Carbon Monoxide and Your Chimney
A chimney exists to take combustion gases out of the house. When it cannot, carbon monoxide stays inside.
How a chimney lets carbon monoxide in
Carbon monoxide enters a home from a chimney in three ways: a blockage, a leak, or backdrafting.
- Blockage. A nest, fallen tiles or heavy soot stop the exhaust leaving, so it comes back down.
- Leak. Cracks and gaps in the liner let exhaust seep into the house through the masonry.
- Backdrafting. The flue is too large, too cold, or the house is under negative pressure, and air flows down the chimney instead of up.
Why gas appliances are the bigger risk
Wood smoke is visible and you can smell it. The exhaust from a gas furnace or water heater has little smell and cannot be seen, so a venting problem can go unnoticed. About 70% of homes in Jackson County heat with gas, according to the Census Bureau’s 2024 American Community Survey, and many vent through an old masonry chimney.
The common fault in older homes is a water heater left venting alone into a large flue after the furnace was replaced. The exhaust is too cool to rise properly, and it condenses and spills. A liner sized to the appliance corrects it; see chimney liner installation.
Warning signs around the house
Signs of a venting problem include rust or water streaks on the vent pipe or the top of the water heater, soot around the appliance or fireplace opening, moisture on windows near the appliance, and white staining on the chimney in the basement or attic.
If a carbon monoxide alarm sounds: get everyone outside into fresh air and call 911. Do not go back in until responders say it is safe. Symptoms of carbon monoxide poisoning include headache, dizziness, nausea and confusion.
How to prevent it
Prevention comes down to four habits.
- Yearly inspection. Have every flue inspected once a year, including flues that serve only a furnace or water heater.
- Alarms. Fit carbon monoxide alarms on every level and near bedrooms, and test them.
- Check after changes. Have the chimney looked at whenever a furnace, water heater, stove or insert is replaced.
- Keep the cap on. A screened cap prevents the most common blockages.
Flue not inspected this year?
Call (816) 239-8989A chimney inspection starts at $119 for Level 1. Our chimney care guide explains how draft works and why flue size matters.
Sources
- National Fire Protection Association, NFPA 211: Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances.
- U.S. Census Bureau, American Community Survey 2024 1-year estimates, table B25040, Jackson County, Missouri.
Common questions
Can a fireplace cause carbon monoxide poisoning?
Yes. Any fire produces carbon monoxide, and a fireplace is safe only because the chimney carries it outside. If the flue is blocked, the damper is closed too soon, or the house pulls air down the chimney, carbon monoxide comes into the room. Never close the damper until the ashes are cold.
Can carbon monoxide come from a chimney when the fireplace is not in use?
Yes, if a furnace or water heater vents through the same chimney. Those appliances run all year, and a blocked or leaking flue can spill their exhaust into the house even when the fireplace has not been lit for months.
What does backdrafting mean?
Backdrafting is when air flows down a chimney or vent instead of up, carrying combustion gases into the house. It happens when the flue is cold or oversized, or when exhaust fans and dryers lower the air pressure indoors. Signs include soot or moisture around the appliance's draft hood.
How often should a furnace flue be inspected?
Once a year, the same as a fireplace flue. NFPA 211 calls for yearly inspection of chimneys and vents. Have it checked again whenever the furnace or water heater is replaced, since a new appliance often needs a different flue size.
Will a carbon monoxide alarm detect a chimney problem?
An alarm detects carbon monoxide once it has built up in the room, so it warns you of the result, not the cause. It is essential protection, but it does not replace an inspection, which finds the blockage or damage before any gas enters the house.