Flue-Gas Combustion Efficiency (Stack Loss)
The third number on the analyzer screen, from the same O2 and stack-temperature readings.
Example
You enter
- Fuel natural_gas
- Flue-gas O2 (%, dry, undiluted) 5
- Stack temperature (°F) 400
- Combustion-air temperature (°F) 70
You get
- Dry stack loss (Siegert) 9.27%
- Combustion efficiency, net (LHV) 90.7%
- Approx. gross (HHV, US analyzer basis) 81.8%
Details, formula, and sources
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.
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.
The Siegert stack-loss relation and its per-fuel coefficients 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: fuel, flue_o2_pct, stack_temp_f, air_temp_f, stack_loss_pct, eff_net_pct, eff_gross_pct
- Siegert coefficients A1 / B / CO2max: natural gas 0.37 / 0.009 / 11.7%, propane 0.475 / 0.000 / 13.7%, #2 oil 0.50 / 0.007 / 15.4%DIN combustion-analysis practice (analyzer fuel tables)
- Basis conversion net (LHV) is the European convention; US analyzers display gross (HHV), approximated as net x LHV/HHV (0.902 gas / 0.920 propane / 0.939 oil)fuel heating values / TSI Combustion Analysis Basics