A furnace is a forced-air heating system that burns fuel (natural gas, propane, or oil) or uses electricity to heat air, then distributes it through ductwork using a blower. Gas furnaces are the most common heating system in the United States.
A gas furnace consists of a heat exchanger (where combustion gases heat the air without mixing with it), burner assembly, draft inducer, heat exchanger, blower, and controls. High-efficiency condensing furnaces add a secondary heat exchanger that extracts additional heat from exhaust gases, achieving 95-98% AFUE.
Furnace maintenance includes annual inspection of the heat exchanger (a cracked heat exchanger can allow carbon monoxide to enter living spaces), burner cleaning, filter replacement every 1-3 months, and checking the flue system. Average furnace lifespan is 15-20 years.