Carbon monoxide (CO) is a gas produced whenever fuel burns, and it has no color, no smell and no taste. A furnace, boiler or water heater that is burning cleanly and venting correctly makes very little of it and sends what it does make up the flue and outside. When combustion goes wrong, or the exhaust has nowhere to go, CO can build up indoors without anyone noticing. That is why a working CO alarm matters more than any warning sign you could spot yourself.
If a CO alarm goes off, get everyone out of the house, pets included, and call 911 from outside. Do not stay behind or go back in to open windows, shut off the furnace or look for the source. The fire department will check the house and tell you when it is safe to go back in. Then leave the heating system off until a technician has checked it, even if the alarm stopped once the house aired out.
How a furnace or boiler produces carbon monoxide
When natural gas, propane or oil burns with the right amount of air, the result is mostly carbon dioxide and water vapor, with very little CO. Starve the flame of air, or let the burner drift out of adjustment, and combustion becomes incomplete. That is when CO production can climb. Dirty or rusted burners, a blocked combustion air supply, incorrect gas pressure, or an oil burner that needs cleaning and adjustment can all push a furnace or boiler in that direction, and none of them is something you can see from the outside of the cabinet.
How CO gets from the equipment into your living space
Producing CO is one problem. Getting it into the house is another. A furnace is built so that combustion gases stay on one side of a metal wall and the air you breathe stays on the other, with the flue carrying the combustion side outdoors. CO reaches the living space when that arrangement breaks down.
A cracked heat exchanger
The heat exchanger is the metal chamber that separates the burner flame and its exhaust from the air the blower sends through your ducts. Years of heating and cooling, overheating from poor airflow, and corrosion can crack it. A crack can let combustion gases mix into the house air, and air from the blower can push through it the other way and disturb the flame, which can make combustion worse at the same time. Cracks are often too small to see without a camera or mirror, so they are found by inspection and combustion testing rather than by guesswork. Boilers heat water rather than air, so a cracked boiler usually shows up as a water leak, but venting and combustion problems affect them just as much.
A blocked or disconnected flue
Older furnaces and boilers, and most oil-fired equipment, vent through a metal flue pipe into a chimney. If a section of that pipe separates, rusts through or pulls loose where it enters the chimney, exhaust can spill into the basement. Chimneys block up too: a collapsed liner, fallen masonry, debris or a nest at the top. A water heater often shares the same chimney, so one blockage can affect two appliances.
Backdrafting
Chimney-vented equipment relies on warm exhaust rising up the flue. If the house is depressurized, the exhaust can be pulled back down into the house instead. Bath fans, a kitchen range hood, a clothes dryer and a fireplace all push air out of the house, and in a tight house with several of them running at once, the furnace or water heater can lose that contest. Technicians test for this by checking the draft with the home's exhaust fans running. It is worth asking for after air sealing work or after installing a powerful range hood.
Snow-blocked vent pipes on high-efficiency equipment
High-efficiency furnaces, boilers and many newer water heaters vent through PVC pipes out a side wall, often low on the house. Drifting snow, a roof slide or a shoveled pile can bury them. These units have a pressure switch that should shut them down if the exhaust is fully blocked, but that safety device is not a reason to leave snow in place. Massachusetts' carbon monoxide alarm law is named for Nicole Garofalo, a seven-year-old who died in January 2005 when snow drifts blocked a heating vent outside her family's home in Plymouth. After every storm, check that the intake and exhaust pipes for your heating equipment and water heater are clear, along with the dryer vent.
Symptoms of carbon monoxide poisoning
CO keeps your blood from carrying oxygen. At lower levels, the symptoms look a lot like the flu without a fever: headache, fatigue, dizziness, nausea and shortness of breath. Higher exposure brings vomiting, chest pain, confusion and eventually loss of consciousness. People who are asleep or have been drinking can be overcome before they notice anything, which is why alarms near bedrooms matter so much. Infants, older adults and people with heart or lung disease are more vulnerable.
The pattern often says more than any single symptom. Symptoms could be from CO rather than a virus when you feel better after leaving the house and worse after coming back, when several people in the home get sick at the same time, when the people who spend the most time at home are hit hardest, or when symptoms appear soon after a fuel-burning appliance starts running. Pets are affected too. A dog or cat that turns unusually drowsy or sluggish at the same time is another clue, since human flu does not pass to pets. If you suspect CO poisoning, get outside into fresh air, call 911, and tell the medical staff you suspect carbon monoxide.
