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.
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