The Hidden Flaw in Some High-Efficiency Furnaces

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Hint: Furnaces use two forms of energy

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You can tell how energy-efficient a furnace is by its official efficiency rating, the Annual Fuel Utilization Efficiency(AFUE) Widely-used measure of the fuel efficiency of a heating system that accounts for start-up, cool-down, and other operating losses that occur during real-life operation. AFUE is always lower than combustion efficiency. Furnaces sold in the United States must have a minimum AFUE of 78%. High ratings indicate more efficient equipment.
(AFUEAnnual Fuel Utilization Efficiency. Widely-used measure of the fuel efficiency of a heating system that accounts for start-up, cool-down, and other operating losses that occur during real-life operation. AFUE is always lower than combustion efficiency. Furnaces sold in the United States must have a minimum AFUE of 78%. High ratings indicate more efficient equipment.
). It’s a measure of how much of the heat originally in the fuel that’s being burned is available for delivery to the home. The more heat that gets lost up the flue or through the cabinet, the lower the AFUE.

But that rating doesn’t capture all the ways a furnace can lose efficiency. Some, like how well the heat gets distributed to the house, aren’t related to the furnace itself. But there’s one big one that is related to the furnace.

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