Replace it if the heat exchanger is cracked, if it is past 15 to 20 years and needs a major part, or if a repair would cost more than a third of a new furnace. Replace a working furnace early only if the efficiency gain is large and you will stay long enough to collect it.
If the repair costs more than a third of a new installed furnace and the unit is past 12 to 15 years, replace. Under that, repair. Add the fuel bill to the math: a 1990s furnace at 78% to 80% AFUE replaced by a 95%+ unit saves about 15 to 18 cents on every heating dollar. In a cold climate that can be several hundred dollars a year; in a mild one it is not enough to justify replacing a working furnace on its own.
Size matters more than brand. A new furnace should be sized from a load calculation, not matched to the old nameplate, because insulation and windows have usually changed since the house was built. A high-efficiency furnace vents through PVC pipe to the side of the house and needs a condensate drain; that is normal. If you also have an aging central AC on the same blower, replacing both at once is cheaper than two separate jobs.
Any gas furnace past 15 years should have a working CO alarm on each level of the house. If an alarm sounds, leave and call the gas utility or fire department. Do not wait to decide about replacement.
The decoder on the home page reads the build date out of the serial number for 53 brands.
How long does a furnace last? · How long does a heat pump last?
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The age you just looked up is the kind of fact that gets lost. A written record of serial numbers, install dates, repairs and who did the work saves the next owner, inspector or insurer from guessing.
A lifetime logbook for the house, organized by system, with room for serial numbers, install dates, repairs and contractors. It stays with the house for the next owner.
Galvanized, polybutylene, cast iron and the sewer line: what fails, what it costs, what insurance won't cover and how to decide.
A buyer's guide to older homes: wiring, pipes, lead, asbestos, insurance refusals and what the repairs actually cost.
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