How the Department of Energy Measures the Efficiency of Furnaces and Boilers
The Department of Energy says that a central furnace or boiler's efficiency is measured by annual fuel utilization efficiency (AFUE). AFUE is a measure of how efficient the appliance is in using fossil fuel (gas or oil) or electricity (for an electric furnace) over a typical year of use.
An all-electric furnace or boiler has no flue loss through a chimney. The AFUE rating for an all-electric furnace or boiler is between 95% and 100% The lower values are for units installed outdoors because they have greater heat loss through the jacket.
The efficiency of manufactured furnaces is governed by the National Appliance Energy Conservation Act of 1987 and regulated by the U.S. Department of Energy. The minimum allowed AFUE rating for a non-condensing, fossil-fueled, warm-air furnace is 78%. The rating for a fossil-fueled boiler is 80%. The rating for a gas-fueled steam boiler is 75%.
A condensing furnace or boiler condenses the water vapor produced in the combustion process and captures the heat which is released from this condensation. The AFUE rating for a condensing unit can be much higher (by more than 10 percentage points) than a non-condensing furnace. Although a condensing unit costs more than a non-condensing unit, the condensing unit can save you money in fuel costs over the 15- to 20-year life of the unit.