Should You Repair or Replace Your Furnace?
Replace a furnace if the heat exchanger is cracked, it is 15+ with a major fault, or the repair tops half a new unit. Under ~12 years, most faults are a cheap part.
Repair, replace, or get a second opinion
Repair it if…
The furnace is under about 12 years old, the heat exchanger is sound, and the fault is a common part — an igniter ($150–$300), a flame sensor ($75–$250), a capacitor ($75–$200), or a blower motor ($400–$700). It is the first repair, and it is a cheap one.
Replace it if…
The heat exchanger is cracked (always — carbon monoxide), the furnace is 15+ with any significant fault, the repair quote tops half a new furnace (roughly $2,000+), or age × repair cost is over $5,000. Two repairs in two years counts as a fault.
Get a second opinion if…
One contractor says "cracked heat exchanger, must replace" and you have no CO alarm going off — a crack is a real safety call but also the easiest way to upsell a $6,000 job. Ask to see it, and get a second technician to confirm before you commit.
Repair a furnace if it is under about 12 years old and the fault is a common part. Replace it if the heat exchanger is cracked, it is 15+ with a significant fault, or the repair quote is more than half the price of a new furnace — roughly $2,000. A cracked heat exchanger is a replacement at any age, because it can leak carbon monoxide.
This is for a furnace that has actually failed or is failing. If yours is just old and you are planning ahead, how long a furnace lasts covers the timeline.
The signs that mean replace, not repair
Any one of these, and you are replacing regardless of the repair quote:
- A cracked heat exchanger. It vents combustion gas — including carbon monoxide — into the air the blower pushes through the house. It is not safely repaired. Shut the furnace off.
- The furnace is 15 or more years old and facing anything beyond a $200 igniter or sensor.
- A second failure within two years. The first repair bought you time; the second is the unit telling you it is done.
- Rooms that will not heat evenly no matter how you set the thermostat, on an older furnace — usually a tired blower and a heat exchanger that has lost capacity.
- The repair costs more than half a new furnace. Half of $4,800 is $2,400; a quote near or above that on an old unit is money thrown after a lost cause.
Common furnace repairs and what they cost
On a furnace under ~12 years, these are the usual faults, and all of them are worth fixing:
| Part | Cost (installed) | What it does |
|---|---|---|
| Flame sensor | $75–$250 | Tells the board the burner lit; a dirty one shuts the furnace down after seconds |
| Hot-surface igniter | $150–$300 | Lights the burner; a brittle part that fails on a cold morning |
| Run capacitor | $75–$200 | Starts the blower or inducer motor |
| Inducer motor | $250–$500 | Pulls exhaust through the heat exchanger before the burner lights |
| Blower motor | $400–$700 | Moves the heated air; the priciest common repair |
| Control board | $300–$600 | The furnace's brain; fails from heat, age, or a power surge |
| Gas valve | $300–$600 | Meters gas to the burner |
The cracked heat exchanger — always replace
The heat exchanger is the metal chamber the burner fires into; house air passes over the outside of it and picks up heat without touching the combustion gas. When it cracks — from years of expanding and contracting, or from overheating behind a clogged filter — that separation is gone, and exhaust including carbon monoxide mixes into the air going to your rooms.
You cannot patch it, and swapping just the exchanger costs nearly as much as a new furnace once labor is in. If a technician says yours is cracked: ask to see it or see the camera footage, get a CO alarm on that floor immediately, and get a second opinion before authorizing a replacement — a crack is both a genuine safety issue and the single most common way a marginal furnace gets turned into a $6,000 sale.
The math: age, repair cost, and the $5,000 rule
A quick test: multiply the furnace's age by the repair quote. Over $5,000, replace it.
- 10-year-old furnace, $400 repair → $4,000 → repair
- 14-year-old furnace, $400 repair → $5,600 → replace
- 8-year-old furnace, $650 blower motor → $5,200 → borderline; repair if the rest of the furnace is clean
It is a rule of thumb, not a law — but it tracks the point where you are pouring money into a unit that owes you nothing, and where that same money is a down payment on 20 years of a furnace that is 15 points more efficient. When the answer is replace, what a new furnace costs is the next read, and if you do not know the age, decode the serial or read the date off the rating plate.
Frequently Asked Questions
Is it worth repairing a furnace?
Under about 12 years old, yes — most furnace faults are a $75 to $700 part and the unit has years left. Past 15, or if the repair is more than half the price of a new furnace (roughly $2,000), replace it. A cracked heat exchanger is always a replacement, at any age, because it can vent carbon monoxide.
What is the most expensive part to repair on a furnace?
The heat exchanger — but it is almost never repaired, because a cracked one is a safety failure and the labor to swap it approaches the cost of a new furnace. After that, the blower motor ($400 to $700) and the control board ($300 to $600) are the priciest common repairs.
How do I know if my heat exchanger is cracked?
Signs include a yellow or flickering burner flame, soot around the furnace, a strong chemical or formaldehyde smell when it runs, and a CO alarm that trips. A technician confirms it with a visual inspection, a combustion analyzer, or a camera. If it is cracked, shut the furnace off and do not run it until it is replaced.
What is the $5,000 rule for furnace replacement?
Multiply the age of the furnace by the repair quote. If the result is over $5,000, replace it. A $400 repair on a 10-year-old furnace is $4,000 — repair it. The same $400 repair on a 14-year-old furnace is $5,600 — replace it. It is a rough guide, not a law, but it tracks the point where you are spending real money on a unit that owes you nothing.
Sources
- Carrier — Furnace Repair vs Replacement: Costs & Decision Guide — replace if the repair exceeds ~50% of a new furnace, if the unit is near the end of its 15–20 year life, or if the heat exchanger is cracked; the "age × repair cost > $5,000" rule of thumb
- U.S. Consumer Product Safety Commission — carbon monoxide / heating systems — a cracked furnace heat exchanger can leak carbon monoxide into the home; CO alarms and annual inspection of fuel-burning appliances