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A boiler heats your home by burning gas or fuel oil to heat water or steam that circulates through radiators, baseboards or radiant floor systems. Boilers do not use a duct system. The efficiency of a boiler is expressed by the Annual Fuel Utilization Efficiency (AFUE), a rating of how efficiently the system converts fuel to space heat in proportion to the amount of fuel entering the boiler. This is commonly expressed as a percentage. A rating of 90 AFUE means that approximately 90 percent of the fuel is utilized to provide warmth to a home, while the remaining 10 percent escapes as exhaust.

The current U.S. Federal minimum energy efficiency standard requires that new residential hot-water boilers have a minimum AFUE of 80%, and steam boilers a minimum AFUE of 75%. The AFUE is typically listed on the units yellow “Energy Guide” label.

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