If a carbon monoxide alarm sounds, get everyone out into fresh air and call 911. Do not wait to find out whether the furnace is the cause.
What it does
Burners heat the inside of the heat exchanger, and the combustion gases leave through the vent. The blower pushes household air across the outside of it, where it picks up heat without ever touching the flame or the exhaust.
Heating and cooling every cycle expands and contracts the metal. Over many years, or faster when a furnace overheats because of poor airflow, the metal can crack.
Signs a technician looks into
- A flame that wavers or changes when the blower starts
- Soot or rust around the burners
- A furnace that trips its safety limit repeatedly
- A carbon monoxide alarm, or unexplained headaches that ease away from the house
How it is checked
A technician inspects the heat exchanger visually, often with a camera, and may test combustion and look at how the flame behaves. Not every crack is visible, so the findings and how they were reached are worth asking about.
What happens next
A furnace with a confirmed crack is normally shut off until it is dealt with. Heat exchangers can sometimes be replaced, but on an older furnace replacing the whole unit is often the more sensible choice.