Cracked Heat Exchanger: Warning Signs, Risks and What to Do

Quick answer: A cracked heat exchanger can let combustion gases, including carbon monoxide, escape from the furnace, so treat it as a safety issue. Warning signs include soot, a yellow or wavering flame, a tripped rollout switch, unusual odors and CO alarms. If a CO alarm sounds, get everyone outside and call 911. Otherwise, turn the furnace off and book an inspection through furnace and heating repair. A real diagnosis includes evidence you can see, such as photos or camera footage. A confirmed crack usually means replacing the heat exchanger or the furnace.

This is one of the most serious things a technician can find, and also one of the most misunderstood. Here's how to recognize the warning signs and how to make sure a diagnosis is genuine.

What the heat exchanger does

Inside your furnace, burners heat a set of metal chambers. Hot combustion gases travel through the inside of those chambers and out the vent. Your home's air blows across the outside, picks up the heat and travels through your ducts. The metal wall between them is what keeps the combustion gases out of the air you breathe.

Why they crack

Metal expands when it heats and contracts when it cools. Over thousands of cycles, that stress can cause cracks. These conditions make it worse:

• Restricted airflow: clogged filters and closed vents make the metal run too hot
• Oversized furnace: frequent short cycles mean more stress
• Undersized ductwork
• Corrosion: from condensation or chemicals in the air, such as those from laundry products and paint stored nearby
• Age: gas furnaces commonly last about 15 to 20 years
• Poor installation or incorrect gas pressure

Changing your filter on schedule is one of the simplest ways to protect it. See the air filter guide.

Warning signs

No single sign proves a crack, and a crack can exist with no obvious signs. Take these seriously:

• Carbon monoxide alarm sounding
• Soot around the furnace or burners
• Flame changes: yellow, wavering or "dancing" when the blower starts
• Rollout switch tripping: flames are escaping where they shouldn't
• Water or rust around the base of the furnace
• Unusual odors: a sharp or exhaust-like smell
• Visible cracks seen during an inspection
• Headaches, dizziness or nausea that improve when you leave the house

See HVAC smells and what they mean and carbon monoxide safety.

What to do right away

If a CO alarm is sounding or anyone feels ill:

1. Get everyone, including pets, outside into fresh air
2. Call 911 from outside
3. Don't go back in until emergency responders say it's safe
4. Have the furnace inspected before using it again

If you only suspect a problem:

1. Turn the furnace off at the thermostat and service switch
2. Make sure your CO alarms work
3. Call for an inspection
4. Use safe ways to stay warm, and protect your pipes (see no heat in freezing weather)

If you smell gas, leave the house and call 911 or your gas utility from outside.

How a technician confirms it

A proper inspection goes beyond a quick look:

1. Check the flame and watch for changes when the blower starts
2. Inspect the heat exchanger visually, often with a camera
3. Run a combustion analysis
4. Measure carbon monoxide in the flue and in the air around the furnace
5. Check airflow, temperature rise and gas pressure
6. Check venting and the rollout and limit switches
7. Document what's found

Ask to see the evidence. Photos or video of the crack should be easy to provide. Some companies have been criticized for using this diagnosis as a sales tool, so a clear explanation is fair to expect. Equally, some cracks are hard to see, and a technician who finds real signs of trouble is right to take it seriously.

A second opinion

If you're told your heat exchanger is cracked and you're unsure, a second opinion is reasonable. Keep the furnace off in the meantime if the first technician found a real safety concern. Share any photos and readings with the second technician.

Your options

Labor can make a heat exchanger swap expensive, even when the part is covered. On an older furnace, a new unit is usually the better value. Check the warranty paperwork and whether the furnace was registered.

If you're replacing, see furnace replacement in New Jersey and ask about $500 off a new system (terms apply).

How to prevent it

• Yearly heating tune-up: includes safety checks and carbon monoxide testing
• Change filters on schedule
• Keep vents and returns open
• Size replacements correctly
• Store chemicals away from the furnace
• Fix short cycling early (see short cycling)

Protect your family

Install carbon monoxide alarms on every level and near bedrooms, test them monthly and replace them by the date on the label. Alarms are your safety net whether or not you've had an inspection, because carbon monoxide has no color or smell.

Older furnaces deserve a closer look

If your furnace is past the 15-year mark and hasn't been inspected in a while, this season is a good time. An $89 heating tune-up (terms apply) includes a safety inspection and gives you real information about what you have. See signs you need a new furnace.

What affects the cost

If you do need work, these factors shape the quote:

• Warranty status: a covered part still usually has labor costs
• Part availability, particularly on older furnaces
• Labor: the furnace often needs substantial disassembly
• New furnace details: size, efficiency, venting and ductwork

Ask for both prices in writing. Seeing them together usually makes the choice clear.

Common myths

• "I'd smell it." Carbon monoxide has no smell. Don't rely on your nose.
• "My CO alarm hasn't gone off, so the furnace is fine." An alarm is a safety net, not an inspection. Alarms also expire.
• "Every old furnace has a cracked heat exchanger." Not so. Age raises the risk, but plenty of older furnaces pass inspection.
• "I can just run it until spring." If a crack has been confirmed, that isn't a risk worth taking.

What to tell us when you call

These details help us respond correctly:

• Whether a CO alarm has sounded and what it read, if it has a display
• Any soot, odor or flame changes you've noticed
• Whether anyone in the home has felt unwell
• The furnace's age and when it was last serviced
• Any paperwork or photos from an earlier inspection

If anyone feels ill, skip the call to us and call 911 first.

Keep any earlier inspection reports with your furnace records, so the next technician can compare them with what they find.

Furnace inspection and repair near you

Our work is performed under NJ Master HVACR License 19HC00767800, by our own licensed team. We serve all 153 towns in Ocean, Monmouth, Burlington and Middlesex counties, including Manchester, Howell, Piscataway and Willingboro, and we show you what we find.

Concerned about your furnace? Book online or call (732) 779-7106. We answer 24/7.