A gas furnace usually lasts 15 to 20 years. Well-maintained units in mild climates reach 25; units that run hard with dirty filters fail sooner.
A furnace is a burner, a heat exchanger, a blower and a set of controls. The heat exchanger is the part that decides when the furnace is done: it is a metal chamber that heats and cools thousands of times a season, and when it cracks, combustion gases can leak into the house air. A cracked heat exchanger on a furnace past its mid-teens is almost always a replace, not a repair.
Blower motors, igniters, flame sensors, pressure switches and control boards fail more often but are ordinary repairs. The question with each repair is the age of the unit around it. Industry guides and manufacturers put the typical life at 15 to 20 years, with 25 at the top end.
Filter habits set the pace. A clogged filter starves the blower and overheats the heat exchanger, which is the fastest way to shorten a furnace's life. High-efficiency condensing furnaces (90%+ AFUE) also have a secondary heat exchanger and a condensate drain that add two more things that can fail.
Find the serial number and decode it: Rheem · Ruud · Goodman · Amana (heating and cooling) · Daikin · Carrier · Bryant · Payne · Day & Night · Heil / Tempstar / Comfortmaker / Arcoaire / KeepRite · Lennox · Armstrong Air / AirEase / Concord / Allied · Ducane · Aire-Flo · Trane · American Standard (heating and cooling) · York · Luxaire · Coleman / Evcon · Fraser-Johnston / Champion / Guardian · WeatherKing · Nordyne / Intertherm / Miller / Maytag / Frigidaire / Tappan / Westinghouse / Kelvinator / Gibson / Broan · Comfort-Aire / Heat Controller / Century
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