Combustion Processes and Combustion Efficiency

Typical furnace combustion efficiencies in fireplaces, space heaters, boilers and more

Combustion is an exothermic reaction between a fuel and an oxidant.

Combustion of carbon and oxygen to carbon dioxide can be expressed as:

C + O2 > CO2

Combustion of methane and oxygen to carbon dioxide and water can be expressed as:

CH4 + 2 O2 > CO2 + 2 H2O

Combustion of hydrogen and oxygen to water can be expressed as:

H2 + 1/2 O2 > H2O

Typical combustion process efficiency can be summarized to

  • home fireplace: 10 - 40 %
  • space heater: 50 - 80 %
  • gas boiler: 70 - 80 %
  • residential gas furnace with low efficiency atmospheric burner: 70 - 80 %
  • oil burner heating system: 70 - 85 %
  • gas powered boiler: 75 - 85 %
  • high efficiency gas or oil condensing furnace: 85 - 95 % 

Furnace Performance Indices

The performance of a furnace is usually assessed by the following indices

Thermal Efficiency

μt = 100 Eo / Ei (1)


μt = thermal efficiency

Eo = fuel energy output (W, Btu/h)

Ei = fuel energy input (W, Btu/h)

Annual Fuel Utilization Efficiency - AFUE

AFUE measures the annual amount of heat actually delivered compared to the amount of fuel supplied to the furnace.

AFUE = 100 Eao / Eai (2)


AFUE = annual fuel utilization efficiency

Eao = annual fuel energy output (W, Btu/h)

Eai = annual fuel energy input (W, Btu/h)

Steady-state Efficiency - SSE

Steady State Efficiency measures how efficiently a furnace converts fuel to heat, once the furnace has warmed up and is running steadily and can be expressed as

SSE= 100 (Ei - El) / Ei (2)


SSE = steady-state efficiency

Ei = fuel energy input (W, Btu/h)

El = fuel energy loss (W, Btu/h)

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