Combustion & Flue Gas

Every combustion question comes back to one number: λ, the air ratio. Too little air makes carbon monoxide and soot; too much heats the atmosphere and carries the energy up the stack. The flue gas tells you which, and because CO₂ and O₂ move in opposite directions with excess air, either one identifies λ.

Analyser readings are dry — the sample is chilled and the water removed before it reaches the cell — so the CO₂ and O₂ figures here are on the same basis. The Siegert flue-loss formula is the shortcut every combustion analyser implements, not a full heat balance; it excludes losses from unburnt fuel, blowdown and (for condensing appliances) the latent heat recovered from the flue water. Net (LHV) and gross (HHV) efficiency differ by about 10% for natural gas — the same appliance is "92% efficient" in Europe and "83%" in the US, which is why a condensing boiler can legitimately exceed 100% on a net basis.