Warning signs at the equipment
None of these proves there is a CO problem, and equipment can produce dangerous levels of CO without showing any of them. They are reasons to turn the system off and have it checked.
- A yellow or lazy flame. On equipment where you can see the flame, a gas burner should burn mostly blue and steady. A yellow, floating flame that seems to drift off the burner points to poor combustion, and a flame that jumps or changes shape when the blower comes on can point to a cracked heat exchanger.
- Soot. Black streaks or soot around the burner area, the flue pipe or the chimney, or soot and debris falling from them.
- Excess moisture. Heavy condensation on the inside of windows that is new or unusual for your house, especially while the heat is running.
- A sharp or unusual smell when the heat runs. CO itself is odorless, but poor combustion produces other gases that can smell sharp, acrid or like exhaust. A rotten egg smell is different: that is gas, and the response is to leave the house and call the gas company or 911 from outside.
- Rust or water streaks on the flue pipe or chimney, loose or disconnected vent connections, or a furnace panel that is loose or missing.
- A pilot light that keeps going out, which can point to a draft or venting problem.
CO alarms and what Massachusetts requires
Massachusetts General Laws chapter 148, section 26F½, known as Nicole's Law, requires working carbon monoxide alarms in any residence that has fossil fuel burning equipment, such as a furnace, boiler, water heater or fireplace, or that has enclosed parking within the structure, such as an attached garage. Every home heated with gas, oil or propane falls under it. According to the state Department of Fire Services, alarms must be installed on every level of the home, including habitable portions of basements and attics, and on levels with sleeping areas they should be within 10 feet of the bedroom doors. Allowed types include battery alarms with battery monitoring, plug-in alarms with battery backup, low-voltage and wireless systems, and qualified combination smoke and CO alarms. When a home is sold, the local fire department inspects it for compliance.
The alarm itself needs a little care. Test it monthly with the test button. Check the manufacturing date printed on the back: depending on the make and model, CO alarms need replacing after 5, 7 or 10 years, and the manufacturer's instructions say which applies to yours. If your alarms use alkaline batteries, the state fire marshal's office recommends replacing them twice a year when the clocks change. Newer models with a sealed 10-year lithium battery do not need battery changes, and the whole unit is replaced when it reaches the end of its life. Learn the difference between your alarm's low battery or end-of-life chirp and a full alarm, and treat a full alarm as real every time.
Prevention: the habits that matter most
The most useful single step is a yearly inspection of every fuel-burning appliance, ideally before the heating season starts. A thorough check on a furnace or boiler goes beyond cleaning. It includes looking over the heat exchanger, checking the venting and chimney connection, and measuring combustion with an analyzer, which shows how much CO the burner is producing and whether it is getting the right amount of air. That measurement catches problems no homeowner could see. Our HVAC maintenance checklist covers what a heating tune-up should include. If an older furnace turns up a cracked heat exchanger, our article on how long a furnace lasts can help you think through what comes next.
Through the winter, keep the outdoor intake and exhaust pipes clear of snow and ice, keep stored items away from the furnace or boiler so it has the combustion air it needs, and keep the furnace panels in place. Never run a generator, a grill or a car in a garage, basement or any enclosed space, even with the door open. Portable generators belong outdoors, at least 20 feet from the house, with the exhaust pointed away from windows, doors and vents. And never use a gas oven or range to heat the house.
Most carbon monoxide problems from home heating are preventable, and most of the prevention is routine: a working alarm on every level, clear vents and a yearly combustion check. If your furnace has not had that check this season, or you have noticed any of the signs above, you can arrange an inspection through our furnace repair page, with the estimate given before any work starts.
Sources: M.G.L. c. 148, section 26F½; Massachusetts Department of Fire Services, Smoke and Carbon Monoxide Alarms; Massachusetts Department of Fire Services, When You Change Your Clocks, Check Your Alarms; State Fire Marshal winter advisory on CO alarms and snow-blocked vents; CDC, carbon monoxide poisoning; U.S. CPSC, The Invisible Killer; U.S. EPA, Preventing Carbon Monoxide Poisoning.