid level.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Short cycling causes

Refrigeration

Thermostat exposed to vibration (foot traffic, machinery) — mount solidly; loose starting-relay connections; capacitor discharge arcing contacts (use caps with bleed resistors).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Shorted winding sound

Refrigeration

A shorted winding may add a "beat" to the normal hum and raises amps / drops power.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Split-phase & PSC need balanced pressures

Refrigeration

Low-torque motors (split-phase, PSC) can only start a compressor when high and low side have equalized (cap-tube systems) or an unloader is fitted; otherwise the motor overheats.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Thermostat bypass test

Refrigeration

Jumper the stat terminals: compressor starts → stat is bad; doesn't → look elsewhere. Leave a data logger 24 h if a return visit is possible.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Three-phase internal overload

Refrigeration

Wye-connected motors have the protector at the common point so all three windings open together.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Trend insulation readings

Refrigeration

Log megger readings in table + graph from day one (new motor); re-check a few weeks after maintenance; falling trend = warn owner of impending failure.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Two-pole motor test limit

Refrigeration

Two-pole compressors must be tested with a proper relay; a manual start winding energized >2–3 s overheats them.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Wattmeter shows a bad start circuit

Refrigeration

Analog wattmeter on a hermetic: needle swings right, pauses at combined start+run watts, then drops to run watts. If it never pauses at the higher value, the start winding or relay is open. Digital meters are too fast to show this.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Airflow & ducts

Airflow, filters, static pressure, ducts and ventilation.

Cover the outlet on first start after cleaning

Air conditioner

Loosened dirt blows out of the grille.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Return-air bypass for dehumidification

Air conditioner

Bypassing some return air around the coil lets the system keep running to dry the air without overcooling.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Tell occupants not to set the stat too low

Air conditioner

Evaporator freezes, airflow drops, more ice.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Thermal overload reacts to ambient too (Pro Tip)

Air conditioner

An external overload senses heat around the shell. A compressor boxed into a hot closet with poor airflow trips repeatedly — fix the airflow before condemning anything.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Winterizing a window unit

Air conditioner

Block grilles with cardboard/plastic, storm sash or plywood, per manual.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Boiler no-fire: check venting first

Boiler

Blocked vent, damper not rotating, and tripped rollout/blocked-vent switches are the usual suspects; never bypass those switches (CO risk).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Tight electric-heat houses need HRV/ERV

Electrical

No combustion draft to remove moisture; humidity complaints are a ventilation problem.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Annual furnace inspection checklist

Furnace

Louvers clear; burners for rust/dirt/water marks; flue pipe joints; burned wires; blower door seals return; return duct sealed to furnace; run test; analyze flue gas. Tell owner to shut down for new noises, odours, excess moisture, burned/rusty parts, or flu-like symptoms that clear when away (CO).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Blower service

Furnace

Clean blower yearly; a few drops SAE 30 once or twice a year if oilable; summer cooling usually uses the high blower tap; replace glazed/cracked belts.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Door interlock switch

Furnace

Blower door must be fully seated or the interlock kills power; "dead furnace" is often just the door.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Filter arrow points toward the furnace

Furnace

(direction of return airflow) — teach the customer.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

G doesn't run the blower in heat mode

Furnace

On most furnaces — a temp or time-delay switch does, so cold duct air isn't blown.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Heat exchanger 3-point check (GAMA)

Furnace

(1) watch flame before/after blower starts: any change = HX/gasket breach; (2) visual with mirror & light; (3) tracer-gas/chemical test at supply registers. Cracks may only open when hot. HX wall thinned ≥50 % = replace. Combustion gas in supply air can also come from mechanical-room depressurization, not just a crack.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

LP pilot failure pools gas in low spots

Furnace

LP is heavier than air; smell = gas present; ventilate basements/crawlspaces. LP systems must be 100 % shutoff.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

PSC speed taps

Furnace

More winding in the circuit = slower; fewer turns = faster. Change speed by moving the lead to a different tap.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Record baseline at install

Furnace

Fuel type, model, filter size, manifold pressure, stack temp; leave a copy with owner and on file.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Stack effect

Furnace

Heating: warm air rises, upper floors slightly positive, lower floors negative pulling cold air in. Cooling: reverse. Combustion appliances pull house negative → add dedicated fresh-air duct/fan.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

V-belt width must match pulleys

Furnace

On belt-drive blowers check belt rides fully in both grooves; wrong width = slip and wear.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Ballpark sizing is out

General

Use two-stage furnace, variable-speed compressor and ECM blower sized to a real load calc. Most codes mandate Manual J (res) / Manual N (comm); ACCA-approved third-party software or services are acceptable.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Basement ventilation logic

General

Dry outdoor air → floor-level exhaust fan (watch negative pressure, hot make-up air rising). Hot humid outdoor air → exhausting makes it worse; use AC or a dehumidifier.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Blower door prep list

General

Write down every damper/register you change so you can restore it. Close exterior doors/windows, open interior doors, close all dampers/flues/HRV ports, kill combustion and exhaust appliances, clean fireplace ashes, open all supplies. Fan blows OUT; take baseline with fan blocked, outdoor tube away from wall out of wind.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Blower table rule

General

Pick design ESP from the MEDIUM speed row. ESP too high = not enough air; too low = too much.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Close the bypass damper in summer

General

On bypass humidifiers.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Commercial IAQ checks

General

Outside intake near pollution source? Blocked vents? Fiberglass-lined ducts? Vehicle exhaust? Restroom odors escaping? Fume hoods used?

