What Causes a CO Alarm to Go Off When the Furnace Seems to Work Fine?

Q: My carbon monoxide alarm went off, but the furnace seems to be heating normally. What should I do?

A: Treat it as a real emergency first. A furnace can appear to “work fine” and still have a combustion or venting problem, and the alarm could also be detecting carbon monoxide from another source in or near the home. If a CO alarm sounds, get everyone to fresh air and call 911 before doing any troubleshooting.

Common Symptoms to Watch For

• Headache
• Dizziness
• Nausea or vomiting
• Weakness or unusual fatigue
• Confusion
• Shortness of breath
• Chest pain
• Feeling sleepy or “off” for no clear reason

Safety First

• If the CO alarm sounds, do not ignore it.
• Move everyone outside or to fresh air immediately.
• Call 911 from outside or another safe location.
• Do not assume it is a false alarm just because the furnace is still running.
• Do not restart the furnace or other fuel-burning equipment until the home is declared safe.

If anyone has severe symptoms, collapses, or is hard to wake, call 911 immediately and treat it as an emergency.

Quick Checks You Can Do

Only do these after everyone is safe and emergency guidance has been followed.

1. Check whether the alarm is a CO alarm or combo smoke/CO alarm: This helps confirm what alert you heard.
2. Check the alarm age: CO alarms expire and can chirp/end-of-life alert. Follow the manufacturer label and instructions.
3. Replace batteries if applicable: Low battery alerts can be confused with alarm events, but do not assume that was the issue until you confirm.
4. Note what was running: Furnace, gas water heater, fireplace, generator, vehicle in attached garage, or other fuel-burning equipment.
5. Check for obvious issues only: Blocked supply/return vents, dirty filter, or unusual furnace cycling. Do not open burner or venting components.

Why a CO Alarm Can Go Off Even if the Furnace Still Heats

Furnace combustion problem

A fuel-burning furnace can still produce heat while also operating unsafely. “Heat output” does not prove combustion is safe.

Venting or flue problem

If exhaust gases are not venting outdoors correctly, carbon monoxide can build up indoors even while the furnace appears to run normally.

Another appliance or source

The furnace may not be the only possible source. Water heaters, fireplaces, generators, attached garages with running vehicles, and other combustion sources can trigger a CO alarm.

Alarm placement or alarm end-of-life issue

Alarms need correct placement and timely replacement. Some alerts are maintenance/end-of-life warnings, but you should always treat a full CO alarm as real until proven otherwise.

Intermittent exposure

CO issues can happen only under certain conditions, such as startup, windy weather, venting backdraft conditions, or specific appliance run cycles.

Important Clarification for Florida Homes

Many Florida homes use heat pumps or electric heat instead of fuel-burning furnaces. Electric heat does not create CO from combustion. However, homes can still have CO risk from other sources, including gas water heaters, fireplaces, generators, or attached garages.

Prevent This Next Time

• Install CO alarms on every level and near sleeping areas
• Test alarms regularly and replace batteries as required
• Replace alarms at manufacturer end-of-life intervals
• Schedule regular heating system maintenance
• Never run generators in or near enclosed spaces
• Do not idle vehicles in attached garages