If the carbon monoxide alarm went off, get everyone out of the building now. Call 911 or your gas utility from outside, not from inside. Do not reset the alarm and do not restart the appliance.
Carbon monoxide has no smell, no colour and no taste, which is the entire reason the alarm exists. It is detecting something your senses cannot. The correct response is not to investigate. Get everyone out of the building, leave a door open behind you if that is quick to do, and call 911 or your gas utility from outside. Do not reset the alarm, do not open windows to clear it first, and do not restart the appliance to see whether it happens again. Once the immediate situation has been handled by people with instruments, the second stage begins: something produced that carbon monoxide, and it needs finding. With hearth appliances the usual culprits are a blocked or disconnected vent, a cracked heat exchanger, incomplete combustion from a poorly adjusted or dirty burner, or a flue that cannot draft because the house is under negative pressure.
Checks you can do safely
- Do these only after the building has been cleared and checked by responders
- Check the age of the alarm: detectors expire and an old one may be failing
- Check exterior vent terminations for snow, ice, nests or debris
- Note which appliances were running when it sounded, and for how long
- Note whether a bathroom fan, range hood or dryer was also running
- Do not use the appliance again until it and its venting have been inspected
Carbon monoxide alarm: causes in the order worth checking
Emergency: leave the building and call 911 or your gas utility from outside
| Possible cause | Rank |
|---|---|
| A blocked, disconnected or damaged vent or flue | Most common |
| Incomplete combustion from a dirty, misadjusted or obstructed burner | 2 |
| Gas logs moved out of their specified position | 3 |
| A cracked heat exchanger on a furnace or a gas appliance | 4 |
| House negative pressure pulling combustion products back into the room | 5 |
| A blocked direct-vent termination: snow and drifts are a Vermont regular | 6 |
| An unvented appliance used in a space too small or too tight for it | 7 |
Services this maps to
Related problems
- I Think I Had a Chimney Fire: What to Do Next
- Chimney Draft Problems: My Stove or Fireplace Will Not Draw
- Smoke Is Coming Into the Room Instead of Up the Chimney
- There Is an Animal or a Bird in My Chimney
If what you have is not carbon monoxide alarm after all, the full list of chimney problems is grouped by urgency, and the service triage tool takes a symptom and returns the stop condition and the service it maps to.
Common questions about carbon monoxide alarm
The alarm stopped. Can we go back in?+
Only when the responders say so. An alarm that stops may mean the level dropped below its threshold instead of that the source is gone, and the source is still there. Waiting for someone with an instrument is not over-caution here.
Could it be a false alarm?+
It could, and detectors do expire. Most have a service life of several years and then need replacing, not testing. But you cannot establish that from inside the building, so the sequence does not change. Treat every activation as real until someone measures.
How old is too old for a CO alarm?+
Check the manufacture date on the back instead of the battery. Detectors have a stated service life and past it the sensor degrades, so an alarm that looks fine and beeps when tested may no longer detect anything. Replace, not assume.
What are the symptoms of CO exposure?+
Headache, dizziness, nausea, weakness and confusion, and they are easily mistaken for flu. Several people or pets in the house feeling it at once, and improving on going outside, is a reason to leave immediately.
Which appliances can produce it?+
Anything that burns fuel: gas and oil heating, gas hearth appliances, wood and pellet stoves, gas dryers and water heaters. Wood appliances produce it too, and a blocked flue on a wood stove puts it into the room the same way.
Why does snow on the vent matter?+
A direct-vent appliance draws its combustion air and expels its exhaust through the same terminal. Block it and the appliance cannot breathe, combustion goes incomplete, and the products have nowhere correct to go. Low-mounted terminations drift over regularly here.
What is negative pressure and how does it cause this?+
When fans pull more air out of a house than leaks back in, the house goes slightly negative and the easiest replacement path can become the chimney: running backwards. Combustion products then come down the flue into the room instead of leaving.
Where should alarms be placed?+
On every level and near sleeping areas, following the manufacturer's instructions for the specific unit. With hearth appliances, having one in the room with the appliance as well as near bedrooms is sensible. Alarms are inexpensive relative to what they do.
Can you find the source?+
The appliance and its venting get inspected, which covers a large share of the causes. Where the source turns out to be a furnace heat exchanger or a gas supply issue it belongs to the heating contractor or the utility, and we will say so instead of guess.
The gas company came and said it was fine. Now what?+
The gas company will have checked the supply and taken readings. If the alarm sounded and the cause was not identified, the hearth appliance and its venting still warrant inspection: a blocked flue is a chimney finding, not a gas-supply one.
Can I use the fireplace while I wait?+
No. Until the source has been identified, every fuel-burning appliance in the house is a candidate, and using one is how you repeat the event: a stop-use situation without exceptions.
What prevents a repeat of this?+
Annual service on every fuel-burning appliance, venting inspected instead of assumed, terminations kept clear all winter, and detectors still inside their service life. Close to every case traces back to one of those four having been skipped.