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Dehumidifier controls

General

Float switch stops compressor before overflow; frost control stops compressor but keeps fan running to melt frost; electronic stats may include humidistat function.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Dissimilar metal joints

General

Aluminum-to-steel corrodes; slip a zinc or magnesium strip in the gap as a sacrificial anode.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Doors closed balance

General

Rooms with supply but no return need a door undercut/gap; check balance with doors open and closed. Duct booster fan when a run is too long/small/too many elbows.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Draft fixes

General

Aim louvers at unoccupied area, swap to a wider-diffusing register, or damper down (may unbalance).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Drum humidifiers

General

Inspect yearly, clean/replace sleeve with correct part, de-scale float. Rotating disk type suits hard water (nothing to clog). Nozzle type clogs with minerals — filter the water or clean regularly.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

EAC install rules

General

Level and plumb, even airflow across face, turning vanes if near an elbow, fine prefilter upstream; cell can arc/fire if dirt builds up; disconnect before service.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Energy-smart IAQ

General

Cut pollutants at the source before adding ventilation; make-up air for a dirty zone can come from clean conditioned areas.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Filter installation

General

No light showing around the frame; plumb and square; arrows in airflow direction (unidirectional filters clog fast if reversed); replace if torn.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Fitting effective lengths

General

Minor fitting differences change EL — match the Manual D fitting exactly. Apps/duct wheels are fine but understand the steps.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Fresh-air intake screen

General

Strong screen-and-bar guard, filtered, to keep out birds/animals/debris.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

HRV vs ERV

General

HRV (heat only) for cold climates; ERV (heat + moisture) where humidity control matters. Supply to living/bedrooms, exhaust from baths/laundry/kitchen; balanced flows.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Humidifier solenoid leaking when furnace is off

General

#ERROR!

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Installer must verify the layout still works

General

Owners move walls, add stoves beside condensing units, put windows in front of cases — check heat sources, airflow and solar load before blaming the equipment. Route conduit/lines and secure drains to avoid stock damage and trip hazards.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Keep blower running

General

Continuous/variable-speed fan improves ventilation and mixing.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Mark damper position

General

Scribe a line on the damper shaft end showing blade position.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Never duct into plenum end

General

Branches go in plenum sides; an end-mounted duct hogs airflow. Keep blower central, runs short.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

No duct tape

General

Attic heat kills the adhesive; use mastic (brush in, 1" either side; mesh if gap >1/4") or foil/metal tape. Seal registers to drywall/subfloor where ducts pass from unconditioned space.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Odor removal

General

Filters (even electronic) don't remove gases; use activated charcoal or activated alumina + potassium permanganate, UV/oxidation.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Over-tight envelope

General

→ sick building; add mechanical ventilation/HRV-ERV.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Painting galvanized duct

General

Wipe with vinegar (weak acid) to etch first, or let it weather outdoors.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Pitot placement

General

Straight duct, ≥10 diameters downstream; straightening vanes upstream for precision; traverse and average. Manometer must be level.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Positive building pressure

General

Stops infiltration — needs a conditioned fresh-air intake.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Return placement

General

Returns in the stagnant zone: near floor for heating, near ceiling for cooling; always as far from supplies as possible. Starved room (more return than supply) is cold in winter/warm in summer — add supply.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Saddle valves may be prohibited

General

Some codes require a tee and permanent shutoff and limit compression fittings (also applies to ice maker lines). Tap a cold line.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Smoke detector placement near ducts

General

High-velocity air defeats detectors; exposed horizontal run → hang detector under duct just behind grille; concealed ducts → detector far from grilles; never in corners.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Steam humidifiers for heat pumps / cool supply air

General

Evaporative and atomizing types need warm duct air; steam type makes its own heat.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Thermal precipitation

General

Dirt streaks around warm-air grilles/windows come from room air hitting cool surfaces, not from the ducts; insulation/storm windows reduce it.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

VRF doesn't do humidity or fresh air by itself

General

Add ductwork/air handler/ERV.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Wet-bulb needs airflow & shade

General

Rapid air over the wick; shield from radiant heat (up to 15% error).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Whole-house fan free cooling

General

Cool evening + warm house: open windows, run attic/whole-house fan instead of AC.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Zoned systems need 400 cfm/ton

General

Across the coil regardless of how many dampers are closed → bypass (barometric or motorized) damper between supply and return; optional freezestat, supply/return sensors, outdoor reset.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Heat pump supply air feels cooler

Heat pump

Register temps are lower than a furnace; pre-empt the "it's blowing cold" complaint on new installs.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Biocide tablets in ductless pans

Mini split

Regular tablets prevent slime/algae/mold in the cool wet pan.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Ductless indoor coil cleaning

Mini split

Remove panels/filters, hang a plastic bib into a bucket, pressure-wash coil.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Overflow switch on ductless drain pan

Mini split

Add a float/overflow switch so a blocked drain or dead condensate pump shuts the unit down instead of flooding.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Air curtains and drafts

