A cracked heat exchanger, and why it ends the conversation
It is the one furnace fault with no repair option and no wait-and-see. What it is, how it is found, and how to know you are being told the truth.
Ryan DeRue · September 14, 2026 · 2 min read
Of everything that goes wrong with a furnace, this is the one that stops being a repair decision.
What it is
The heat exchanger is the metal wall between the burner flame and the air that goes into your house. Combustion gases, including carbon monoxide, stay on one side. Your air stays on the other. A crack means those two sides can mix.
Why it cannot be repaired
It is a sealed pressure component under constant heating and cooling. Welding it is not a sanctioned repair and no manufacturer will stand behind one. On an older furnace the part alone, where it is even available, usually costs a meaningful fraction of a new system, which is why replacement is nearly always the answer.
What causes it
Metal fatigue from years of expanding and contracting. It happens faster in furnaces that short cycle, so oversizing and restricted airflow both bring it forward. It is the natural end of a furnace's life, and finding it at twenty years is not a surprise.
How to know you are being told the truth
This is an expensive diagnosis, and a small number of contractors have abused it. A good one will:
- Show you. A camera down the exchanger, and you looking at the screen.
- Show you the combustion numbers. A combustion analyser in the supply air picking up what should not be there.
- Red tag it. If it is genuinely unsafe, a gas appliance gets shut off and tagged, which is a formal act with consequences for whoever does it. Somebody unwilling to do that while telling you it is deadly is worth a second opinion.
If all you get is "there is a crack in it" and a quote, get a second opinion. Any honest contractor will tell you the same.
What we do
Every gas appliance we touch gets combustion tested before we leave, on every visit, not only when something is suspected. That is how this gets found in October rather than in January.
Read it and still not sure?
Describe what the system is doing and we will tell you whether it is a five minute fix, a real repair, or a replacement conversation. That costs nothing.
Quick answers
Can I keep using the furnace until I replace it?
If a heat exchanger is confirmed cracked, no. Carbon monoxide gives no warning and it is the one fault worth being inflexible about. If you cannot replace immediately, the furnace stays off and we will help you work out heat in the meantime.
Would my carbon monoxide alarm catch it?
Not reliably. Household alarms are set to trigger at levels well above what causes harm over longer exposure, and a crack can leak intermittently, only when the metal is hot and expanded. An alarm is a last line of defence, not a test.