Excess Air from Flue-Gas O2
Turn a combustion analyzer's oxygen (or CO2) reading into percent excess air: 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.
Formula and source
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.
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