Combustion (Stack) Efficiency: Siegert Formula

Everything a burner wastes leaves through the chimney as hot gas, and the Siegert formula (used across Europe since the early 1900s and built into every combustion analyser since) estimates that loss from two measurements. The net stack temperature (flue minus combustion-air temperature) says how hot the exhaust is; the CO₂ percentage says how much excess air is diluting it. More excess air means more nitrogen dragged through the fire and out the flue, so CO₂ falls and the loss term rises. The fuel constant A is about 0.66 for natural gas, 0.68 for fuel oil and 0.74 for coal, all on a Celsius-degree basis, so enter ΔT in °C or K (°F entries are converted for you).

Worked example: a gas boiler with a 180 °C net stack rise and 10 % CO₂ loses 0.66 × 180 ÷ 10 = 11.9 %, so combustion efficiency is 88.1 %, a healthy non-condensing result. The two levers are visible immediately: cut excess air until CO₂ climbs from 8 % to 10 % and you gain roughly two points; let the heat exchanger scale until stack temperature rises 50 °C and you lose three. The formula's honest limit is that it counts only dry flue loss and stops at the dew point: it cannot describe a condensing appliance recovering latent heat from the water vapour, which is how modern boilers reach 95 %+ and why their analysers switch to a different calculation. The field trap is measuring stack temperature too close to the appliance or in the wrong part of the flue cross-section; take it in the centre of the stream and let the probe stabilise for a full minute.