AFUE or Annual Fuel Utilization Efficiency, is a rating applied to how efficient a combustion appliance uses it’s main fuel source. This applies to boilers, water heaters & furnaces that burn gas, propane, or oil. In short for every dollar you spend, that amount is actually going to heat your place. For example, a 78 AFUE essentially means 22 cents basically is going straight up the chimney as compared to 5 cents for a 95 AFUE. The AFUE accounts for start-up, cool-down, and other operating losses that occur during real-life operation.
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