Reading boiler efficiency from flue gas losses
There are two ways to measure boiler efficiency. The direct method meters both the fuel in and the steam out, while the indirect method subtracts measured losses from 100%. The indirect method is far more practical in the field because a single combustion analyzer supplies everything it needs, and that is what this calculator uses. The largest single loss is the heat going up the stack, which follows from flue temperature and oxygen alone.
Stack loss uses the Siegert equation: qA = (flue temp − air temp) × (A₂ ÷ (21 − O₂) + B), with the temperature difference converted to degrees Celsius because the coefficients are defined that way. The coefficients applied here are A₂ 0.66 and B 0.009 for natural gas, A₂ 0.68 and B 0.007 for both fuel oils, and A₂ 0.74 and B 0.008 for coal. A gas boiler at 356°F stack, 68°F air and 3% oxygen loses about 7.3% up the stack, so with 2% radiation loss the efficiency lands near 90.7%.
Only two levers raise that number: lower the stack temperature or cut the excess air. High oxygen means the burner is heating cold air it never needed, and a high stack temperature usually means soot or scale on the heating surfaces. Pushing oxygen too low is its own hazard, since incomplete combustion sends carbon monoxide up fast, so every burner trim should be verified with an analyzer. For boilers under inspection requirements, have a mechanical engineer or the inspection authority confirm the results.
Frequently Asked Questions
It depends on the fuel and burner, but around 3% is a common target for gas and 3% to 4% for oil. Going lower raises the risk of carbon monoxide and smoke.
Catalog figures are test-stand values at full fire. This calculation reflects the boiler as it runs right now, so part-load operation, fouled surfaces and cycling losses all pull the number down.