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.