Combustion Processes and Combustion Efficiency

Typical furnace combustion efficiencies - fireplaces, space heater, boiler and more

Combustion is the oxidation of carbon-containing material in the presence of sufficient oxygen to complete the process.

C + O₂ > CO₂

Typical combustion process efficiencies can be summarized as

• 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

μ_t = 100 E_o / E_i (1)

where

μ_t = thermal efficiency

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

E_i = 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 E_ao / E_ai (2)

where

AFUE = annual fuel utilization efficiency

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

E_ai = 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 (E_i - E_l) / E_i (2)

where

SSE = steady-state efficiency

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

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

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