Work out combustion efficiency instantly with clear inputs, formula shown and shareable results.
The Siegert formula estimates the heat lost up the stack from net flue temperature and residual oxygen: loss = ΔT x (A2 / (21 - O2) + B), with fuel-specific constants. Efficiency is 100 percent minus that loss, so cutting either excess air or stack temperature improves it directly.
Siegert stack loss
Loss % = (Tstack - Tair) x (A2 / (21 - O2) + B)
Efficiency
η = 100 - stack loss %
Loss % = (Tstack - Tair) x (A2 / (21 - O2) + B). The Siegert formula estimates the heat lost up the stack from net flue temperature and residual oxygen: loss = ΔT x (A2 / (21 - O2) + B), with fuel-specific constants.
Siegert accounts only for dry flue gas loss on a net calorific basis. A condensing appliance also recovers latent heat from the water vapour, which this method does not credit.
This calculator takes 4 inputs: Stack temperature, Combustion air temperature, Flue oxygen, Fuel. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.