Boiler Efficiency — from Flue Gas Losses

Boiler efficiency by the indirect method from flue gas temperature and oxygen, with the loss breakdown.

Unit system

Inputs

FuelFlue gas temperature

Measure AFTER the economiser if there is one. Roughly one point of efficiency per 20 °C.Combustion air temperature

Flue gas oxygen (%, dry)

Typically 2–4 % on gas. High oxygen means excess air: fuel is spent heating air that goes up the stack.Radiation and unaccounted losses (%)

Falls with boiler size: 2–3 % on small boilers, 0.5–1 % on large ones.

Boiler efficiency

0.9072

Verdict

In the typical range; flue temperature and excess air could be reviewed

Computed on the lower heating value (LHV). On a higher-heating-value basis the number looks about ten points lower — make sure two plants are compared on the same basis.
The dry flue gas specific heat is taken as 1.05 kJ/(kg·K); the deviation is small between 100 and 300 °C.
Unburnt losses (CO, soot) are not included; if the analyser shows CO the real efficiency is lower.

A PDF carrying the inputs, results, assumptions and the source. A one-time code is sent to your address.

Results are for engineering pre-assessment and are not a product selection commitment. Final responsibility rests with the user.