Can a Furnace Cause Carbon Monoxide Poisoning? Causes, Signs and What to Do

Last updated: October 2026

CO alarm sounding or someone feeling sick? Get everyone, including pets, out of the house and call 911 from outside. Fairfax County Fire and Rescue advises everyone to get out at once and call the fire department from a neighbor’s house.

Yes. A gas or oil furnace can leak carbon monoxide (CO) when it isn’t burning fuel properly or isn’t venting its exhaust safely. The most common causes are a cracked heat exchanger, a blocked or cracked flue or chimney, blocked vents and poor combustion. Fairfax County Fire and Rescue lists defective gas or oil furnaces and cracked chimney flues among the common sources of CO in the home, and recommends a yearly furnace inspection by a qualified technician and working CO alarms as the best protection.

Carbon monoxide is serious. According to the Centers for Disease Control and Prevention (CDC), more than 400 Americans die from unintentional, non-fire-related CO poisoning each year, more than 100,000 visit an emergency department and more than 14,000 are hospitalized.

Worried about your furnace? AVS provides 24/7 furnace repair and inspections across Northern Virginia and Maryland. Call 703-457-9028 (VA) or 301-686-7129 (MD).

How does a furnace produce carbon monoxide?

Furnace Cause Carbon Monoxide is produced by combustion. When a gas or oil furnace burns fuel, it creates exhaust gases that must leave your home through the flue or vent pipe. If the fuel doesn’t burn completely, more CO is produced, and if the exhaust can’t escape properly, it can end up in your living space.

Fairfax County Fire and Rescue explains that carbon monoxide combines with the hemoglobin in your blood and robs it of the oxygen your body needs. Because CO is colorless, odorless and tasteless, you can’t tell it’s there without an alarm.

Electric furnaces don’t burn fuel, so they don’t produce carbon monoxide themselves.

Common causes of furnace carbon monoxide leaks

A cracked heat exchanger is one of the most serious causes, because the crack lets combustion gases mix with the air heading to your rooms. Learn the signs your furnace heat exchanger may be cracked.

Fairfax County’s carbon monoxide safety guidance also reminds residents to make sure vents for the furnace, dryer, stove and fireplace stay clear during and after a snowstorm, which is especially important for high-efficiency furnaces with low wall vents.

7 warning signs of a furnace carbon monoxide problem

1. Your CO alarm sounds. Treat every alarm as real.
2. Flu-like symptoms with no fever. Headache, dizziness, nausea or fatigue without the fever that usually comes with the flu, as DB Heating & Cooling notes.
3. Symptoms that improve when you leave home. DB Heating & Cooling points out that CO symptoms improve away from home, while flu symptoms stay the same wherever you are.
4. Several people or pets feel sick at once. DB Heating & Cooling calls this a major red flag.
5. A yellow or orange burner flame. Fire safety guidance from the City of San Marcos, based on San Diego Gas & Electric’s natural gas safety information, says a burner flame should be about 90 percent blue, and a yellow flame means the appliance isn’t working right and could be giving off harmful fumes.
6. Blocked vents after snow or storms. Snow, ice or debris over a high-efficiency furnace’s vent pipes can stop exhaust from leaving the home.
7. A cracked chimney flue or damaged vent pipe. Fairfax County Fire and Rescue lists cracked chimney flues as a common source of CO.

Symptoms of carbon monoxide poisoning

According to the CDC, the most common symptoms of CO poisoning are headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion. Fairfax County Fire and Rescue adds that early symptoms are similar to the flu, and confused thinking can stop victims from thinking clearly enough to get help. Without treatment, a person can lose consciousness and die.

Comparison based on DB Heating & Cooling’s CO safety guide.

If anyone in your home has these symptoms and you suspect CO, get everyone outside to fresh air and call 911.

What to do if your carbon monoxide alarm goes off

1. Get everyone out. Leave immediately with everyone in the home, including pets.
2. Call 911 from outside. Fairfax County Fire and Rescue recommends calling the fire department from a neighbor’s house.
3. Don’t go back inside until the fire department tells you it’s safe.
4. Get medical help for anyone with symptoms. Tell responders that a CO alarm went off.
5. Have your furnace inspected before you use it again. Fire crews can confirm whether CO was present, but a heating technician needs to find and fix the source.

Gas leak vs carbon monoxide: what’s the difference?

