Furnace Carbon Monoxide: Signs, Alarms and When to Leave
⚠️ CO alarm sounding or someone feels sick? Get out, then call 911
The U.S. Consumer Product Safety Commission: “Never ignore an alarm when it sounds. Get outside immediately. Then call 911.” Take everyone, including pets, into fresh air, and don’t go back in until the fire department or a qualified technician says it’s safe. If anyone has headache, dizziness, nausea, confusion or chest pain, tell the 911 operator.
Quick answer: A gas or oil furnace can release carbon monoxide (CO) into your home if it burns fuel badly or its exhaust can’t escape: a blocked or disconnected flue or vent, a cracked heat exchanger, or a furnace that hasn’t been serviced. CO has no smell or colour, so you can’t detect it without an alarm. Protect your home with CO alarms on every level and outside sleeping areas, test them monthly, keep vents clear of snow and debris, and have the furnace inspected every year.
How a furnace produces carbon monoxide
The NFPA describes CO as an “odorless, colorless gas created when fuels burn incompletely.” A properly working furnace sends its combustion gases out through the flue or vent pipe. The EPA lists “leaking chimneys and furnaces” and “back-drafting from furnaces” among indoor CO sources. Common furnace causes:
- Blocked or disconnected vent: a bird’s nest, debris, snow on high-efficiency PVC pipes, or a flue pipe that has come apart. Goodman warns never to “block, close or patch a vent pipe.”
- Cracked heat exchanger: Carrier says a cracked heat exchanger “can leak carbon monoxide, posing a significant safety risk.”
- Poor combustion: dirty burners or a gas pressure problem make the flame burn badly.
- Back-drafting: exhaust fans or a blocked chimney pull combustion gases back into the house.
Signs your furnace needs a CO check
None of these prove there is CO, but each is a reason to have the furnace inspected promptly:
- A CO alarm that has sounded, even once.
- Soot or scorch marks on or around the furnace or vent pipe.
- Yellow, lazy or lifting burner flames instead of steady blue ones.
- A disconnected, rusted or sagging flue pipe, or water dripping from it.
- Snow, ice or debris blocking the vent pipes outside.
- Household members who feel better when they leave the house.
Furnace short cycling or overheating can also be linked to venting or heat exchanger problems; see furnace short cycling.
Carbon monoxide poisoning symptoms
The CDC lists the most common symptoms as “headache, dizziness, weakness, upset stomach, vomiting, chest pain, and confusion.” The CPSC notes they can feel “similar to the flu (but without the fever).” Higher levels cause loss of consciousness and death; the CDC says more than 400 people in the U.S. die from unintentional CO poisoning each year. The CDC adds that “infants, the elderly, and people with chronic heart disease, anemia, or breathing problems” are more likely to get sick. If symptoms ease when you go outside and return indoors, suspect CO and call 911.
CO alarms: where, which and how often to test
- Where: the CPSC recommends CO alarms “on every level of the home and outside sleeping areas.” The NFPA says a central location outside each sleeping area and on every level. Mount each alarm where its instructions say.
- Which: choose an alarm that “meets the requirements of the current UL 2034 safety standard” (CPSC). Battery or battery-backup models keep working in a power cut.
- Interconnected: the CPSC recommends interconnected alarms, so “when one sounds, they all sound.”
- Testing: “Test CO and alarms monthly” (CPSC) and replace alarms as the manufacturer says; the NFPA says the same.
- Not a substitute: the CPSC says a CO alarm is “no substitute” for maintaining fuel-burning appliances.
What CO levels set off an alarm?
Alarms meeting UL 2034 sound based on both the CO level and how long it lasts, so they don’t go off for brief, low readings. According to First Alert’s maker, BRK:
| CO level | Alarm must sound within |
|---|---|
| 70 ppm | 60 to 240 minutes |
| 150 ppm | 10 to 50 minutes |
| 400 ppm | 4 to 15 minutes |
That means an alarm that sounds has usually detected CO for some time. Treat every alarm as real.
How to prevent furnace CO problems
- Annual inspection: the CDC says heating systems should be “serviced by a qualified technician every year,” and the EPA recommends having a professional “inspect, clean and tune-up central heating system (furnaces, flues and chimneys) annually.”
- Keep vents clear: the CPSC says to “clear snow away from the outside vents for fuel-burning appliances such as furnaces.”
- Change the filter as your manual recommends so the furnace doesn’t overheat.
- Don’t block the furnace with storage or close it into a small room without the air supply it was installed with.
- Never use ovens, grills or generators for heat indoors or in an attached garage.
If your furnace is leaking water rather than gas, see furnace leaking water.
What CO-related furnace work usually costs
| Item | Typical cost | Source |
|---|---|---|
| Average furnace repair | $133 to $505 (about $318 average) | HomeAdvisor, June 2026 |
| Heat exchanger replacement | $500 to $1,500 | Angi, Sept 2026 |
| Inducer motor (venting fan) | $200 to $1,500 | HomeAdvisor, June 2026 |
A CO alarm itself is inexpensive compared with any of these; replace alarms on the schedule printed on them.
Frequently asked questions
Can a furnace leak carbon monoxide?
Yes. A blocked or disconnected vent, a cracked heat exchanger or poor combustion can let CO into the home. That is why annual inspections and working CO alarms matter.
What does carbon monoxide smell like?
Nothing. CO is odorless and colorless. A rotten-egg smell is natural gas, which is a different emergency; see furnace gas leak.
What should I do if my CO alarm goes off?
Get everyone outside into fresh air immediately, then call 911. Don’t go back in until responders say it’s safe, and have the furnace and other fuel-burning appliances checked before using them.
Where should CO detectors be placed?
On every level of the home and outside sleeping areas, per the CPSC, mounted as the alarm’s instructions say. Interconnected alarms are best.
How often should a furnace be checked for carbon monoxide?
Once a year, before the heating season, by a qualified technician. The CDC, CPSC and EPA all recommend annual inspection.
Can an electric furnace produce carbon monoxide?
No. Electric furnaces don’t burn fuel. CO comes from fuel-burning appliances such as gas or oil furnaces, water heaters, stoves and generators.
Related help: furnace gas leak · furnace short cycling · furnace won’t ignite · furnace not heating
Sources: CPSC: carbon monoxide questions and answers · CPSC: winter heating safety · CDC: carbon monoxide basics · CDC: furnace safety fact sheet · EPA: carbon monoxide and indoor air · NFPA: carbon monoxide safety tips · Goodman: carbon monoxide · Carrier: furnace short cycling · BRK / First Alert: CO alarm levels · HomeAdvisor: furnace repair cost · Angi: furnace repair cost.
Updated October 22, 2026. Written and edited by Jordan Hale. General educational guidance; in an emergency, call 911.