Refrigeration

Open cases must be shielded from HVAC grilles, unit heaters and fans — drafts break the curtain, raise run cost and cause defrost problems; check the total electrical load when cases are ganged.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Clumped cube ice

Refrigeration

Cubes bridging into big slabs = ice-thickness sensor set too thick. Odd-shaped cubes on spray (inverted mould) machines = scaled nozzles/high mineral water; check reservoir probe, drain, filters.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Medium-temp coils ice at the bottom

Refrigeration

Melt water from the top re-freezes on the lower fins and blocks airflow — look low on the coil when a medium-temp box has airflow problems.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Reading evaporator frost

Refrigeration

Frost only near the TXV with low suction = superheat set too high. Spotty frost = uneven airflow or a defrost element out. Full ice blanket = defrost failure (timer contacts, heater, limit).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Combustion & gas

Gas pressure, flame, venting and combustion readings.

Air vent caps must be loose

Boiler

A fully tightened cap blocks venting; painted-over vents in living spaces are a common failure.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Test the LWCO by draining

Boiler

Shut makeup water, open boiler drain, burner must stop immediately; also lift the relief lever briefly — if nothing comes out, shut down and replace.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Burner firing with inducer off ruins the pressure switch

Furnace

Overheated diaphragm; don't jumper the switch.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Check gas pressure before adjusting air

Furnace

High manifold pressure mimics "not enough air", low pressure mimics "too much air" on the analyzer.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

CO means air is wrong either way

Furnace

Too little OR too much combustion air makes CO; use CO2/O2 to tell which.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Flame colour read

Furnace

Blue inner cone with light-blue outer = good; yellow tips = short on primary air (CO risk); orange flecks = dust burning, not an air problem; noisy, lifting flame = too much primary air.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Flame rod = ionization electrode = flame electrode

Furnace

Same part, several names.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Flame rod cleaning

Furnace

Oxide film on the rod causes burners to light then drop out; clean with sand cloth/steel wool, check ceramic for cracks, don't crack it on reinstall, rod is hot.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

LP regulators are usually external

Furnace

NG valves have integral regulator; LP often has a separate one.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Modulating furnaces need the matching communicating stat

Furnace

With a plain stat they run as single-stage.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

NG→LP conversion = change spuds

Furnace

LP is heavier, flows less through the same orifice; swap orifices and set 10.5 in. WC.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Rollout switch is manual reset — never just reset it

Furnace

Tripped rollout = cracked HX, rusted burners, poor draft, wrong flue height, blocked passages; find the cause.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Starved combustion in tight houses

Furnace

A negative-pressure house starves the burner and lowers efficiency; consider a makeup air unit or two-pipe direct vent.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Trial for ignition settings are mfr-specific

Furnace

On selectable-TFI modules, check the manual before changing.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Yellow flame = IAQ clue

Furnace

Yellow instead of blue burner/pilot flames on gas appliances is on the residential IAQ pollutant checklist.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Cast iron pipe is prohibited for gas

General

; black iron or CSST typical. Label exposed gas pipe "Gas" every 5 ft (not required for steel or in the appliance room).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Hybrid gas/electric chillers

General

Let owners switch to the cheaper energy.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Leave the first two threads bare

General

When doping gas pipe so compound doesn't plug orifices.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Rubber mallet tap on a stuck steam trap/vent/valve

General

Can free it temporarily to confirm which component is at fault.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Two gas shutoffs on RTUs

General

One outside the cabinet for emergency; gas pipe ≥ 3.5" above roof.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Valve names don't matter, function does (Pro Tip)

General

Hot-gas bypass, liquid injection, head pressure control are functions; the hardware may be an AXV-style, TXV-style or EEV-style body. Identify what it does in the circuit.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Water valve problems by symptom

General

Too little flow: leaking/chattering/sediment-bound/misadjusted valve, clogged screen, leaking bellows. Too much: supply pressure too high (add regulator), valve leaking or opened too far; water running during off cycle = leaking valve, flush debris from seat. Constant flow hides air in condenser and ruins hot-gas defrost.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Heat pump "gas line" not "suction line"

Heat pump

The big line carries gas both ways; the small line always carries liquid.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Excess flash gas in the liquid line hurts TXVs

Refrigeration

A liquid-to-suction heat exchanger subcools liquid and warms suction to prevent it.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Hot-gas bypass needs an accumulator

Refrigeration

If bypass gas cools and partly condenses, liquid reaches the compressor; an accumulator on the suction line protects it.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Hot-gas defrost solenoid stuck

Refrigeration

Rap the valve body sharply with timer closed; hear the hot-gas surge / line warms = it freed. Warm evap + warm line with timer open = stuck open; rap again, low-side pressure should start dropping. Else replace.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Hot-gas defrost valve calibration

Refrigeration

Low-pressure-adjusting hot-gas valves have an adjustable bellows — check the manufacturer's pressure table before calibrating.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Hot-gas defrost vs hot-gas bypass (Pro Tip)

Refrigeration

Defrost valve dumps a lot of hot gas to clear a coil; bypass valve feeds a small, regulated amount to the low side for capacity control / low-load protection. Different jobs, often different valves.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Keep hot-gas defrost drain lines from freezing

