This is the one furnace topic where the instructions matter more than the explanation. Read the first section first.
If you suspect carbon monoxide right now — a CO alarm sounding, headaches or dizziness that ease when you leave the house, a specific smell you cannot place — leave the house and call the gas utility or 911. Do not wait for a service appointment, do not try to diagnose it yourself, and do not go back in to check. Handle that first; the rest of this guide is background, not a substitute.
A gas furnace's heat exchanger keeps the combustion process — burning gas and its byproducts — physically separate from the air that gets blown through your ducts and into your rooms. Air passes over the outside of the exchanger and picks up heat; it never touches what is burning inside it, as long as the exchanger is intact.
A crack in the heat exchanger can let combustion byproducts, potentially including carbon monoxide, reach the airstream that gets distributed through the house. This is a combustion-safety issue, not a comfort issue, and it is treated accordingly by anyone qualified to inspect one.
A cracked heat exchanger is not reliably found by looking at it through an access panel. A proper inspection involves combustion analysis and other checks a licensed contractor is equipped to perform, and a furnace with a suspected crack should not keep running on the assumption that it is probably fine.
A working carbon monoxide detector on every level of the house is one of the most effective safety measures available, and it does not conflict with or replace anything else here. Regular maintenance that includes a combustion check is the other half — it is meant to catch developing problems before they become the situation described at the top of this page.
Furnace 801 is a referral service. We match homeowners with a licensed contractor for inspection and repair. We are not equipped to handle an active carbon monoxide emergency, and neither is any contractor arriving on a normal service timeline — that is what the gas utility and 911 are for.
Describe what the furnace is doing and we'll match you with a licensed contractor who can act on it.
Get a Furnace Contractor OutA short description is enough — we'll follow up to confirm the details and get someone scheduled.