When to Replace a Furnace
Replace a furnace right away if it has a cracked heat exchanger or a carbon monoxide problem. Otherwise, start planning once it's past about 15 years and needs a major repair. Here's how to decide.
By Suraj Jha · Updated 2026-10-26
Replace now if…
- A technician finds a cracked or corroded-through heat exchanger.
- Carbon monoxide alarms go off and a furnace problem is confirmed. Leave the house, call the gas utility or 911, and don’t use the furnace until it’s inspected.
- The furnace is red-tagged (shut off) by the utility or a technician as unsafe.
These are safety issues, not budget decisions.
Start planning if…
| Signal | Why it matters |
|---|---|
| Age over about 15 years | Most furnaces last 15–20 years; failures get more likely each year |
| A major repair quote | Inducer motor, control board, gas valve or blower motor on an old unit |
| Repairs in each of the last two winters | A pattern usually continues |
| Rising gas bills | Efficiency drops as burners, blowers and controls wear |
| Uneven heating or short cycling | Often a sizing or duct issue; a replacement is a chance to fix it |
| Yellow, lazy flames or soot | Combustion problems; have it checked promptly |
Repair or replace: two rules of thumb
The 50% rule. If a repair costs more than half the price of a new furnace, replace.
The $5,000 rule. Multiply the furnace’s age by the repair cost. Over about $5,000, replacing usually makes more sense. A $400 repair on a 10-year-old furnace (4,000) is worth doing; the same repair on a 16-year-old (6,400) is borderline.
Both rules are blunt. They ignore efficiency savings, how long you’ll stay in the home and whether the rest of the system (ducts, AC, coil) also needs work, but they’re a sensible starting point.
The efficiency argument
Furnace efficiency is measured as AFUE, the share of the fuel that becomes heat in the house:
| Furnace | Typical AFUE | Gas used vs a 96% furnace |
|---|---|---|
| Older standing-pilot furnace | 60–70% | about 40–60% more |
| Standard 80% furnace | 80% | about 20% more |
| Condensing furnace | 92–98% | baseline |
If you spend $1,200 a year on heating gas with an 80% furnace, a 96% model would cut that to around $1,000. That alone won’t pay for a furnace quickly, but it shifts the balance when you’re already facing a big repair.
What a replacement involves
- Load calculation: a contractor should check the size (Manual J) rather than copying the old one. Many older furnaces are oversized.
- Venting: condensing furnaces vent through PVC pipe and need a condensate drain. Switching from an 80% furnace can mean new venting and sometimes a liner for a water heater left alone in the old chimney.
- Permits and inspection: usually required for gas appliances.
- Matching the AC coil: the indoor coil sits on the furnace; if the AC is old, consider replacing both.
- Heat pump option: in many climates a heat pump, alone or paired with a gas furnace (dual fuel), is worth pricing as an alternative.
Making the old furnace last until you’re ready
Change the filter, keep the area around the furnace clear, make sure the condensate drain on a high-efficiency furnace is flowing, and book a tune-up in the fall. Fit carbon monoxide alarms on every level of the home whatever the furnace’s age.
Frequently asked questions
At what age should a furnace be replaced?
There's no fixed age, but most gas furnaces last 15 to 20 years, and ENERGY STAR suggests considering replacement past about 15 years. A furnace that still runs safely and efficiently doesn't need replacing just because of age.
Is a cracked heat exchanger always a reason to replace?
In practice, almost always on an older furnace. The heat exchanger separates combustion gases from the air you breathe; a crack can let carbon monoxide into the home, and replacing the part costs a large share of a new furnace.
How much can a new furnace save on gas bills?
Going from an old 80% furnace to a 96% condensing furnace cuts the gas used for heating by roughly a sixth (1 − 80/96 ≈ 17%). From an older 65–70% furnace, the saving is closer to a third.
Should I replace my AC at the same time as my furnace?
Often it's worth considering if the AC is also more than about 10 to 12 years old. The indoor coil sits on the furnace, and replacing both together ensures matched equipment and avoids paying for installation twice.
Next steps
Look up the age of your furnace and air conditioner
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