How long does a furnace last?

Standard 80% AFUE gas furnace: 18–25 years. High-efficiency (95%+ AFUE) condensing gas furnace: 15–22 years (the secondary heat exchanger is more sensitive to condensate corrosion). Oil furnace: 20–30 years. Electric resistance furnace: 25–35 years (no combustion, fewer failure modes). The single biggest life-shortener is undersized return-air ducts, which run the blower hot and burn out the heat exchanger early. Annual professional service (combustion analysis, blower clean, filter management) extends furnace life 30–40%. Replace at year 18–20 if you've had two service calls in the last 2 years.