How is the efficiency of a furnace calculated?

The efficiency of a furnace is based on the total amount of fuel energy being used by the unit, relative to the amount of generated heat that actually stays in the home. There are two general methods of calculating efficiency: steady state efficiency and seasonal efficiency, or Annual Fuel Utilization Efficiency (AFUE).

Steady State Efficiency

Steady state efficiency is the efficiency of the unit when it is running at its peak operating temperature. It is calculated by comparing the input heating capacity of a furnace (i.e. the amount of actual heat energy generated by the burning flames) to the output capacity, which is the heat energy that is actually harnessed to heat the home. The output capacity of a furnace is less than the input capacity and takes into consideration the amount of heat generated by the burners that is vented up the chimney with the exhaust gases, as well as the interior house air that is used for combustion of the gas.

Seasonal Efficiency

The seasonal efficiency is the efficiency of the unit determined over the course of the whole heating season, taking into consideration off-cycle losses. These off-cycle losses include the warm house air that is lost through the chimney when the furnace is not in operation and the heat that is produced from the pilot light that is continuously burning.

For example, a furnace with a steady state efficiency rating of 80% indicates that for every dollar spent on fuel, 80 cents is being used to heat the home when the system has reached its peak temperature. Unfortunately this “steady state efficiency” occurs infrequently during the heating season, after the furnace has been in operating for a period of time. This same furnace may have a seasonal efficiency of 60%, due to the off-cycle losses. Therefore, for every dollar spent for gas, realistically only 60 cents is used to heat the home and 40 cents is being wasted up the chimney.

High-efficiency furnaces can have a typical minimum AFUE of 90% and maximum AFUE of approximately 97%, while mid-efficiency furnaces have an AFUE of 80-82%. Conventional furnaces can have an AFUE as low as 55%.


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