A well-installed, annually maintained furnace typically runs 15–20 years. A poorly sized one that short-cycles, or one that never sees a tune-up, can die in 10–12 — installation quality and maintenance are the whole difference.
Watch the trajectory, not just the age: rising gas bills at the same thermostat setting, a second repair in two winters, longer run times, and uneven rooms all say the same thing — efficiency is leaving before the furnace does.
Past year 15, plan the replacement on your schedule instead of the furnace's. A planned install in October beats an emergency one during a January cold snap in every possible way, including price.
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