A heat exchanger is the metal barrier between the combustion gases inside your furnace and the air that heats your home. When it cracks — and in older furnaces, it's almost always when, not if — the combustion gases mix with your indoor air. That includes carbon monoxide, which kills about 400 Americans a year and sends 50,000 more to emergency rooms.
The crack is usually invisible to the naked eye, and the soot patterns that give it away are hidden inside the burner box. We find it with combustion analyzer readings: measuring O₂, CO, and stack temperature at the flue, and comparing them against manufacturer specs. A healthy 95%-efficient furnace reads 2-5 ppm CO in the flue; a cracked heat exchanger pushes that into the hundreds, and those gases are leaking into your home's air handler.
Heat exchangers crack from thermal cycling stress — expansion and contraction every time the furnace fires. Fifteen to twenty years of cycling is the typical failure point. If your furnace is in that age range, a combustion analysis is the only way to know for sure. We include it in every fall tune-up, and the replacement-vs-repair math tilts decisively toward replacement once a crack is confirmed.
A healthy 95%-efficient furnace reads 2-5 ppm CO. A cracked exchanger pushes that into the hundreds.
If your furnace is under 10 years old and still shows warranty coverage, a cracked exchanger is often a free part swap (we handle the paperwork). Over 10 years, the manufacturer warranty is usually expired and the repair cost can exceed 50% of a new furnace — at which point replacement is the right call. Either way, running a furnace with a confirmed CO leak is not an option, and we will not re-light a unit that fails combustion analysis.
