Fridge, freezer & room A/C data

Normal readings, drier and cap tube sizes, bench-test temperatures.

Domestic refrigerator operating readings (70 °F room)

Typical R-134a fridge with a freezer. About 5 to 10 psig on the low side and about 150 psig on the high side is a normal reading. A restriction pulls the low side into a vacuum and sends the head above 200 psig.

Typical 18 ft³ fridge-freezer by room temperature

Many fridges run about one third of the time (for example 5 minutes on, 10 minutes off). Any unexpected change in cycling is worth checking.

Electrical checks on a fridge

• Outlet voltage: nameplate 115 V ±10% (about 103 to 127 V) under load.
• A drop of 5 to 10 V between no load and running points to a dirty condenser, an unbalanced fan or a part drawing too much current. More than 10 V points to shorted windings, a locked fan motor or wiring that is too light.
• A start relay should drop out in about 3 seconds. Slower means the compressor is overloaded.
• A switch or thermostat must break the hot leg, never the neutral.
• Ice maker: the mold must be below 15 °F or its thermostat will not close.
• Compressor pumping test: pinch the suction line and it should pull 25 to 28 in. Hg in seconds. A vacuum that decays from 20 to 10 in. Hg means leaking discharge valves. Run it only briefly.
• Discharge line temperature of 350 °F or higher breaks the oil down and risks burnout.
• Clean the condenser about every 3 months.

Thermostat bench test

• Ice water is 32 °F. With the control set to 32 °F, the contacts should be open (OL), and close as the water warms.
• A 45 °F bath with the control set to 45 °F: the contacts open and should begin opening just below about 45.5 to 46 °F.
• Freezer thermostats (0 to −20 °F) need dry ice, not ice water.
• Fridge control example: cut-in 45 °F, cut-out 32 °F gives a range of 32 to 45 °F and a differential of 13 °F.
• Chattering contacts mean pitted points: replace the thermostat.

Filter-drier size for small systems

Replace the drier whenever the circuit is opened. Never use liquid antifreeze-type driers. Desiccant holds about 16% of its weight (silica gel), 12% (activated alumina) or 16% (synthetic silicate).

Replacement capillary tube for room A/C (inside diameter × length)

Cut a capillary tube by filing a notch and snapping it. A tube cutter squeezes the inside. A replacement must match the diameter and length exactly.

Room and split A/C

• Typical R-22 readings: evaporating 40 to 44 °F (69 to 75 psi), superheat 2 to 11 °F; discharge 270 to 380 psi, liquid subcooled 2 to 11 °F. Values change with size, temperatures and load.
• Comfort cooling evaporating temperature is generally 40 to 50 °F.
• A window unit must slope about 1/4 in toward the outside so the condensate drains. A unit that drips inside has the wrong slope or a plugged drain hole.
• Do not restart a cap-tube unit right after shutdown: wait for the pressures to equalize.
• Condensing unit: pad 12 to 24 in from the building, at least 4 in thick; at least 24 in from walls; not in a corner or under an eave. Line set holes are about 3 in; keep outdoor units within the manufacturer's line set limit (up to about 50 ft).
• Condenser above the evaporator: add a trap or inverted loop in the suction line for oil return. A coil bonnet wider than 12 in needs at least 6 in above the furnace.
• Inverter compressors run from about 30 Hz up to 310 Hz and can slow to 10% of rated speed.
• Ductless: add an overflow switch on the drain pan and use biocide tablets to prevent slime.

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Calculation aids and reference figures for licensed technicians. Always follow the manufacturer’s data plate and manual for the exact model, and local codes. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Pressures are typical saturation values and change with load and airflow.