Carbon Monoxide Furnace Leaks: Signs and What to Do Now

Yes. A gas, oil or propane furnace can leak carbon monoxide (CO) when it burns fuel poorly and a blocked or disconnected vent, a bad chimney or a damaged heat exchanger lets exhaust into your rooms. If a CO alarm sounds or people feel sick, get everyone outside and call 911. The CPSC says not to search for the source first.

The key numbers come from the CPSC. Most people have no symptoms from long exposure at 1 to 70 parts per million (ppm). Above 70 ppm, headache, fatigue and nausea become more noticeable. Sustained levels above 150 to 200 ppm can cause disorientation, unconsciousness and death. Kidde describes its alarm as time-weighted: the higher the CO level, the sooner it sounds. Kidde's alarm manual lists 60 to 240 minutes at 70 ppm, 10 to 50 minutes at 150 ppm and 4 to 15 minutes at 400 ppm.

CO poisoning is common enough to plan for. The CDC says more than 400 people in the US die each year from CO poisoning not linked to fires, and more than 100,000 visit an emergency department.

What to do if your carbon monoxide detector is going off

Treat a full alarm pattern as real, even if nobody feels sick. The CPSC says never to ignore a sounding CO alarm. Then follow these steps in order.

1. Get everyone outside now. Do not hunt for the source. The CPSC says to go outside to fresh air.
2. Call 911 from outside. National Grid tells customers who suspect CO to go outside immediately and call 911 or its gas emergency number.
3. Count heads. The CPSC says to do a head count from outside.
4. Stay out. Do not go back in until responders give permission.
5. Tell responders about symptoms. Headache, dizziness, nausea or confusion in anyone matter. Kidde's manual says that if you suspect CO poisoning, move to fresh air and get emergency help.
6. Leave the furnace off. The CPSC says not to run an appliance you suspect until trained personnel service it. Kidde says never to restart the source of a CO problem until it has been corrected.
7. If the alarm sounds again within 24 hours, repeat these steps. Kidde's manual says to then call a qualified appliance technician to check every fuel-burning appliance.

Airing out can fool you. Kidde's manual notes that open doors and windows may clear the CO before help arrives. The house seems fine, but the source is still there. In at least one furnace case the CPSC reviewed, the CO reading was taken after the home had been aired out.

A single chirp is a different signal. On Kidde's CO-only alarms, one quick beep every 60 seconds means a low battery, and one every 30 seconds means an error. A real CO alarm is 4 quick beeps, then 5 seconds of silence, repeating. Combination smoke and CO alarms are covered further down.

Can a furnace leak carbon monoxide? How it happens

It usually takes two things going wrong at once. CPSC staff reviewed 28 in-depth investigations of CO incidents with modern gas furnaces from 1987 to 2001, linked to 13 deaths. Their memo says a leak "typically occurs as a result of two failures." First, something makes the furnace produce high CO. Second, something opens a path from the combustion chamber or flue into the living space.

Many incidents involved more than one condition, so the counts add up to more than 28. New furnaces were not exempt. The CPSC memo found furnaces from under a year to 13 years old, and 21% were a year old or less. In 12 investigations a CO alarm was present and sounded. In 13 of the 19 cases with a recorded level, the highest reading topped 100 ppm.

Kidde's manual adds everyday conditions that can spill exhaust for a while. It names strong wind, exhaust fans that put a house under negative pressure, and several fuel-burning appliances competing for the same air. Vent connections on furnaces and water heaters can also vibrate loose. A cracked heat exchanger is one possible leak path. Our cracked heat exchanger guide covers how a technician proves a crack and what the CO readings on the report mean.

Carbon monoxide symptoms and signs at the furnace

You cannot see, smell or taste CO. The CDC lists the most common symptoms as headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion. The CPSC says mild poisoning can feel like the flu without a fever. The EPA adds a clue: symptoms that clear up after you leave home.

Some people are at higher risk. The CDC names infants, older adults, and people with chronic heart disease, anemia or breathing problems. It also warns that people who are asleep or drunk can die before they have any symptoms. That is the gap an alarm outside the bedrooms is there to cover.

What CO levels are dangerous?

Know which number you are looking at. A room reading, an alarm threshold and a flue reading on a technician's report mean different things. The room and alarm numbers below come from the EPA, the CPSC and Kidde's manual.

The EPA also notes that low levels can cause chest pain in people with heart disease. For flue readings and "air-free" CO on a service report, see the readings table in our cracked heat exchanger guide.

Are CO detectors legit if mine always reads 0?

A steady 0 is normal. On Kidde's KN-COPP-B-LP, a display model, the screen shows "0" whenever CO stays between 0 and 30 ppm. Kidde's manual rates the display at plus or minus 30% from 30 to 999 ppm. A peak button shows the highest reading since the last reset, from 11 ppm up. Other models differ, so check yours.

Home alarms are built to stay quiet at low levels. Kidde says this alarm "has not been independently investigated for carbon monoxide detection below 70 PPM." It also says CO between 1 and 30 ppm "can often occur in normal, everyday conditions." People with medical problems may want a device that signals below 30 ppm, the manual adds.

The test button checks the alarm, not the air. The CPSC says it tests whether the circuitry works, not the accuracy of the sensor. Kidde tells owners to press it weekly. Alarms also wear out. The Kidde alarm in that manual beeps twice every 30 seconds after 10 years, and then it no longer detects CO. Check the replace-by date on the label.

Is my smoke alarm also a carbon monoxide detector?

Only if it is a combination unit. Kidde's manual says plainly that CO alarms are not smoke alarms, and a CO-only alarm will not sense smoke. Combination smoke and CO alarms exist, and the model building code allows them in place of separate CO alarms, per the IRC summary from Anne Arundel County, Maryland. Kidde prints the model number on the back of the alarm, so take it down and look it up.

