Method and limits

Efficiency is the Siegert approximation: η = 100 − k · (T_flue − T_air) / CO₂%, with the fuel-specific Siegert constant k and, where oxygen is measured instead, CO₂ = CO₂_max · (21 − O₂) / 21. Excess air follows from EA = O₂ / (21 − O₂).

Siegert accounts for the dry flue gas (sensible heat) loss only. It is a net or lower-calorific-value efficiency and excludes the latent heat of the water vapour in the products of combustion, so it does not describe a condensing boiler, where gross efficiencies above 100% on a net basis are normal and the figure here will be badly pessimistic. It also excludes radiation and casing losses, blowdown, and unburnt-fuel loss beyond the CO check. For a certified figure, use an indirect-method calculation to EN 12952-15 or an instrument that reports both sensible and latent terms.

The annual loss figure assumes continuous firing at the entered rate for 8,760 hours against a reference efficiency of 95% net, and is an order-of-magnitude indication for prioritising a tuning visit — not a savings guarantee. Driving oxygen down to chase efficiency without watching CO is dangerous: the tool flags CO above 100 ppm, and any reading above 400 ppm needs the burner shut down and investigated.