When should you replace a furnace?

This guide focuses on whether the furnace is reaching the end of its useful life, not on the immediate choice between one repair and a new system. Age, declining efficiency, repeated breakdowns, poor parts availability, worsening reliability, comfort changes, corrosion, and heat-exchanger or venting concerns all deserve attention. Age alone is not a verdict: a newer unsafe furnace needs prompt diagnosis, while an older system that runs safely may still be serviceable. For a decision about paying for a specific repair versus buying a new furnace, see the furnace repair vs replacement guide. When replacement is approaching, compare a properly sized installation quote covering equipment, venting, airflow, thermostat, permits, disposal, and warranty.