Cracked heat exchanger: CO steps first, fix $1,000 to $3,000

National figures. The three guides differ because a heat exchanger swap is labor-heavy and the part is often under warranty on a newer furnace.

Do this now: if you suspect a cracked heat exchanger

1. CO alarm sounding, or anyone with a headache, dizziness, nausea or confusion? Everyone outside, then 911 from outside. CPSC: do not search for the source, do not go back in until responders say so.
2. Smell gas? Same thing: leave without touching switches, then call 911 or the gas utility (Kansas Corporation Commission).
3. Otherwise, shut the furnace off. Lennox says to shut it off to stop exposure to combustion gases, open doors and windows, and call for a combustion analysis or visual inspection. Use space heaters with care until it is checked.
4. Do not run it to "see if it is really cracked." The exchanger is the wall between the flame and the air you breathe.
5. Ask to see the crack. Trane says to ask to see the cracked unit before agreeing to replace it. A second company's inspection costs $80 to $100 in HomeAdvisor's figures.

Is it an emergency? Cracked heat exchanger and carbon monoxide

A cracked heat exchanger is an emergency the moment carbon monoxide shows up, and the CDC counts more than 100,000 emergency room visits a year for CO poisoning; with no alarm and no symptoms it is a stop-using-it-and-call situation.

• 911 now: the CO alarm is sounding or anyone has CO symptoms. The CDC lists headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion.
• Furnace off, call today: a metallic smell, soot, yellow flames, or a technician's finding. Lennox says shut it off.
• No CO alarm in the house? The CDC says to install battery-operated or battery-backup detectors near every sleeping area; CPSC says one on every level. Replace them about every 5 years (CDC).
• What happens when the alarm goes off, step by step: carbon monoxide alarm going off.

Cracked heat exchanger symptoms

Lennox and Trane list eight signs between them, and the first two mean leave the house, not call a company:

Lennox (Nov 2025), Trane (2026), CDC, CPSC and ConsumerAffairs (Jul 2026).

Lennox adds that short cycles and uneven temperatures can come from a crack, and Trane lists short cycling first. Our furnace short cycling page covers the more common causes of that symptom, which is why a technician checks the exchanger when short cycling will not stop.

What causes a heat exchanger to crack

Trane gives four main causes: years of wear, overheating, an oversized furnace and rust or corrosion; Lennox adds poor duct design and moisture.

• Wear. Lennox says the metal expands and contracts with every heating cycle and fatigues over years. Trane says a heat exchanger should last 10 to 15 years or more with good maintenance.
• Overheating. Restricted airflow from dirty filters makes the furnace run hot (Lennox, Trane). Carrier's short cycling page says the limit switch exists to protect the exchanger from exactly this.
• Oversized furnace. It short cycles, which Trane links to cracking. This one was built in on day one.
• Corrosion. Moisture, including condensate on high-efficiency models, weakens the metal (Lennox).

Heat exchanger replacement cost

Heat exchanger replacement costs $1,000 to $3,000 in HomeGuide's 2025 furnace guide and in Trane's own owner page, while HomeAdvisor's June 2026 table lists the heat exchanger at $100 to $1,500. Trane says the price depends on the furnace model, exchanger type, location and labor rates.

• Why the gap. Trane says the price depends on the furnace model, the exchanger type, location and labor rates, and HomeGuide says most technicians replace rather than repair because of the CO risk; a quote at the low end is usually a warranty claim with labor only, which is the next question to ask.
• Repair is off the table. Lennox says repairs are "rarely recommended" because the exchanger is a sealed component; Trane says it would not recommend trying to repair one.
• Ask about the warranty before paying. Have the company check the serial number against the manufacturer's warranty, and confirm in writing what it covers and what the labor charge is.

Replace the heat exchanger or the furnace?

A new furnace costs $2,823 to $6,897 in HomeAdvisor's 2026 data, $4,814 on average, against $1,000 to $3,000 for the exchanger alone, so HomeAdvisor's rule applies: if the repair is more than half the price of a new furnace, start shopping.

• Under warranty: replace the exchanger and have the company file the claim; ask what labor you would owe.
• 15 years or older: Lennox says to consider replacing a furnace over 15 years old, and HomeGuide says replacement is usually best for units needing frequent repairs or older than 20 years. A $3,000 exchanger in a $5,000 furnace is more than half.
• Ask to see it. Trane's advice: ask to see the cracked unit before agreeing to replace it, in person or on the technician's camera.
• Get a second inspection when the finding comes from a tune-up that turned into a sales call. $80 to $100 (HomeAdvisor) is cheap next to $4,814.
• Full replacement prices by fuel are on furnace replacement cost.

Preventing a cracked heat exchanger

Annual service is the prevention every source agrees on: CPSC says to schedule a yearly professional inspection of fuel-burning heating systems, the CDC says the same, and Lennox recommends an annual tune-up, which HomeGuide prices at $70 to $200 with a CO test and heat exchanger check included.

• Change the filter every 1 to 3 months (Trane) so the furnace does not overheat.
• Keep supply registers and return vents open (Carrier); closed vents raise the heat in the exchanger.
• Put CO alarms on every level and near sleeping areas (CPSC, CDC) and replace them about every 5 years (CDC).
• Book the tune-up before the heating season; see furnace tune-up.

Related: furnace repair, gas leak, how long a furnace lasts and HVAC repair. Calls go to an independent heating and cooling company that serves your area and takes this kind of job; hours vary. How calls work.