Combustion Efficiency Calculator — Free Online Calculator
The Siegert formula converts three field measurements — flue gas temperature, combustion air temperature, and flue oxygen — into the sensible stack heat loss percentage and net combustion efficiency of any gas, propane, or oil-fired boiler. Enter the readings below for an instant efficiency result with excess-air and safety flags.
Enter flue gas readings
Net Efficiency
Stack Loss
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Excess Air
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See the breakdown
Warning
Results use the simplified Siegert method. Radiation and convection losses are estimated at 1.5%. For certification, use a calibrated flue gas analyser per EN 15502.
Boiler efficiency benchmark
| Net Efficiency | Rating | Typical system |
|---|---|---|
| Under 75% | Poor | Old cast-iron boiler, no maintenance |
| 75–80% | Fair | Standard-efficiency pre-1990s boiler |
| 80–85% | Good | Mid-efficiency gas boiler |
| 85–90% | Very Good | High-efficiency condensing boiler (non-condensing mode) |
| 90%+ | Excellent | Condensing boiler operating in condensing mode |
Sources & standards: ASHRAE Handbook — Fundamentals; EN 304 / EN 14394 boiler efficiency test methods; BRE Domestic Heating Design Guide.
The Siegert formula, explained
The Siegert method was developed by German engineer Karl Siegert and remains the basis for all portable flue-gas analyser efficiency readings. It splits boiler losses into three components: sensible stack heat carried out by hot flue gas, latent heat in uncondensed water vapour, and fixed radiation and convection losses from the boiler casing. The sensible stack loss dominates in non-condensing boilers and is directly measured using flue gas temperature and oxygen percentage.
Why flue oxygen?
Oxygen remaining in flue gas directly indicates excess air. Lower O₂ means less excess air and less sensible heat wasted — but below 1–2% O₂, the risk of incomplete combustion and CO formation increases sharply.
Fuel factor f
The fuel factor accounts for the specific heat capacity and molecular weight of each fuel's combustion products. Natural gas (f=0.66) and propane (f=0.63) are close; fuel oil (f=0.68) has slightly heavier combustion products.
Condensing benefit
When flue gas temperature drops below the dew point (~55°C for natural gas), the latent heat of water vapour is recovered, boosting efficiency above 100% on a gross calorific basis.
O₂ vs CO₂
Many older analysers measure CO₂ instead of O₂. The relationship is: CO₂,max − CO₂,measured = O₂ × (CO₂,max / (21 − O₂,stoich)). For natural gas, CO₂,max ≈ 11.7%.
Worked examples
Three real-world readings — a condensing boiler at low stack temperature, a conventional gas boiler, and a neglected oil boiler with high flue losses.
Efficient condensing boiler — 60°C flue gas, 20°C air, 4% O₂, Natural Gas
η = 100 − 1.55 − 11 − 1.5 = 85.95% ≈ 86%
Result: High efficiency — low stack temperature is the hallmark of condensing-mode operation.
Conventional gas boiler — 200°C flue gas, 20°C air, 5% O₂, Natural Gas
η = 100 − 7.4 − 11 − 1.5 = 80.1%
Result: Typical for a well-tuned conventional boiler — acceptable but limited by the non-condensing flue design.
Neglected oil boiler — 300°C flue gas, 25°C air, 6% O₂, Fuel Oil
η = 100 − 12.5 − 0 − 1.5 = 86%
Result: Note the high stack temperature flag — fouled heat exchange surfaces are wasting thermal energy.
Frequently asked questions
Common questions about boiler combustion efficiency and the Siegert method.
What is boiler combustion efficiency?
Combustion efficiency measures how completely a boiler converts fuel into usable heat. It is the percentage of the fuel's heat content that ends up in the building's heating system, after subtracting sensible stack losses, latent water-vapour losses, and casing radiation. A well-tuned natural-gas boiler typically achieves 80–92% combustion efficiency, with condensing boilers exceeding 90% when the flue temperature is below the dew point.
What is the Siegert formula?
The Siegert formula calculates sensible stack heat loss — the heat wasted by hot flue gas escaping up the chimney — from three measurable field quantities: flue gas temperature, combustion air temperature, and flue oxygen percentage. Developed by Karl Siegert in the 1930s, it remains the basis for all portable flue-gas analyser efficiency readings.
What is a good flue gas oxygen percentage?
For natural gas, 3–5% O₂ represents a well-adjusted burner with adequate excess air to prevent CO formation but low enough to limit stack losses. Below 1.5% O₂ there is a risk of carbon monoxide; above 8% O₂ there is excessive dilution air that carries heat up the flue.
Why does stack temperature matter?
Stack gas temperature is the single biggest driver of stack heat loss. Every 10°C reduction in flue temperature saves roughly 0.4–0.5% in stack losses. Elevated stack temperatures above 250°C typically indicate fouled heat-transfer surfaces, scale on waterside heat exchangers, or insufficient combustion gas baffling.
What is excess air in combustion?
Excess air is the percentage of air supplied above the stoichiometric minimum needed for complete combustion. It is calculated from flue O₂ as: excess air (%) = O₂ / (21 − O₂) × 100. Typical targets are 10–20% excess air (3–4% O₂) for gas boilers; higher excess air increases stack losses.
How is combustion efficiency different from seasonal efficiency (AFUE)?
Combustion efficiency is a real-time, point-in-time measurement under steady operating conditions. AFUE (Annual Fuel Utilization Efficiency) is a laboratory-rated seasonal figure that also accounts for pilot losses, cyclic start/stop losses, and jacket losses over a whole heating season. A boiler can have 82% combustion efficiency at steady state but a lower AFUE once cycling losses are included.
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