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 %
- 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)
where
μt = thermal efficiency
Eo = 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)
where
AFUE = annual fuel utilization efficiency
Eao = annual 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)
where
SSE = steady-state efficiency
Ei = fuel energy input (W, Btu/h)
El = energy loss (W, Btu/h)