The sound tells you which hazard it is. These are the signals in the user guide for Kidde's KN-COSM-IBA combination alarm. Other brands and models differ.

When smoke and CO are both present, the smoke signal takes precedence on this model. If a bedroom alarm keeps sounding a full alarm pattern, treat each one as real until responders clear the house. If it only chirps, it wants a battery or a replacement.

Can an air conditioner, emergency heat or a boiler cause carbon monoxide?

Air conditioners and heat pumps

Not on their own. CO comes from incomplete burning of fuel, the CPSC says, and a central air conditioner or heat pump runs on electricity. If a CO alarm sounds in summer, something else is burning fuel. Kidde lists water heaters, gas ranges, gas dryers, fireplaces, generators and cars in attached garages among the sources. Respond the same way: get out and call 911.

Emergency heat

It depends on your backup. Lennox explains that emergency heat shuts off the heat pump and runs only the backup source. For most homes that is electric resistance heat, which burns no fuel. Some dual-fuel systems use a gas or oil furnace instead. Then emergency heat means a furnace is running, with the same CO risks as any furnace.

Boilers

Yes. A gas boiler burns fuel too. A 2002 CPSC staff memo reviewed 56 investigations of CO incidents with residential gas boilers from 1987 to 2001, linked to 41 deaths. Vent problems showed up in 33 of them and blocked chimneys in 13. CPSC has also announced boiler recalls, such as a 2007 Crown Boiler recall of about 400 units that could make excessive CO in the flue gases. For pressure and pump problems, see our boiler not heating guide.

Where should CO alarms go, and are they required?

The CPSC recommends alarms on every level of the home and outside sleeping areas. It says interconnected alarms are best, because when one sounds they all sound. Keep them away from heating vents and from spots where furniture or drapes can cover them. The CPSC does not recommend kitchens or spots above fuel-burning appliances.

So do not mount one on the furnace. Kidde's manual says to avoid kitchens, garages and furnace rooms, and places colder than 40°F or hotter than 100°F, such as attics and crawl spaces. With only one alarm, Kidde says to put it near the bedrooms, not in the basement or furnace room. It notes that CO in the basement may never reach an alarm on the second floor.

Rules vary by state and city. The 2021 International Residential Code, as summarized by Anne Arundel County, Maryland, requires CO alarms in new homes with a fuel-fired appliance or an attached garage that opens into the house. They go outside each separate sleeping area, near the bedrooms. A bedroom with a fuel-burning appliance in it or its bathroom gets its own. Permitted work on an existing home can trigger the rule too. In California, PG&E notes that all single-family homes must have CO detectors.

How to prevent carbon monoxide from your furnace

1. Book a yearly inspection. The CPSC, the CDC and the EPA all recommend professional service of the heating system every year. The EPA's advice names furnaces, flues and chimneys.
2. Ask what the visit includes. Carrier says an inspection typically covers the heat exchanger, burners, ignition, blower, electrical parts and venting, with a camera on the heat exchanger. For gas furnaces it can include combustion and CO testing. If you want CO tested, ask for it by name.
3. Keep outdoor vents clear. National Grid warns that snow and ice around fresh air and exhaust vents can cause equipment malfunction and dangerous CO buildup. Check them after snow.
4. Change a dirty filter and keep the panel on. CPSC investigators listed a dirty filter, and a filter panel not sealed or fully on, among the conditions in furnace CO cases.
5. Do not starve the furnace of air. Inadequate combustion air and back-drafting appear in the CPSC furnace memo, and Kidde notes that exhaust fans can depressurize a house. Ask your technician whether the furnace space gets enough air.
6. Look at the vent pipe. With the heat off, follow the pipe from the furnace to the wall or chimney. Note any joint that has pulled apart, holes or heavy rust. Do not patch it yourself. Disconnected and improperly installed vents appear in both CPSC memos, so this is a technician's job.
7. Never idle a car in an attached garage. The EPA lists car exhaust from attached garages as a CO source.
8. Test alarms weekly and replace them by the date on the label, as Kidde's manual directs.

Modern furnaces also carry safety switches. The CPSC memo counts pressure or thermal switches that meet the standard's blocked-vent rules as a mark of a modern furnace. If a pressure switch keeps shutting yours down, our pressure switch guide covers the causes.

What a CO check and furnace inspection cost

Some gas utilities check appliances for free. PG&E, for example, offers free in-home gas safety checks covering furnaces, water heaters and ovens. Its November 2025 release said it had responded to more than 6,000 suspected CO incidents since the start of that year. Ask your own utility what it offers. Paid visits vary, and Bob Vila and Carrier report these ranges.

These are national ranges, and the Bob Vila figures date from 2023. Repairs depend on what the technician finds. A loose vent connection is a different job from a heat exchanger. Our cracked heat exchanger guide lists reported costs for that repair and for a new furnace. If the furnace is tagged off while you wait, our no heat guide covers staying warm safely.

Call 911 or a pro now if

• A CO alarm sounds its full pattern. Get out and call 911 from outside.
• Anyone has a headache, dizziness, nausea or confusion that eases outside. Call 911.
• You smell gas. Leave, then call 911 and the gas utility from a safe distance.
• The alarm sounds again after responders cleared the house. Have a qualified technician check every fuel-burning appliance.
• The furnace vent pipe has come apart, has holes or is rusted through.
• You see fresh soot on or around the furnace or its vent.
• An outdoor vent was buried in snow or ice and the furnace is acting up after you cleared it.
• A technician or the utility tagged the furnace. Leave it off and book the repair or a second opinion.

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