Carbon monoxide alarm going off near the furnace

Symptoms

• CO detector alarm sounding
• Possible headache, dizziness, nausea, or confusion in occupants
• May coincide with furnace running

Likely causes

• CommonIncomplete combustion in the furnaceA furnace burning fuel improperly can produce dangerous carbon monoxide.
• CommonBlocked or cracked flue/exhaust ventExhaust that can't escape properly can back up into the living space.
• PossibleCracked heat exchangerA crack can let combustion byproducts mix into the heated air stream.
• PossibleDetector nearing end of life or low battery causing false alarmDetectors can occasionally malfunction, but this should never be assumed first.
• PossibleOther fuel-burning appliance nearby (water heater, stove) contributingCO can come from any combustion appliance in the home, not just the furnace.

What to do

1. Get everyone and pets out of the house into fresh air immediately; don't stop to investigate.
2. Call 911 or your local emergency number from outside and report a carbon monoxide alarm.
3. Don't re-enter the house until fire department or emergency responders say it's safe.
4. Do not attempt to turn the furnace off or investigate the source yourself before getting out.
5. Once cleared, have a technician inspect the furnace, flue, and all fuel-burning appliances before using any of them again.
6. Replace the CO detector's batteries or the unit itself as part of the follow-up, but only after the emergency is resolved.

When to call a pro

Emergency responders first, then a licensed HVAC technician to inspect the furnace and flue before it's used again.

Safety