Combustion Efficiency Calculator (Natural Gas)
Turn a combustion analyzer’s oxygen and stack-temperature readings into combustion efficiency and excess air, and estimate what a tune-up saves, using the U.S. Department of Energy’s natural-gas table.
Method reviewed October 26, 2026. Version 1.0.0. How it works
This is an estimate. It uses a simplified method and typical assumptions, all listed below. Use it to plan and to check quotes; equipment should be sized from a full room-by-room load calculation (ACCA Manual J) by a qualified professional.
Result
- Combustion efficiency
- 78.2%
- Excess air
- 44.9%
7 % O₂ with a 400 °F stack temperature rise is about 78.2 % combustion efficiency (45 % excess air).
- This table assumes complete combustion. Carbon monoxide is not part of the calculation: a CO reading must be checked separately against the appliance manufacturer’s limits. Burner adjustment is work for a qualified gas technician.
- DOE notes that well-designed natural-gas systems can reach about 10 % excess air, and that boilers often run with more excess air than they need.
- Heat lost up the stack
- 21.8%
- Stack minus combustion-air temperature
- 400°F
Show the working
- Temperature rise = stack − combustion air470 − 70 = 400 °F
- Efficiency interpolated in the DOE natural-gas table (O₂ and temperature rise)7 % O₂, 400 °F → 78.2 %
- Excess air interpolated from the same table44.9 %
What the readings mean
Burners need some air beyond the exact amount the fuel needs, to burn it completely. Too little causes soot and carbon monoxide; too much carries heat up the flue in extra hot gas. The oxygen left in the flue gas shows how much excess air there is, and the stack temperature shows how hot the gas leaves. Together they set how much of the fuel’s heat stays in the appliance.
The DOE Steam Tip Sheet #4 tabulates natural-gas combustion efficiency for 2 % to 10 % oxygen and for a stack temperature 200 °F to 600 °F above the combustion air. This calculator interpolates between those points. It does not extrapolate beyond them.
Estimating savings from tuning
Enter a second reading, for example after a technician adjusts the burner, and the calculator applies the DOE relation: fuel saved = 1 − efficiency before ÷ efficiency after. In the DOE example, efficiency goes from 78.2 % at 44.9 % excess air and a 400 °F rise to 83.1 % at 9.5 % excess air and a 300 °F rise. That cuts fuel use by 5.9 %.
Limits and safety
- Natural gas only, assuming complete combustion. Carbon monoxide is not part of the table; a CO reading must be checked separately against the appliance manufacturer’s limits.
- Combustion efficiency is a steady-state figure. It is not AFUE, which also counts cycling and standby losses.
- Condensing appliances run below the table’s temperature range and are not covered.
- Combustion testing and burner adjustment are work for a qualified gas technician with a calibrated analyzer. Never adjust a gas burner yourself.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Flue-gas oxygen (O₂)
- 7 %
- Flue-gas (stack) temperature
- 470 °F
- Combustion-air temperature
- 70 °F
- Compare with a second reading?
- No
Result
- Combustion efficiency
- 78.2 %
- Excess air
- 44.9 %
- Heat lost up the stack
- 21.8 %
- Stack minus combustion-air temperature
- 400 °F
7 % O₂ with a 400 °F stack temperature rise is about 78.2 % combustion efficiency (45 % excess air).
Assumptions
- Natural gas only. Efficiency is interpolated linearly between the points of the U.S. DOE combustion-efficiency table, which assumes complete combustion and dry combustion air.
- Excess air is read from the same table (2 % O₂ = 9.5 %, 10 % O₂ = 81.6 %).
Limitations
- Combustion (stack) efficiency is not AFUE or seasonal efficiency: it excludes jacket, cycling and standby losses.
- Readings outside 2–10 % O₂ or a 200–600 °F temperature rise are outside the table and are not extrapolated. Condensing appliances with low stack temperatures are not covered.
- Never adjust a gas burner yourself; testing and adjustment are for a qualified technician with a calibrated analyzer.
Frequently asked questions
What is a good excess-air level?
DOE notes that well-designed natural-gas systems can reach about 10 % excess air (around 2 % oxygen), and that boilers often run with more than they need. The right setting for a particular appliance comes from its manufacturer.
Why does a lower stack temperature mean higher efficiency?
Less heat leaves with the flue gas. DOE quotes a rule of thumb of about 1 % efficiency for every 40 °F reduction in stack temperature, or every 15 % reduction in excess air.
Can I use this for oil or propane?
No. The DOE table is for natural gas. Other fuels produce different amounts of flue gas and water vapour, so their efficiencies at the same readings differ.
Sources
- Improve Your Boiler’s Combustion Efficiency (Steam Tip Sheet #4, DOE/GO-102012-3405, January 2012) — U.S. Department of Energy, Advanced Manufacturing Office. Combustion efficiency for natural gas versus flue-gas oxygen (2, 3, 5, 7, 10 %; excess air 9.5–81.6 %) and flue-gas minus combustion-air temperature (200–600 °F), assuming complete combustion and dry combustion air; worked example 78.2 % → 83.1 % and annual savings = fuel use × (1 − E1/E2).
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