Excess Air from Flue-Gas O2
Turn a combustion analyzer's oxygen (or CO2) reading into percent excess air.
Example
You enter
- Flue-gas O2 (%, dry air-free) 4
- Fuel CO2max (%: 11.7 gas / 13.7 propane / 15.3 oil) 11.7
You get
- Excess air 23.7
Details, formula, and sources
EA = O2 / (20.9 - O2) x 100, or from carbon dioxide EA = (CO2max / CO2 - 1) x 100 (CO2max ~ 11.7% gas, 13.7% propane, 15.3% oil). The O2 form assumes complete combustion, so measurable CO understates it; a gas appliance targets ~3-4% O2 (15-25% excess air). A tuning aid, not a certified combustion test.
EA_pct = measured_co2 > 0 ? (co2max / measured_co2 - 1) x 100 : measured_o2 / (20.9 - measured_o2) x 100.
ASME PTC 4.1 / combustion analysis practice excess air from flue-gas oxygen, by name.
The excess-air relations are standard combustion-analysis practice; the analyzer and appliance manufacturer instructions govern the actual tune.
Estimate. AHJ and licensed professional govern.
Field names used by the API: measured_o2_pct, co2max_pct, excess_air_pct
- Complete combustion the O2 form assumes complete combustion; measurable CO understates the excess aircombustion analysis practice
- Ultimate CO2 CO2max ~ 11.7% natural gas, 13.7% propane, 15.3% #2 oilfuel stoichiometry