Maintaining a gas boiler.

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Environmental Impact

Energy Efficiency

Energy efficiency in furnaces is measured by AFUE—Annual Fuel Utilization Efficiency—and manufacturers are required to display the AFUE score on the units they sell. That number tells you how much of the fuel that the furnace consumes actually gets turned into heat for your house. An old furnace might send a quarter of the heat it produces up the chimney. But a modern, high-efficiency furnace can be have an AFUE of 96 percent or even higher, meaning hardly any heat is wasted.

The U.S. Department of Energy (DOE) warns, however, that AFUE doesn't calculate heat lost through the duct system and piping of your home, which can be as much as 35 percent when ducts are located in the attic.

Still, DOE says that upgrading the furnace or boiler from 56 percent to 90 percent efficiency in an average cold-climate house will save 1.5 tons of carbon dioxide emissions each year if the house is heated with gas, or 2.5 tons if it’s heated with oil. It’ll also cut the energy bill by 38 percent.

DOE’s Energy Star program certifies furnaces with AFUEs of 85 percent and higher. The AFUE rating for an all-electric furnace or boiler is between 95 and 100 percent, and condensing furnaces are rated at or above 90 percent. Note that heat pumps aren’t rated using AFUE, but with two other systems: seasonal energy efficiency ratio (SEER), and heating system performance factor (HSPF), which measure cooling and heating output in btus divided by energy input in watt-hours.

The DOE recommends replacing old coal burners that have been switched over to oil or gas, as well as gas furnaces with pilot lights rather than electronic ignitions.

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