Flue-Gas Combustion Efficiency (Stack Loss)
The third number on the analyzer screen, from the same O2 and stack-temperature readings: flue CO2 = CO2max x (1 - O2 / 20.9), dry stack loss qA = dT_C x (A1 / CO2 + B) (Siegert, per-fuel A1/B), efficiency = 100 - qA on the net (LHV) basis, plus an approximate gross (HHV) conversion because US analyzers display HHV - a gas furnace at 5% O2 and a 400 F stack over 70 F air is 90.7% net but about 81.8% on a US analyzer, right where a healthy non-condensing furnace reads. Sample dry in the undiluted flue; a condensing appliance recovers latent heat this counts as lost. A tuning aid, not a certified combustion test or an AFUE rating.
Formula and source
co2_pct = CO2max x (1 - flue_o2_pct / 20.9); qa_pct = (stack_temp_f - air_temp_f) x 5/9 x (A1 / co2_pct + B); eff_net_pct = 100 - qa_pct; eff_gross_pct = eff_net_pct x LHV/HHV.
Siegert stack-loss method (DIN combustion-analysis practice as implemented by flue-gas analyzers), by name.
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