Homeowners often confuse these two dangers, but they’re detected and handled differently.

Because carbon monoxide has no smell, a working CO alarm is the only reliable way to know it’s present.

CO detectors: where to install them

Working alarms save lives. UL Standards & Engagement reports that a Consumer Product Safety Commission investigation found no alarm was present in 94% of carbon monoxide deaths, and that a CPSC in-home survey found 45% of homes had no CO alarm.

Where to put them:

• On every floor of your home. Fairfax County Fire and Rescue recommends CO alarms on every floor, mounted according to the manufacturer’s instructions.
• Outside every sleeping area. Fairfax County’s guidance calls for alarms in a central location outside each sleeping area.
• Within about 15 feet of bedrooms, but not right beside the furnace. The Illinois State Fire Marshal advises installing a CO detector within 15 feet of any room used for sleeping, and not near the kitchen or garage or in a room with a furnace.

When you buy an alarm, make sure it carries a UL label and is labeled specifically for carbon monoxide. A CO alarm doesn’t detect smoke, so you still need smoke alarms.

Annual furnace safety inspection

Fairfax County Fire and Rescue recommends having a qualified technician inspect your gas furnace and other gas appliances every year, and every fall is the ideal time. A safety-focused inspection checks the heat exchanger for cracks, the flue and venting for blockages, and the burners and combustion for proper operation. Book your yearly furnace tune-up before the heating season starts.

Northern Virginia winter CO safety

Winter brings extra CO risks beyond the furnace. Fairfax County Fire and Rescue warns residents to:

• Never use a gas oven or range to warm a room.
• Never run a generator inside your home, basement or garage, or less than 20 feet from any window, door or vent.
• Never let a car run in an enclosed area, especially a garage attached to your home.
• Clear outdoor vents after snow, including vents for the furnace, dryer and fireplace.

Why Northern Virginia homeowners trust AVS with furnace safety

AVS provides 24/7 furnace repair across Northern Virginia and Maryland, with same-day service and no extra charge for evenings, weekends or holidays. Site visits start at $90.

If another company has told you your heat exchanger is cracked, AVS offers a free second opinion on cracked heat exchangers before you decide on a costly repair or replacement.

• Founded in 1991, with more than 50,000 residential and commercial customers served across the Washington DC metro area
• Licensed in Virginia (HVAC 2705039269A) and Maryland (HVAC 520714137895, Home Improvement 4192876)
• Call 703-457-9028 (VA) or 301-686-7129 (MD)

Frequently Asked Questions

How long does it take to get sick from carbon monoxide?

It depends on how much CO is in the air. BRK Electronics, which makes First Alert alarms, explains that 100 ppm of CO for 20 minutes may not affect a healthy adult, but the same level for four hours may cause headaches. At 400 ppm, headaches can start after about 35 minutes, and exposure can cause death after two hours.

Can carbon monoxide poisoning happen while you sleep?

Yes. According to the CDC, people who are sleeping, intoxicated or under the influence of other substances can die from CO poisoning before they ever have symptoms. That’s why CO alarms belong outside every sleeping area, where they can wake you before dangerous levels build up.

Why didn’t my CO alarm go off if I feel sick?

Home CO alarms aren’t designed to sound at low levels. Under the UL 2034 standard, alarms must sound at 70 ppm within one to four hours, 150 ppm within 10 to 50 minutes, and 400 ppm within 4 to 15 minutes, and are designed not to sound at 30 ppm. If you feel sick and suspect CO, leave the home and call 911 anyway.

How long do carbon monoxide alarms last?

Most last between five and ten years, depending on the model. Many alarms use an electrochemical sensor rated for five to seven years, while some newer models include an end-of-life warning that sounds after about ten years. Check the date on the back of your alarm, and replace the entire unit when it reaches the end of its life.

Does Fairfax County offer free carbon monoxide alarms?

It has. Fairfax County Fire and Rescue’s carbon monoxide alarm program has offered a limited number of CO alarms to county residents who need one, with the local fire and rescue station coming out to install it. Contact Fairfax County Fire and Rescue to check whether the program is currently available.

Where should I place a carbon monoxide detector in my home?

A carbon monoxide detector should be installed on every floor of your home and outside each sleeping area. Keep the alarm within about 15 feet of bedrooms, but avoid placing it directly beside a furnace, in the kitchen, or near a garage. Always follow the manufacturer’s installation instructions for the most accurate placement.