Refrigeration

Route part of the hot-gas line under the drain pan/pipe or add a small heater, or the melt water refreezes in the drain.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Liquid injection / desuperheater valve

Refrigeration

When bypassed hot gas is too hot, a TXV-style valve sensing suction superheat injects liquid to desuperheat it. Same body as a TXV, different job.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

ORI / ORD head pressure valves

Refrigeration

ORI (condenser pressure regulator) at condenser outlet closes as condenser pressure drops → condenser floods → pressure rises. ORD (receiver pressure regulator) opens on ~20 psi differential between discharge and receiver to feed hot gas into the receiver. LAC = both in one body with an ambient-sensing dome. Aliases: holdback, limiter, discharge vapor bypass, condenser bypass.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

SLHX protects compressor and prevents sweat-back

Refrigeration

Suction-liquid heat exchanger superheats suction (no frost back, no liquid to compressor) and subcools liquid (no flash gas in the liquid line).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Other

Everything else worth knowing at the appliance.

Blank-off plates

Air conditioner

Support the coil and force all air through it; evaporator must be level.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Closed-loop towers

Air conditioner

Need far less condenser tube cleaning.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Combination liquid line/cap tube

Air conditioner

Never shorten or lengthen it.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Condensate drain

Air conditioner

Open drain with air break, 4" U-trap optional, keep plastic drain away from hot furnace parts; some jurisdictions require a pump.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Condensate dripping on the heat exchanger rusts it

Air conditioner

Check pan/drain; blow out clogs with CO2 drain gun.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Condensing unit siting

Air conditioner

24" from walls, no inside corners, not under eaves, discharge with prevailing summer wind, north side if possible, away from bedrooms.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Cooling tower water loss

Air conditioner

#ERROR!

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Don't short-cycle a cap-tube unit

Air conditioner

Restarting immediately after shutdown stalls the compressor because pressures haven't equalized; wait for equalization.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

EEV initialization and lost steps

Air conditioner

Stepper EEVs have no position feedback; the controller over-drives closed at power-up (and periodically) to re-zero. Clicking/driving at start-up is normal, not a fault.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Hot/humid climates may need a larger line diameter

Air conditioner

Follow the manufacturer's line-sizing.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Hunting cause

Air conditioner

Hunting (surging) is often a valve too large for the coil, or wrong superheat; minimize hunting but still use the whole evaporator.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Insulate the far end of the stat bulb

Air conditioner

On room AC bulbs at the evaporator inlet, insulating the portion farthest from the coil makes it track coil temperature better, cuts overcooling and lets it stop the unit if the coil ices.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Internal overloads and manual reset buttons

Air conditioner

Some compressors have an internal overload in the windings (auto reset after cooling — can take a long time) and some have a shell-mounted manual reset button.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Oversized vs undersized TXV

Air conditioner

Undersized: evaporator starved no matter the setting. Oversized: hunts, suction line sweats/frosts before the bulb can close it; raising superheat to stop hunting then starves the coil most of the time.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

PTAC/PTHP facts

Air conditioner

Common 42"×16" wall sleeve; electric heat PTACs usually need 240 V; PTHP efficient above ~35°F (2°C), resistance heat assists below; PTACs usually R-134a or R-410A with cap tube or TXV.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Pump-down control

Air conditioner

Stat closes liquid-line solenoid, compressor pumps low side down until LP switch opens; HP switch also in series with starter coil.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Read the moisture indicator near 75°F (Pro Tip)

Air conditioner

Hot liquid lines read "dry" with far more moisture present (see table A25).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Seal the sash gap

Air conditioner

Notch foam strips between upper and lower sash; check for daylight at seams and seal.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Sequenced compressors

Air conditioner

Microprocessor stages second compressor/condenser fan only at high load.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Suction line should be cold, not warm

Air conditioner

Superheated suction vapor is still low-temp; the line should feel cool/cold. Insulate it — sweating means it's picking up heat outside the conditioned space and loading the system.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Temperature probes must be strapped tight and insulated

Air conditioner

For both superheat and subcooling readings, otherwise ambient skews the number.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

TXV controls superheat, not pressure

Air conditioner

The compressor sets suction pressure; the TXV only meters to hold superheat. Don't try to "fix" low suction pressure with the TXV adjuster.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

TXV upgrade on old fixed-orifice systems (Thinking Green)

Air conditioner

Replacing a piston with a TXV on an older comfort system can noticeably improve efficiency under varying load.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Window unit tilt

Air conditioner

Sleeve must slope ~1/4" down toward outside so condensate drains; unit dripping indoors = wrong slope or plugged drain hole.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Air separator location

Boiler

Place downstream of the boiler, upstream of the pump (hottest, lowest pressure water releases the most air).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Circulator pumps away from the expansion tank

Boiler

Tank can be on supply or return but the point of no pressure change must be on the pump suction side.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Eccentric reducers in hydronic piping

Boiler

Keep the top of pipe aligned when changing size so no air pocket forms.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Fan convectors pull heat from cooler water

Boiler

Kickspace/fan coils suit low-temp (condensing/radiant) loops where plain radiators won't.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Foaming boiler water fools the LWCO

Boiler

Clean and deaerate before commissioning.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Glycol systems need a transfer pump purge

