Skip to content
ENExplore Calculators

Combustion Efficiency Calculator

Work out combustion efficiency instantly with clear inputs, formula shown and shareable results.

About Combustion Efficiency Calculator

The Siegert formula estimates the heat lost up the stack from net flue temperature and residual oxygen: loss = ΔT x (A2 / (21 - O2) + B), with fuel-specific constants. Efficiency is 100 percent minus that loss, so cutting either excess air or stack temperature improves it directly.

Formulas

Siegert stack loss

Loss % = (Tstack - Tair) x (A2 / (21 - O2) + B)

A2
0.66 gas, 0.68 oil, 0.65 coal
B
0.009 gas, 0.007 oil, 0.005 coal

Efficiency

η = 100 - stack loss %

Worked example

With the default values below, Combustion Efficiency Calculator returns Combustion efficiency of 82.51%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Stack temperatureminimum 50 °F
400 °F
Combustion air temperatureminimum -40 °F
70 °F
Flue oxygenaccepts 0.1–20 %
6 %
Fuelone of Natural gas, Light fuel oil, Coal
Natural gas

Results

Combustion efficiency
82.51% %
Dry flue gas loss
17.49% %
Excess air
40.3% %
Net stack temperature
330.0 °F

Step by step

  1. 1Values used
  2. 2Siegert stack loss
  3. 3Efficiency
  4. 4Combustion efficiency= 82.51 %
  5. 5Dry flue gas loss= 17.49 %
  6. 6Excess air= 40.3 %
  7. 7Net stack temperature= 330.0 °F

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Combustion Efficiency figure worked out?

Loss % = (Tstack - Tair) x (A2 / (21 - O2) + B). The Siegert formula estimates the heat lost up the stack from net flue temperature and residual oxygen: loss = ΔT x (A2 / (21 - O2) + B), with fuel-specific constants.

Why can a condensing boiler beat this number?

Siegert accounts only for dry flue gas loss on a net calorific basis. A condensing appliance also recovers latent heat from the water vapour, which this method does not credit.

What values do I need to enter?

This calculator takes 4 inputs: Stack temperature, Combustion air temperature, Flue oxygen, Fuel. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

Related Calculators

Combustion Efficiency Calculator | Calcrivo