Combustion Efficiency Calculator
A Quick Field Estimate From Flue Gas Readings
The Siegert method estimates combustion efficiency directly from a portable flue gas analyzer reading — flue gas temperature, ambient temperature, and CO2 percentage — without needing a full heat balance.
The Formula
Efficiency % = 100 − K × (Flue Gas Temp − Ambient Temp) / CO2%
K is a fuel-specific constant: roughly 0.5–0.6 for natural gas, ~0.68 for oil, ~0.74 for coal (defaults to 0.58).
Worked Example
Flue gas at 220°, ambient air at 20°, with 10% CO2:
Efficiency = 100 − 0.58 × (220 − 20)/10 = 88.4%
How to Use This Calculator
- Enter the Flue Gas Temperature and Ambient Temperature.
- Enter the CO2 percentage measured in flue gas.
- Optionally adjust the K constant for your fuel type.
- Select Calculate to get estimated combustion efficiency.
Related Calculations
Need the direct-method boiler efficiency instead? See the Boiler Efficiency Calculator, or check flue gas oxygen in the Flue Gas Oxygen Calculator.