Boiler

Recirculate from bucket back into the supply-side drain so the glycol is reclaimed, keep both hose ends submerged.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Never add air to an expansion tank with a compressor

Boiler

Bicycle pump only, check pressure every few strokes; over-pressure ruptures the bladder.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Never purge a hot boiler

Boiler

Cold fill water into a hot cast iron boiler can crack it; let it cool first.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Oversize flow-control valves on fast boilers

Boiler

Low-mass boilers that heat quickly may need larger flow checks to handle thermal expansion.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

PRV has many names

Boiler

Pressure-reducing valve = boiler feed valve = fill valve; clarify.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Purge hose into a bucket of water

Boiler

Bubbles show when the air is gone; close shutoff downstream of the drain so flow is forced through the hose.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Purge valve = shutoff + drain in one body

Boiler

On systems with separate valves, close the shutoff downstream of the drain while purging.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Relief valve discharge pipe cut on an angle

Boiler

So nobody can cap or plug it; always pipe the relief to a safe spot.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Steam radiators work on hot water, not the reverse

Boiler

Don't put a hot-water-only radiator on a steam system.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

"Function on a rise"

Electrical

Control names describe what happens on a rise: heating control opens on rise, cooling closes on rise, most safeties open on rise. Refrigeration stats close on a rise.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Element test

Electrical

Power off, lift wires, continuity terminal-to-terminal (must have), terminal-to-ground (must not).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Never mix AC and DC equipment

Electrical

Don't connect AC appliances/meters to DC circuits or vice versa.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Never swap a manual-reset safety for an auto-reset

Electrical

The manual reset forces someone to look at why it tripped.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

NO/NC and poles/throws

Electrical

NO contacts close only when actuated (e.g. call for heat); NC open only when actuated. Pole = moving part; throw = number of positions.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Overload vs short circuit

Electrical

A short bypasses the load → extreme current → instant trip. An overload follows the proper path but draws too much → may trip slowly or not at all.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Range vs differential

Electrical

Range adjustment shifts both set points equally (differential unchanged). Differential adjustment changes the gap and also the range/average. Only adjust differential if you understand the effect on the system.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Reading connections

Electrical

Darkened/oxidized terminals, blue-green tint or charred insulation = overheating from a loose/dirty connection.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Resistance checks only with power off

Electrical

; keep clear of fan blades on restart.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Second (manual-reset) limit on electric heat

Electrical

Trips at a higher temp than the auto limit; a fusible link can't be reset — find the overheating cause before replacing.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Bimetal basics

Furnace

Copper expands more than steel so the strip bends with heat. Bimetal discs snap over and are used as furnace high-limit and rollout switches.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Condensate pumps (Code Alert / Pro Tip)

Furnace

Local code dictates condensate disposal, materials and piping. Pumps are self-priming with a float switch, vents must stay uncovered; many have two inlets — one for the evaporator drain, one for the acidic high-efficiency furnace condensate. Clean coils, pans and drains often to stop mold/algae.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Makeup air inlet placement

Furnace

Outdoor intake needs clearance from appliance exhaust terminations and may need to be within a set distance of the furnace; check mfr sheet.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Absorption time delay

General

Circulation waits for generator warm-up and continues after satisfy; don't mistake for a fault. Plastic pipe not directly on a furnace-mounted chilled water coil — use copper extensions; insulate drain lines.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Ask the customer three things

General

What happened, when first noticed, how many times. Note previous problems and any interest in upgrades on the work order.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Check the economizer on every RTU call

General

Stuck-open outdoor louvers wreck comfort and efficiency.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Class 2 circuits

General

(NEC Art. 725) — some municipalities require a licensed electrician even for 24 V.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Closing a stat call

General

Remove jumpers, replace covers, restore switches, offer the old part, leave manuals, show the customer.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Code applicability

General

Adding AC to an unconditioned building, mixed-use, or heritage buildings changes which code applies; verify first.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Comfort trade-off

General

High RH feels hotter; a lower temp plus more air movement compensates; in winter low RH needs higher temp.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Drain condensate from steam main before opening supply valve

General

Avoids slugging.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Enthalpy control matters

General

60°F/100% rh outdoor air holds more heat than 80°F/15% indoor air; sometimes better to just run the compressor.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Evaporator vs cooling coil

General

"Cooling coil" properly means a brine/water coil that doesn't evaporate; useful when reading manuals for geothermal/secondary-loop systems.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

GFCI locations

General

GFCI required outdoors, garages, pools, kitchens/baths; trips at ~6 mA leakage and can be reset.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Heat stress

General

OSHA: light clothing, 1 pint water (50–60°F) per hour, shaded rest breaks, avoid alcohol/caffeine.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Inspection mirror for hidden parts

General

Telescoping mirror checks fan alignment, condenser dirt, evaporator; keep it away from moving parts.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Inverter soft start

General

Low starting frequency = low torque = energy savings; inverter DC bus also powers EEV and fans.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Ionizers make ozone

General

Some ionizing purifiers emit ozone; don't use ozone-generating cleaners.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Ladder diagram on the door

General

Ladder diagrams show components in the order they energize (top first); usually glued inside the access panel. Each rung = one load circuit.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Life-cycle cost

General

Purchase/installation is often < 10% of lifetime cost; sell on efficiency and maintenance.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Low-e windows can focus reflected heat

