AFUE

The term AFUE, or annual fuel utilization efficiency, is a standardized measure of efficiency for furnaces and boilers.

AFUE percentage is calculated by dividing the annual heat output of a furnace or boiler by the total annual fossil fuel energy it consumes. So, if you have a furnace with a 90% AFUE rating, that means 90% of the energy from the fuel becomes heat in your home and the other 10% escapes up the vent and elsewhere. Having a system with a high AFUE means you stay cozy and so does your wallet.