A furnace’s heat exchanger is the sealed metal chamber, or set of tubes, where the burner flame and its exhaust heat the metal. The blower pushes house air across the outside of it, while the combustion gases stay inside and leave through the vent pipe. A crack or hole breaks that separation. It can let combustion gases, including carbon monoxide (CO), reach the air you breathe, and air from the blower can push through the opening and disturb the flame. That is why a suspected crack is a safety problem first and a repair question second: the furnace stays off until it has been inspected.
If a CO alarm is sounding now
- Get everyone out of the house, pets included. Don’t stop to look for the cause.
- Call 911 from outside.
- Stay out until the responders say it is safe to go back in.
- Tell the dispatcher if anyone has a headache, dizziness, nausea or confusion. Those are symptoms of CO exposure.
- Leave the furnace off afterwards. Never restart a furnace with a suspected cracked heat exchanger, and never restart one that has been shut off and tagged as unsafe.
If you smell gas instead, the order is the same: leave first, then call the gas utility or 911 from outside.
Signs that point to the heat exchanger
| What you notice | What it may mean | What it takes |
|---|---|---|
| CO alarm sounds while the furnace runs | Combustion gases reaching the rooms: heat exchanger, vent or burner problem | Evacuate, then a full inspection with CO readings |
| Soot or dark streaks around the burner area | Incomplete combustion, or flame escaping the burners | Burner, vent and heat exchanger inspection |
| Yellow, lazy or flickering flame instead of steady blue | Poor combustion, or air entering through a breach | Combustion check and camera inspection |
| Flame shifts or wavers the moment the blower starts | Blower air reaching the flame through an opening | Camera inspection of the heat exchanger |
| A sharp, unusual odor when the furnace runs | Combustion byproducts in the airflow | Furnace off until inspected |
| Headaches or nausea at home that ease once you leave | Possible CO exposure | Leave, call 911, furnace off |
| Crack seen during a maintenance visit | A confirmed breach | New heat exchanger or new furnace |
Why heat exchangers crack
- Age and thermal stress. Every heating cycle expands the metal and every pause lets it shrink. Over many years the metal fatigues, usually at seams, bends and welds. This is normal wear, and it is one reason an inspection matters more once a furnace is past about 15 years old.
- Overheating from restricted airflow. A clogged filter, closed registers or a return that is too small let the metal run hotter than it was designed for. A furnace that keeps tripping its high-limit switch, as described on our furnace short cycling page, is overheating.
- Improper sizing. A furnace too large for the house cycles on and off more often, and one too large for its ducts runs hot.
- Poor maintenance history. Dirty burners and a flame no one has looked at in years add stress.
- Manufacturing defect. Occasionally a design is weak, which is when the warranty paperwork matters.
How a crack is confirmed
Most cracks can’t be seen from outside the cabinet, so a CO alarm that stays quiet does not prove the heat exchanger is sound. A technician looks inside with a camera or by removing the burners, watches the flame as the blower starts, and measures CO in the flue gas and at the supply registers. A crack is a serious finding and an expensive one, so you should see the evidence: an image of the crack and where it is, or the readings that point to it. We explain what we found and why. If you want a second opinion before you replace a furnace, that is a reasonable request.
Repair or replace
Patching or sealing a crack is not an accepted repair. The choice is a new heat exchanger or a new furnace. Heat exchangers often carry a longer manufacturer’s warranty than the rest of the furnace, so find the model number, serial number and install date before deciding. Even when the part is covered, the labor is substantial, and on an older furnace a furnace replacement usually makes more sense. Our repair-or-replace guide walks through how to weigh it.
What you can do, and what you shouldn’t
You can:
- Keep CO alarms on every floor and near the bedrooms, and test them.
- Look, without opening anything, for soot, scorch marks or rust flakes around the furnace and its vent pipe.
- Watch the burner flame through the viewing window if your furnace has one. Steady and mostly blue is normal.
- Keep the filter clean, and ask for a heat exchanger inspection at each furnace tune-up.
Don’t:
- Run the furnace “just to check” once a crack is suspected.
- Remove the burner cover or run the furnace with its panels off.
- Heat the house with a gas oven or range while you wait. Use electric space heaters, kept clear of curtains and bedding.
Local note
In much of Daly City the furnace sits in the ground-floor garage with bedrooms directly above it, and in many mid-century homes that furnace is decades old. Put CO alarms in the rooms over the garage, not only in a hallway at the far end of the house.