HVACopediaBoiler & oil burner data
Boiler temperatures, expansion tanks, glycol, water limits and oil burner numbers.
Hot water boilers
| Item | Value |
|---|---|
| Supply and return difference at the boiler (design) | about 20 °F; much less points to circulation or air trouble |
| Conventional boiler return water | keep above 140 °F to stop flue gas condensing (some tolerate down to 105 °F) |
| Operating range | 90 to 200 °F |
| Mixing valve settings | about 140 °F return side, 160 °F supply side |
| Expansion tank air precharge | about 12 psi. Check with a tire gauge with the makeup water isolated |
| Water pH | 7 to 10 |
| 50% propylene glycol | protects to about −30 °F but reduces heat output to about 90% |
| Condensing boilers | above 90% efficient; recover about 970 BTU for every pound of flue gas condensed |
| Steel pipe growth | about 3/4 in per 100 ft per 100 °F rise |
| Copper pipe growth | about 11/16 in per 100 ft per 100 °F rise |
Boiler water limits (ppm)
| Substance | Limit |
|---|---|
| Sodium sulfite | 1.0 |
| Sodium chloride | 10 |
| Sodium phosphate | 25 |
| Sodium sulfate | 25 |
| Silica | 0.2 |
| Total dissolved solids | 50 |
Boil-out: about 1 lb of trisodium phosphate per 50 gallons, circulate about 4 hours, drain, clean strainers, refill.
Tips and checks
- Add air to an expansion tank with a hand pump only, a few strokes at a time. A compressor can burst the bladder.
- A tank that is hot from top to bottom is waterlogged. A relief valve that weeps every cycle means a waterlogged or undersized tank.
- The circulator should pump away from the expansion tank. Put the air separator downstream of the boiler and upstream of the pump.
- Air vent caps must be loose. Painted-over vents are a common cause of air-bound radiators.
- Never purge a hot cast-iron boiler with cold water: it can crack.
- Test the low-water cut-off by shutting the makeup water and opening the drain: the burner must stop right away.
- Never use automotive antifreeze, and never use glycol in galvanized pipe.
- Steam boiler sight glass should be 1/3 to 1/2 full. Residential steam systems are tested at 50 psig.
- Never bypass a rollout or blocked-vent switch.
- Black wire should be hot and white neutral at the boiler service switch. Reversed polarity upsets flame-rectification controls.
Oil burner numbers
| Item | Value |
|---|---|
| No. 2 oil heat content | 140,000 BTU per gallon |
| Pump pressure (standard) | 100 psi |
| Draft over the fire and in the flue | −0.02 to −0.03 in. WC |
| CO₂ | 8 to 12% (under 8% is incomplete combustion) |
| Stack temperature range | 300 to 900 °F |
| Electrode gap | 1/8 to 3/16 in |
| Electrode height above nozzle centre | 1/2 to 5/8 in |
| Ignition transformer | 10 to 14 kV (14 kV preferred) |
| Solid-state igniter | 14 to 20 kV at 20 kHz |
| Single-stage pump suction limit | 12 in. Hg |
| Two-stage pump suction limit | 17 in. Hg |
Never raise the pump pressure to fix a problem and never fit a higher-capacity nozzle: that overfires the burner. A dull orange or red flame means not enough air. Take readings after the flue has stabilized, about 5 minutes.
Oil furnace efficiency by net stack temperature
| Net stack (°F) | Efficiency |
|---|---|
| 1000 | 65 to 69% |
| 800 | 70 to 73% |
| 600 | 76 to 79% |
| 400 | 82 to 84% |
Oil line and tank
| Item | Value |
|---|---|
| Line, 50 to 100 ft | 3/8 in copper |
| Line, 200 to 300 ft | 1/2 in copper |
| Minimum supply / return | 3/8 in / 1/4 in inside diameter |
| Gravity tank to furnace (one-pipe) | at least 7 ft away, less than 25 ft of elevation |
| Fuel pickup above tank bottom | 3 to 4 in |
| Tank vent, up to 660 gal | 1 1/4 in |
| Tank vent, 661 to 3000 gal | 1 1/2 in |
| Tank vent, 3001 to 10,000 gal | 2 in |
Cold oil is thick: viscosity doubles at 0 °F. Blow oil lines out with nitrogen only, never compressed air or oxygen. Check all three filters: the line filter, the pump strainer and the nozzle strainer.
Take it with you
All of these tables, plus the calculators, are in the HVACopedia app, offline.
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.