Drawing No. EH–TH–001 // Thermal Engineering & HVAC
Boiler Efficiency / Stack-Loss Calculator
Reviewed August 2026
Estimate combustion efficiency from flue-gas temperature, CO₂ percentage and an empirical Siegert fuel factor, then see the approximate fuel input for a specified useful heat demand.
What problem does this solve?
The biggest energy loss in most boilers is heat carried away in the hot flue gas going up the stack. The Siegert method estimates this loss — and therefore combustion efficiency — from just three field-measurable quantities: stack temperature, ambient temperature, and flue gas CO2 percentage, making it a fast way to spot-check boiler tuning without full calorimetric testing.
Inputs
Results
Background
Stack loss (%) = K × (Tstack − Tambient) / CO₂%, where K is an empirical fuel/convention-specific factor. Combustion efficiency (%) ≈ 100 − stack loss. This is a well-established field-estimation method used by combustion analyzers worldwide.
A hotter stack means more sensible heat is being carried away unused, directly increasing loss. For a given fuel, a higher measured CO₂% generally corresponds to lower excess air (closer to stoichiometric combustion while still retaining the required safe excess air), so less unnecessary air is heated and discharged up the stack. Boiler tuning aims to run CO₂ as high as safely possible (approaching the fuel's theoretical maximum) while keeping stack temperature as low as practical without risking flue gas condensation.
This simplified formula captures the dominant sensible flue-gas loss but omits radiation and convection losses from the boiler's outer shell, incomplete combustion losses (CO, unburned fuel), and blowdown losses on steam boilers — a full efficiency test accounts for all of these separately.
Frequently asked questions
Practical questions about inputs, assumptions and interpretation.
There is no single universal target: expected stack loss depends on boiler type, firing rate, excess air, fuel, whether the appliance is condensing, and the adopted test convention. Compare the measured result with the boiler manufacturer’s commissioning guidance or the combustion analyzer’s applicable method rather than treating one percentage band as universal.
No — combustion (stack-loss) efficiency ignores shell radiation/convection losses and, for steam boilers, blowdown losses. Overall fuel-to-steam or fuel-to-water efficiency is always somewhat lower than combustion efficiency alone, though stack loss is usually the largest single loss category.
As flue gas cools, at some point (the acid dew point for sulfur-containing fuels, or the water dew point for natural gas) water vapor and acidic compounds in the flue gas begin condensing inside the flue or stack, risking corrosion damage — condensing boilers are specifically designed to handle this and recover that latent heat safely, but conventional boilers must keep stack temperature above this dew point.