HVACopediaGas appliance data
Combustion targets, flames, gas properties, furnace classes and venting.
Combustion targets (natural gas and propane)
| Reading | Natural gas | Propane |
|---|---|---|
| CO₂ | about 7.6% | about 8.9% |
| Ultimate (undiluted) CO₂ | 11.9% | 13.9% |
| Oxygen | 6 to 9% (about 7.5% ideal) | 6 to 9% (about 7.5% ideal) |
| Manifold pressure | 3.5 in. WC | 10.5 in. WC |
| Carbon monoxide | under 50 ppm normal; absolute limit 400 ppm air-free | same |
Low CO₂, high oxygen and CO: too much excess air or gas pressure too low. High CO₂, low oxygen and CO: not enough air or gas pressure too high. Check the manifold pressure before changing the air.
Stack temperature
- Net stack = flue gas temperature − combustion air temperature.
- Non-condensing furnace: 325 to 500 °F net stack. Condensing: flue gas under 140 °F (about 115 to 118 °F is typical).
- About 1% efficiency is lost for every 40 °F rise in stack temperature.
- Flue pipe on a conventional appliance can run 500 to 600 °F.
Combustion efficiency of gas (percent)
| Excess air | O₂ | 200 °F | 300 °F | 400 °F | 500 °F | 600 °F |
|---|---|---|---|---|---|---|
| 9.5% | 2% | 85.4 | 83.1 | 80.8 | 78.4 | 76.0 |
| 15% | 3% | 85.2 | 82.8 | 80.4 | 77.9 | 75.4 |
| 28% | 5% | 84.7 | 82.1 | 79.5 | 76.7 | 74.0 |
| 45% | 7% | 84.1 | 81.2 | 78.2 | 75.2 | 72.1 |
| 82% | 10% | 82.8 | 79.3 | 75.6 | 71.9 | 68.2 |
By flue oxygen and net stack temperature. Interpolate between values.
Flame conditions
| Flame | Cause | What to check |
|---|---|---|
| Soft blue, no tipping, no lifting | Good combustion | Nothing |
| Yellow | Not enough primary air | Reset the primary air shutter, look for blockage, dust or a bad burner |
| Lifting | Gas speed faster than the flame speed | Reduce gas pressure or primary air |
| Popping (flashback at shut-off) | Flame travels back into the burner | Raise gas pressure, reduce primary air or orifice size, check valve and burners |
| Floating (very dangerous, makes CO) | Incomplete combustion | Check gas flow, flue, burners, secondary air and primary air |
| Rollout (very dangerous) | Blocked flue, poor draft, not enough air, overfiring | Same checks as floating; find the cause before resetting the rollout switch |
Orange flecks are usually dust burning, not an air problem.
Gas properties
| Item | Natural gas | Propane |
|---|---|---|
| Heating value | 1000 to 1100 BTU/ft³ | 2500 BTU/ft³ |
| Air needed for burning (theory) | 10 ft³ per ft³ | 24 ft³ per ft³ |
| Air with about 50% excess | about 15 ft³ per ft³ | about 35 ft³ per ft³ |
| Ignition temperature | 1100 to 1200 °F | 920 to 1020 °F |
| Flammable range (percent gas in air) | 4.0 to 14.0% | 2.4 to 9.5% |
| Weight compared to air | Lighter, rises | Heavier, pools in low spots |
Butane: 3200 BTU/ft³, boils at 32 °F. 28 in. WC is about 1 psi. LP is much heavier than air: gas from a failed pilot collects in low places, so ventilate basements and crawlspaces.
Furnace classes
| Class | AFUE | Features |
|---|---|---|
| Low efficiency | 68 to 72% | Natural draft, standing pilot |
| Mid efficiency | 79 to 83% | Spark or hot-surface ignition, induced draft, 3 in. flue, Class B vent or approved liner |
| High efficiency (condensing) | 90 to 97% | Secondary heat exchanger, PVC vent |
Condensate forms in furnaces above about 83% AFUE. Excess air: 20 to 50% on older furnaces, as low as 10% on high-efficiency ones.
Venting categories
| Category | Vent pressure | Condensing | Vent type |
|---|---|---|---|
| I | Negative | No | Class B vent (most mid-efficiency furnaces); flue gas 140 °F or more |
| II | Negative | Yes | Special vent per manufacturer |
| III | Positive | No | Stainless steel |
| IV | Positive | Yes | PVC; flue under 140 °F |
More gas furnace numbers
- Fan switch: fan on at about 140 °F in the plenum, off at about 90 °F.
- Over 2000 ft (610 m) a smaller burner orifice is needed. Check the manufacturer's chart.
- Pre-purge about 15 seconds. The igniter drops out within seconds of the gas valve opening. If flame is not proven in a few seconds the valve closes and the control retries.
- A soft lockout retries by itself after an inter-purge. A hard lockout needs the power turned off and on.
- Chimney: at least 2 ft above the highest part of the roof it passes through.
- The first two threads of gas pipe are left bare when applying pipe dope so it cannot plug an orifice.
- Drip leg at the furnace and at the meter outlet.
- Local gas code (CSA B149.1 in Ontario) decides pipe sizes, supports and tests. Use the Gas pipe sizing tool for pipe capacities.
- Heat exchanger check: watch the flame before and after the blower starts (a change means a leak), inspect with a mirror and light, and use a tracer-gas test at the supply registers. A heat exchanger wall thinned by half or more is replaced.
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.