· by Michael Blair, Nashville's HVAC
The first cool nights of the season are when gas furnaces get their fall tune-up, and every year some of those visits end with the same news: the heat exchanger is cracked, the furnace is unsafe, and it needs replacing now. That can be exactly right. A breached heat exchanger can let combustion gases, including carbon monoxide, into the air the blower sends through the house. It is also a diagnosis that can be hard for a homeowner to check, which is why it is worth understanding what the part does and what real evidence of a crack looks like.
What the heat exchanger does
Inside a gas furnace, the burners fire into a set of sealed metal chambers. The flame and its exhaust stay inside those chambers and leave through the flue; the house air is blown across the outside of the metal and picks up the heat without ever touching the combustion. The heat exchanger is the wall between the two, and its only job is to keep them apart.
Every time the furnace runs, that metal heats up and expands, then cools and contracts when it shuts off. Over many seasons of that cycling, stress concentrates at bends, welds and crimped seams, and that is where cracks start. A condensing furnace also has a secondary heat exchanger that handles acidic condensate, and it can corrode from the inside if the drain system has been backing up.
What makes one fail early
Heat is the enemy, and too little airflow is the usual source of too much heat. A clogged filter, closed or blocked supply registers, undersized return ductwork or a blower running at the wrong speed all let the metal run hotter than it was designed to, and the high-limit switch cycling the burner off repeatedly is a sign that is happening. A furnace that is much too big for the house short cycles, which piles on more of those expansion and contraction swings than a correctly sized one would ever see.
Age matters too, though there is no fixed number of years at which a heat exchanger fails. Plenty of furnaces from the same era as yours are still sound. What a technician should be looking for is physical evidence on your unit, not a birthday.
What real evidence looks like
A crack you can see is the clearest proof. With an inspection camera fed into the burner openings or the flue path, a technician can usually photograph it, and you should ask to see that image or video before agreeing to anything. Rust through, visible holes, or scorched and discolored areas outside the burner openings are also physical signs worth documenting.
Indirect signs support the diagnosis without proving it on their own: a flame that wavers, lifts or changes color the moment the blower starts, rising carbon monoxide readings in the supply air, soot at the burner openings, or a combustion analysis that shifts when the blower comes on. A single CO reading taken without the context of how the furnace was running is not proof. Several signs pointing the same way, or a photograph, is.
- Ask for a photo or video of the crack itself
- Ask which test was used and what the readings were
- Get the model and serial number written on the report
- Get a second opinion before paying for a replacement
What to do if the furnace is tagged
A technician who finds a genuine breach will shut the furnace off and may tag it, and the gas utility may do the same. Do not restart a furnace that has been tagged unsafe. Have working carbon monoxide alarms on every sleeping level while you sort it out, and use electric space heaters with care rather than an oven or an unvented heater to get through a cold night.
Then check the paperwork. Heat exchangers often carry a longer manufacturer warranty than the rest of the furnace, sometimes for the life of the unit when it was registered, which can make replacing the part itself worth considering on a newer furnace even though the labor is not covered. On an older unit, a replacement usually makes more sense. When our crews find a crack, we photograph it and leave the report with the model and serial number on it, so the next technician you call can confirm it or tell you otherwise.