General

Onto neighbouring objects/structures.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Lower indoor RH in extreme cold

General

To stop window/wall condensation (see A26).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Mark fan hub before removal

General

Scribe matching marks on hub and shaft so the new fan sits in the same position.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Master-slave zoning

General

Slave can't call cooling while master is in heat; replace slave stats only with the manufacturer's part.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

NC vs NO solenoids

General

Most liquid line solenoids are normally closed (gravity/spring closes on power loss). Direct-acting vs pilot-operated: pilot valves use system pressure to move the main valve (reversing valves are pilot-operated).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Patio doors

General

Can be 20% of a home's solar gain/leakage.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Record commissioning data

General

On console/packaged units log inlet/outlet air temps, electrical load and pressures at install; give a copy to owner.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Replace the board, not the part

General

When a device on a PCB fails, replacing the whole board is usually quickest; repair later or discard.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Service receptacle required

General

NEC 210.63 requires a 125 V 15/20 A receptacle accessible at HVACR equipment for service tools.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Size by load, not by the old unit

General

Windows, insulation, shade trees change loads; run a load calc on every replacement.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Smoke + stopwatch for drafts

General

Time smoke puffs; >15–20 fpm = draft. Vane/swinging-vane/pitot are inaccurate at very low velocities.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Soda-can dew point test

General

Half a shiny can, 2/3 water, add ice while stirring; when the outside fogs, water temp ≈ dew point.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Strainer ahead of every water valve.

General

Strainer ahead of every water valve.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Trace a water-circuit blockage

General

Crack the inlet valve, open joints one at a time downstream with a bucket until flow stops — that's the restriction (valve vs condenser vs drain).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

TXV/TEV, AXV/AEV, EEV/EXV naming (Pro Tip)

General

Same valves, different field abbreviations. Metering orifices are also called pistons, flowraters, flow restrictors, accuraters.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Undersized heating plant

General

Can overheat stack/chimney (fire); oversized wastes energy. Use outside dimensions for wall area (conservative).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

UV lamps in walk-ins

General

Turn UV germicidal lamps off when anyone works in the box, or protect skin/eyes.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

UV safety & placement

General

Never look at lit lamps; access doors need interlock; mount in return upstream of air handler or over/inside the coil and drain pan; check manufacturer location; sealed lamps contain mercury. Clean with window cleaner on a cloth, never touch glass; cool 15 min first.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Vane anemometer technique

General

Hold propeller at right angle to flow, let it stabilize ~1 min before recording.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Vapor barrier on warm side

General

Peeling paint near kitchens/baths = missing vapour barrier.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Vaulted ceilings

General

Add the triangular gable wall area. ACCA lists different values for 9'/10' ceilings.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Water hammer fix

General

Single thump when a water valve closes: add a tee with a short vertical dead-leg (air cushion) just upstream of the valve, or an expansion tank.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Water-cooled condenser short cycles on HP switch

General

If water stops, head rises, HP switch opens, pressure falls, restarts — the switch is the symptom; restore water flow. Post signs on hand valves. Rubber hose sections isolate vibration from building plumbing.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Zone panel discharge air sensor / outdoor reset

General

Newer codes require outdoor sensor so staging follows true load.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Zone panels have no serviceable parts

General

; tiny terminals — watch for loose strands/jumpers.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Accumulator can still be overwhelmed (Caution)

Heat pump

An accumulator protects against normal floodback, not massive low-side flooding — a badly overcharged or hunting system can still slug.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Biflow metering TXV bulb goes between reversing valve and compressor

Heat pump

With equalizer on the same line so it reads superheat in both modes.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Defrost "smoke" is steam

Heat pump

Warn new owners that the outdoor unit steaming in cold weather is normal defrost.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Dirty sock syndrome fix

Heat pump

Mold on indoor coil (HP heat not hot enough to kill it); UV germicidal light plus periodic pan/plenum chemical cleaning; shield plastic drain pans from UV.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Drain line flush

Heat pump

Blow out with air, prime trap, flush with 1:4 bleach:water.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Heat pump accumulators have smaller orifices

Heat pump

Don't substitute an AC accumulator; it won't throttle liquid return in winter.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Heat pump check valves bypass the idle metering device

Heat pump

Each TXV/piston works in one mode; the check valve routes flow around it in the other.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Ice around outdoor unit → check base-pan drain heat tape

Heat pump

Frozen drain means the pan heater is dead or missing.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Install outdoor unit on risers above snow line

Heat pump

8–18" typical; tell owner to keep snow clear.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Piston orifice size changes with line-set length

Heat pump

Manufacturers ship alternate pistons; long suction line sets may need a different orifice size. Heat pumps use two pistons (one meters each direction, the other free-flows through its flutes) or one piston outdoors + TXV with check-valve bypass indoors.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Record the LED code before anything else

Heat pump

Status LEDs flash codes keyed to the maker's table (example heat pump board: 1 = heartbeat, 2 = HPS open ≥6 s, 3 = discharge >263°F, 4 = discharge <90°F after 1 h run, 5 = defrost fail, 6 = discharge sensor short, 7 = outdoor sensor open/short, 9 = bonnet sensor fail).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Reversing valve energize state

Heat pump

In the book's example the solenoid is de-energized in cooling and energized in heating; the pilot uses discharge pressure to shift the piston. (Manufacturers vary — check the unit.)

