What does the 80% AFUE rating mean for the GM9S80?
The 80% AFUE (Annual Fuel Utilization Efficiency) rating means that 80 cents of every energy dollar you spend warms your home. This rating system was established by the Department of Energy (DOE) to calculate the annual operating cost of residential gas furnaces.
From the official Goodman GM9S80 Gas Furnace manual, page 5
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