high and low heating values of a fuel

Heating values express how much energy is released on combustion of a given quantity of fuel, for example, joules per kilogram, or Btu per gallon. A “high heating value” includes the heat that can be obtained by condensing the water vapor produced by combustion. A low heating value does not include this heat.

The internal combustion engine in an automobile exhausts the water vapor produced by combustion without condensing it. In this case, it is appropriate to use the low heating value of a fuel such as gasoline.

Most natural gas furnaces are noncondensing; the water vapor produced by combustion goes up the chimney.  But a few high efficiency condensing furnaces are marketed. For these it is appropriate to use the high heating value of the gas.

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