When is it worth repairing a furnace instead of replacing?

Repair when the furnace is under about 12 years old, the fix is under $500, and it's the first major repair. A young furnace with a bad igniter, flame sensor, or blower motor is worth fixing.

What is the $5,000 rule for furnaces?

Multiply the repair cost by the furnace's age in years. If the result is over $5,000, replace. A $600 repair on a 10-year-old furnace is $6,000 — replace. The same repair on a 4-year-old unit is $2,400 — fix it.

At what age should a furnace be replaced?

Gas furnaces last 15–25 years. Past year 15, any repair over $1,000 usually favors replacement. Past year 20, replace on the next significant failure regardless of the quote.

What furnace problems mean replace immediately?

A cracked heat exchanger (carbon monoxide risk), repeated short-cycling on an old unit, or rising gas bills with declining heat. A cracked heat exchanger is a safety red line — replace, don't patch.

What does furnace replacement cost in 2026?

Gas furnace only: $3,000–$7,500. High-efficiency or hard installs run higher. A repair on a dying furnace only buys time; the replacement is coming either way.

Repair or replace your furnace?

A furnace almost never dies at a convenient time. It's a cold morning, there's no heat, and a technician is standing in your basement quoting a repair — while gently suggesting you might just replace the whole thing. Here's the framework to make that call before you're standing in the cold making it.

Start with the age test

Gas furnaces last 15–25 years. Where yours falls in that range changes the whole decision:

Under 12 years: lean toward repair. The unit has plenty of life left, and most failures at this age are single components — an igniter, flame sensor, or blower motor.
12–18 years: the gray zone. Repair small things; replace on anything major.
Over 18 years: lean toward replacement on the next significant failure, regardless of the quote. You're maintaining a unit that's already on borrowed time.

The $5,000 rule

A quick way to price the decision: multiply the repair cost by the furnace's age in years. If the result is over $5,000, replace.

A $600 repair on a 10-year-old furnace = $6,000 → replace.
The same $600 repair on a 4-year-old furnace = $2,400 → fix it.

The math captures something real: money spent on an old furnace buys you fewer remaining years than the same money spent on a young one.

The safety red lines — replace, don't patch

Some failures aren't a cost decision at all:

Cracked heat exchanger: this can leak carbon monoxide into your home. Replace the furnace — patching a cracked exchanger is not a safe repair.
Repeated short-cycling on an old unit: the furnace turns on and off rapidly and can't hold temperature — a sign of a system at the end of its life.
Rising gas bills with declining heat: efficiency is falling, and you're paying for it every month.

What replacement costs in 2026

A gas furnace runs $3,000–$7,500 installed, more for high-efficiency models or difficult installs. See our methodology for how we estimate cost bands. If the furnace is paired with aging AC, read what full HVAC replacement costs — replacing both together is often cheaper than two separate jobs, and code sometimes requires it.

The bottom line

Repair a young furnace with a cheap, isolated failure. Replace an old one facing a big or safety-related repair. The worst outcome is pouring $800 into a 19-year-old furnace that fails again next winter — a decision that's easy to avoid if you know the unit's age before the technician arrives.

Also worth reading: repair or replace your AC and how to budget for home maintenance.