Furnace Manuals & Owner Guides

What AFUE means

Annual Fuel Utilization Efficiency (AFUE) is the share of the fuel a furnace burns over a typical heating season that becomes heat in the home. A 96% AFUE furnace wastes about 4% of its fuel; a 30-year-old furnace may waste a third or more. The federal minimum for typical indoor gas furnaces is 80% AFUE.

ENERGY STAR sets separate requirements for the U.S. South and for northern states, where furnaces run longer and efficiency matters more. Each furnace page shows which regions the model is certified for.

Condensing furnaces, venting and condensate

Gas furnaces rated about 90% AFUE and above are condensing furnaces. They extract so much heat that water vapor in the exhaust condenses, so they vent through plastic pipe instead of a chimney and need a drain for the slightly acidic condensate. Correct vent slope, termination clearances and freeze protection for the drain are essential to reliable operation.

Sizing and staging

Load calculation

A Manual J calculation sizes the furnace to the home's actual heat loss. Most replacement furnaces can be smaller than the units they replace, especially after insulation or window upgrades.

Single-stage

Fires at full capacity every cycle. Lowest cost, shortest run times.

Two-stage

Runs on low fire most of the time, high fire on cold days. Quieter and more even.

Modulating

Adjusts the flame in small steps to match the load for the steadiest temperatures.

Furnace FAQ