- Stack (flue-gas) loss
- 8.2 %
- Combustion efficiency (net, LHV)
- 91.8 %
- Excess air (approx.)
- 17 %
- Net stack temperature
- 180 °C (324 °F rise)
The short answer
Combustion efficiency ≈ 100 − (stack temperature − combustion-air temperature) × (A2 ÷ (21 − O2) + B), the Siegert formula used by flue-gas analysers. For natural gas (A2 0.66, B 0.009), a 180 °C net stack rise at 3 % O2 gives a stack loss of about 8.2 % and roughly 87 % efficiency on a net (lower heating value) basis.
Formula
Stack loss qA (%) = (t_stack − t_air) × (A2 ÷ (21 − O2) + B)Combustion efficiency (%) = 100 − qAExcess air (%) ≈ O2 ÷ (21 − O2) × 100
How to use it
- Choose the fuel.
- Enter flue-gas O2 (dry, %) and stack temperature from the analyser.
- Enter combustion-air (burner intake) temperature.
Assumptions
- Siegert coefficients: natural gas A2 0.66, B 0.009; light fuel oil A2 0.68, B 0.007; propane A2 0.63, B 0.008 (the values used in EN 50379 analysers).
- Net (lower heating value) basis, temperatures in °C. It ignores unburned CO and radiation losses, so fuel-to-steam efficiency is a few points lower.
- Not valid for condensing operation, where the flue gas is below its dew point.
Frequently asked questions
What O2 should a gas boiler run at?
Many packaged gas burners are tuned for roughly 2–4 % O2 at high fire, depending on the burner and turndown. Lower O2 cuts stack loss but raises the risk of CO; tune to the burner maker’s figures with an analyser.
How much does stack temperature matter?
As a rule of thumb, every 40 °F (22 °C) rise in net stack temperature costs about 1 % efficiency. A rising stack temperature at the same load often means fouled tubes or scale.
Is this the same as boiler efficiency?
No. Combustion efficiency only counts flue-gas loss. Fuel-to-steam efficiency also subtracts radiation, convection and blowdown losses.