The short answer
Replace a furnace when it's unsafe (cracked heat exchanger, red tag), when a major repair is needed on a unit over about 15 years old, or when repeated repairs and high bills cost more than a new system would save.
A furnace that's 18 years old and needs an ignitor doesn't need replacing. A 12-year-old furnace with a cracked heat exchanger probably does.
Repair-or-replace scorecard
| Factor | Leans repair | Leans replace |
|---|---|---|
| Age | Under 12–15 years | Over 15–20 years |
| The repair | Minor or moderate part | Heat exchanger, or multiple major parts |
| Safety | No issues | CO concerns or a red tag |
| Repair history | Rare | Repairs in two or more recent winters |
| Efficiency | Already 90%+ | Old 60–80% unit in a cold climate |
| Comfort | Even heat | Chronic cold rooms and noise |
The heat exchanger is the deciding part
The heat exchanger keeps combustion gases separate from the air you breathe. Cracks from age and overheating can let carbon monoxide in. Replacing a heat exchanger is possible, but the labor often makes it closer in cost to a new furnace, especially out of warranty.
If a technician reports a crack, ask to see photos or a camera inspection.
Eight signs it's time
- Furnace is 15–20+ years old and needs a major part
- Cracked heat exchanger or red tag
- Repairs in two or more recent winters
- Rising bills without a change in use
- Yellow, flickering flames or soot
- Parts discontinued for your model
- Loud operation despite repairs
- Uneven heating that duct fixes didn't solve
Planning the replacement
- Get the failing furnace's model and age.
- Ask for a Manual J load calculation (why it matters).
- Compare at least two efficiency levels.
- Check venting and duct changes needed.
- Review warranties and registration requirements.
- Schedule, with a permit and inspection where required.
Don't wait for the coldest night
Replacing a failing furnace in fall gives you time to compare quotes. Waiting until it dies in January means choosing under pressure, possibly with an emergency replacement.
Replacement FAQs
Is the 50% rule useful?
A common rule of thumb says that if a repair costs more than about half the price of a new furnace, and the furnace is past mid-life, replacement is usually the better value.
Can I replace only the furnace and keep my AC?
Often yes, if the AC is newer and its coil is compatible with the new furnace's cabinet and airflow.
Next steps
Ready to get it fixed? Call (888) 555-0146 to reach a licensed provider in your area. Related guides:
Busy winter markets: furnace installation in Michigan · furnace installation in Wisconsin · furnace installation in Ohio · furnace installation in Colorado · furnace installation in Utah · furnace installation in Iowa · all states.