AFUE (Annual Fuel Utilization Efficiency)
The percentage of a gas furnace's fuel that becomes indoor heat over a year — 80% for a standard furnace, 96% for a high-efficiency condensing model.
AFUE measures how much of the fuel a furnace burns actually turns into heat inside the house, expressed as a percentage. A standard 80% AFUE furnace sends the remaining fifth of its gas up the flue as exhaust; a 96% condensing furnace recovers nearly all of it by capturing heat that would otherwise escape, which is why it needs a sealed PVC flue and a condensate drain — real installed labor that shows up in its price.
Stepping from 80% to 96% AFUE costs about $2,300 more installed at the mid of the range ($5,200 to $7,500 on a standard-size furnace), and 96% is effectively the ceiling — a "premium" furnace tier buys a modulating burner for comfort and quiet, not meaningfully higher AFUE. The federal minimum is 80% AFUE nationwide.