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Reversing valve identification

Heat pump

Lone stub = discharge; middle of three = suction. De-energized usually = heating.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Run every mode on a HP PM

Heat pump

Cooling, heating and defrost; a reversing valve that won't shift gets replaced.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Scroll/rotary vs recip for residential

Heat pump

Small hermetic recips and scrolls are chosen for homes because they're quiet, compact and need no service for years; scrolls have fewer parts, less vibration and can modulate via bypass port, variable speed or scroll-separation.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Accumulator inlet/outlet at top

Refrigeration

Liquid drops out and must boil to leave; protects compressor from slugs.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Accumulator with liquid-line coil

Refrigeration

Some accumulators run the liquid line through a coil in the bottom: it boils off trapped liquid and subcools the liquid line at the same time.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Blast/quick chillers vs normal boxes

Refrigeration

Hot food in a standard reach-in runs the compressor nonstop, ices the coil and stalls during defrost — sell/spec a blast chiller instead.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Check valve noise (Pro Tip)

Refrigeration

Normal click/bang on open/close; a "hammering" noise means the valve isn't fully stopping reverse flow. A slow-opening or undersized check valve throttles and starves the coldest evaporator.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Coldest evaporator gets a check valve, not an EPR

Refrigeration

On multi-evap single-suction systems, EPRs go on the warmer coils; the coldest coil gets a check valve at its outlet so warm vapor from an EPR opening on the off cycle can't condense in it.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Defrost timers that count compressor run-time

Refrigeration

Some timers are wired in parallel with the compressor and only count while it runs; others run on the clock — know which type before judging "defrost too often/too seldom".

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Don't touch the discharge line

Refrigeration

It runs very hot while the compressor operates.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Door liners

Refrigeration

Only old blanket-insulated doors have replaceable liners; foamed doors don't.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

EEPR vs EEV

Refrigeration

They look alike; EEPR sits between evaporator and suction line and modulates on box air temperature (thermistor), EEV sits between liquid line and evaporator.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

EEV + power loss

Refrigeration

A stepper EEV holds its last position when power drops; migration protection needs a liquid line solenoid.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Energy saving via range

Refrigeration

Raising the average temp of a cooling system shortens run time and cuts compressor current.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Evaporative condenser capacity control

Refrigeration

Louvers, fan cycling or VFD; shut water off at ≤45°F (7°C) and run dry as an air-cooled condenser; indoor reservoir must hold all the water for freeze protection.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Flake machine noises

Refrigeration

Crunching = auger cutting scale in the evaporator; check auger wear, bearings, bore scoring. Float set too high → auger overload trips and water spills into bin melting the ice.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Fridge start-up sequence

Refrigeration

Control OFF, plug in, set control mid-range, confirm it runs before pushing it into place; check fresh-food temp after a few hours.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Frost and box humidity (Pro Tip)

Refrigeration

Frost is the box's moisture; big TD = more frost and dry product. Lower TD = slower frost and higher humidity.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Gravity-return solenoids must be upright

Refrigeration

Some solenoid valves rely on gravity to drop the plunger; mount vertically.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Head pressure kills compressors

Refrigeration

Most burnouts come from high head pressure, usually a dirty condenser. Clean, check air passages, recharge, recheck head pressure.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Heat reclaim series vs parallel

Refrigeration

Series reclaim condenser only desuperheats; parallel reclaim condenser must carry the whole load (desuperheat, condense, subcool). Three-way solenoid selects.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Heat-gun gasket repair

Refrigeration

A single bad spot on a gasket can be warmed with a heat gun and reformed; silicone grease helps it seal. Soak new gaskets in warm water to make them pliable; loosen (don't remove) retainer screws.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

High side and low side each read one pressure

Refrigeration

Pascal's law: each side is a fixed volume, so suction/evaporator pressure are the same thing, as are head/discharge/condenser pressure.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

High-side restriction symptoms

Refrigeration

Continuous running, no cooling, condenser cooler than normal.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Ice machine drains

Refrigeration

Never tee drains together (especially pressurized ones) — one backs up into another; stopped-up bin drain melts the ice; clogged reservoir drain concentrates minerals → bad ice and scaled nozzles.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Insulate horizontal baffles/drain pans

Refrigeration

Cold on top, warm underneath → condensation and eddy currents if not insulated.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Insulate suction absorbers below 32°F (Pro Tip)

Refrigeration

Frost forming between the corrugated tube and the braid will split the tube — insulate any absorber on a suction line running under freezing.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

King and queen valves

Refrigeration

Receiver outlet service valve = king valve, receiver inlet = queen valve. Most compressors have suction and discharge service valves.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

King valve (LRSV) / queen valve

Refrigeration

King = receiver outlet, queen = condenser-to-receiver inlet; front seat the king valve for pump-down.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Loosen shipping bolts

Refrigeration

Spring-mounted compressors: remove bolts after install; grommet-mounted: back them off 2–3 turns so the compressor can float. Always check.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Louver control logic

Refrigeration

Bellows on head pressure vs atmosphere closes louvers as head pressure falls; fans may run with louvers closed or cut off near closed. Some louver controls are ambient-temperature based instead.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Low-temp coils have wider fin spacing

