A cracked heat exchanger, explained

What the heat exchanger does

Inside a gas furnace, two air streams must never meet. Burners fire inside a sealed metal chamber, and those combustion gases travel through the heat exchanger and out the flue. Separately, your blower pushes household air across the outside of that same metal. Heat passes through the wall; the gases do not.

That metal wall is the only thing between your home's air and the exhaust. If it cracks, combustion products — including carbon monoxide — can be drawn into the air stream and distributed through every room by the blower. That is why this diagnosis ends with the furnace being shut down and tagged rather than scheduled for next week.

Why they crack

Metal expands when heated and contracts when it cools. A heat exchanger does that every cycle for fifteen or twenty years, and eventually metal fatigue wins. That is normal end-of-life. What shortens it:

• Restricted airflow. The single biggest factor. A clogged filter, closed registers or undersized ductwork means less air removing heat, so the exchanger runs far hotter than design. This is why a neglected filter is not a trivial matter.
• An oversized furnace. Oversizing causes short cycling, which multiplies the number of heat-and-cool cycles the metal endures.
• Improper combustion setup, particularly at altitude, where gas appliances must be derated and adjusted. A furnace set up at sea-level assumptions overfires here.
• Condensation in a mid-efficiency furnace that was not designed for it, which corrodes from inside.
• Age. Past fifteen to twenty years, this is a normal failure mode rather than a surprising one.

Signs worth noticing

There is no symptom that confirms a crack from the living room, but these justify an inspection:

• A CO alarm that has sounded, or shows a non-zero reading
• Flames that flutter, change shape or roll out when the blower starts — a classic indicator, since blower air is reaching the combustion chamber
• Burner flames that are yellow or orange rather than crisp blue
• Soot or rust flakes inside the furnace cabinet or around the burners
• An acrid or chemical smell during operation
• Household members with flu-like symptoms that ease when they leave the house
• Unusual moisture or condensation on windows during furnace operation

If a CO alarm has sounded: everyone outside, call from outside, and do not operate the furnace again until it has been inspected. Our carbon monoxide article covers that sequence.

How a real diagnosis is made

A technician should be doing some combination of:

• Visual inspection with the burners and often the blower assembly removed — many cracks are only visible with real access.
• Camera inspection. A borescope reaches the parts of the exchanger nobody can see directly. This is the most common way a crack is genuinely documented, and it produces an image.
• Combustion analysis. A meter in the flue measuring CO, oxygen and efficiency. Readings that shift when the blower engages are meaningful evidence.
• Ambient CO measurement in the supply air stream.
• Flame observation as the blower starts.

You are entitled to see the evidence. Ask to be shown the camera image or the combustion readings. A technician who has genuinely found a crack will usually be glad to show you — it is the whole basis of the recommendation. A diagnosis delivered with no visual, no numbers and immediate pressure to sign for a replacement that day is a reason to get a second opinion. Red-tagging is also a specific act: the furnace is shut down and tagged, and the gas utility may be notified. If someone says "cracked heat exchanger" but leaves the furnace running, those two things do not fit together.

Equally — and this matters — if two independent technicians show you a crack on camera, it is real. Seeking a third opinion in the hope of a different answer is not caution at that point.

Why it cannot be patched

Welding or sealing a heat exchanger is not an accepted repair. The metal is thin, it is already fatigued, it operates at high temperature through constant thermal cycling, and a crack visible in one place generally indicates stress throughout the component. No reputable technician will patch one, and no manufacturer sanctions it.

Replace the exchanger or the furnace?

If the furnace is relatively young and the exchanger is covered by warranty, replacing the component can make sense — though the warranty typically covers the part and not the substantial labour, since getting to it means largely disassembling the furnace. If the furnace is in the second half of its life, putting significant labour into one component of an aging appliance rarely pencils out, and replacement is usually the better value.

Whatever is decided, insist that the replacement is properly sized with a load calculation and set up for altitude — because an oversized, overfired furnace is one of the reasons heat exchangers crack early in the first place. See heating replacement, furnaces, and our furnace buying guide. Annual heating maintenance with combustion analysis is what catches this kind of thing early.

Frequently asked questions