Refrigeration

Wider spacing tolerates frost between defrosts; a medium-temp coil used in a freezer will block up fast.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Manual defrost accelerators

Refrigeration

Pot of very hot water inside, or heat gun/hair dryer kept moving at a distance. Never chip ice with anything sharp — punctured evaporators are rarely worth repairing. Wash cabinet with baking soda solution after.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Mechanical subcooling

Refrigeration

Using an AC-temperature system to subcool a freezer's liquid line pays off because high-temp systems reject heat about 3× more efficiently than low-temp ones; can be retrofitted.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Megger procedure

Refrigeration

Run ≥1 h first; power off; disconnect all wires; wipe terminals; jumper all three terminals together; black lead to bare metal on dome, red to a terminal; energize per maker's time; record; remove red then black.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Multi-evaporator TXVs get liquid-line solenoids

Refrigeration

Each case gets its own TXV and liquid solenoid so idle cases can be isolated and the compressor (with unloader) matches load.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

No alcohol additives in wet systems (Caution)

Refrigeration

Desiccants grab alcohol in preference to water and can release moisture back into the system.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Noncondensables vs overcharge vs dirty condenser

Refrigeration

All raise head pressure; subcooling is highest with air, high with overcharge, NORMAL with a dirty/blocked condenser.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Not enough ice? Do a capacity check

Refrigeration

Time one cycle, weigh the batch, scale to 1,440 min; within 10% of spec = machine is fine, customer needs a bigger bin, a storage freezer or a second machine.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Outdoor condenser in cold wind

Refrigeration

Strong cold wind can defeat louvers, fan cycling and even electric heaters; site and weatherproof the condensing unit (especially the electrical) to minimize wind.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Paper-slip gasket test

Refrigeration

Close door on a slip of paper; if it pulls out easily the gasket isn't sealing. Also look for condensation/mold on gasket and longer run times.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Plate evaporator defrost (Pro Tip)

Refrigeration

Never chip ice with a metal tool — you'll puncture the plate. Shut down, put a pan of hot water in the box, close the door, sponge up the melt.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Pneumatic control systems

Refrigeration

Need monthly checks.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

PWM solenoid EEV looks like a plain solenoid (Fig. note)

Refrigeration

Don't mistake a pulse-width-modulated metering solenoid for an on/off solenoid; the controller is what makes it a metering device.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Receiver heaters

Refrigeration

Electric heat in/around the receiver keeps it warmer than the condenser cabinet so it doesn't act as a second condenser in cold weather.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Replace the cap-tube compression nut

Refrigeration

The special squeeze nut deforms on tightening — always use a new one when the tube is serviced.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Sensing-bulb mounting

Refrigeration

Clamp tightly, clean both surfaces with steel wool, use the evaporator's socket if provided; best spot = last third of the evaporator. Loose bulb = wider actual swing.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Shaft seal is the weak point on open-drive

Refrigeration

Belt tension/misalignment overload the seal; keep pulleys aligned and shafts parallel.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Shell-and-coil condensers can't be rodded

Refrigeration

Mechanical cleaning only works on shell-and-tube (remove end manifolds) and rectangular tube-in-tube; shell-and-coil needs chemical cleaning.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Snap-action EPR

Refrigeration

Fully open or closed, lets the warm evaporator pressure rise before reopening — used where defrost is wanted every cycle (walk-ins, display cases).

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Starting a multi-evaporator system on a hot pull-down

Refrigeration

All TXVs open wide at start and can overload the compressor; close all suction manifold valves, start, open one slowly keeping suction just above cut-in, bring cases on one by one; or throttle the SSV from cracked toward mid-position as pressure drops.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Stat capillary handling

Refrigeration

Don't flex the sensing capillary (work-hardens, cracks); keep it off the evaporator; tape coiled sections.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Three-way solenoid on the TXV equalizer

Refrigeration

Stops off-cycle TXV chatter: when the compressor stops, the solenoid feeds head pressure to the underside of the diaphragm and holds the TXV closed.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Uninsulated suction line costs you

Refrigeration

60°F of extra superheat raises condenser inlet from 180°F to 240°F and the condenser must shed it before condensing — insulate the suction line, minimize superheat for best subcooling and capacity. Liquid line usually uninsulated unless in an attic/hot roof.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Use time-termination as backup only

Refrigeration

Terminate defrost on temperature or pressure; time termination is the fail-safe against over-defrosting. Ice crystals on product = it thawed and refroze.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Walk-in door latch

Refrigeration

Inside safety release must work — check it on every service visit. Mullion/door heaters stop sweating and ice at frames.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Water defrost must drain fully

Refrigeration

Drain the water line completely before restarting or it freezes and blocks.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

Where is the high-side restriction?

Refrigeration

Compare condenser-outlet temp to liquid-line temp at the TXV: close together → suspect the TXV; big difference → restriction in the liquid line between them. Restriction at receiver inlet gives HIGH head, condenser temp and subcooling instead.

Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

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For licensed technicians. Installer menus, test modes and access codes change settings that affect safety; only use them if you are trained on the appliance and understand the result. Gas, electrical and refrigerant work must be done by a qualified, licensed person, and the manufacturer’s manual for the exact model and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.