Sources of Carbon Monoxide Poisoning

What causes carbon monoxide in a house? Furnaces, water heaters, gas stoves, fireplaces, and generators all share one thing in common: they burn fuel — and any of them can produce carbon monoxide (CO) if something goes wrong with combustion or venting. A car running in a garage adds another common risk. Most CO incidents follow predictable patterns tied to specific appliances or situations. This guide covers the most common sources of carbon monoxide poisoning at home and while traveling, the warning signs that a source may be malfunctioning, and what to do if you suspect a problem.

Key Takeaways

• Top home sources: furnaces/boilers, water heaters, gas stoves/ovens, fireplaces, and unvented space heaters.
• Attached garages are a common risk area — vehicle exhaust can seep into living spaces even with the door open.
• Portable generators are a leading cause of fatal CO poisoning during outages; use them outdoors only, far from buildings.
• Warning signs of a malfunctioning source include soot near appliances, unusual flame color, or excess condensation.
• Annual professional inspection of fuel-burning appliances and working CO alarms are your best defenses.

How CO Is Produced

Carbon monoxide forms when carbon-based fuels (gasoline, natural gas, propane, wood, coal, charcoal, kerosene, oil) burn without enough oxygen or with poor venting. A properly installed, adjusted, and vented appliance should send combustion gases outdoors. Problems happen when equipment is faulty, improperly vented, blocked, or used in a confined space — allowing exhaust to accumulate indoors.

Common Home Sources

Below are the most common sources of carbon monoxide in and around homes. These sources are higher risk when appliances are poorly maintained, venting is blocked, or the home has negative pressure that pulls exhaust back inside.

Furnaces, Boilers, and Water Heaters

Fuel-burning central heat systems and water heaters can produce CO if burners are misadjusted, heat exchangers are damaged, or venting/chimneys are blocked. If you suspect a problem, stop using the appliance and arrange a professional inspection.

Signs that a furnace or water heater may be producing CO include a yellow or orange flame instead of a steady blue one, soot or discoloration around the unit or vent connections, a pilot light that repeatedly goes out, or unusual smells when the unit is running. If a water heater vent pipe feels hot to the touch or shows signs of corrosion, that may indicate venting problems. Carbon monoxide from furnace issues is one of the most common sources during heating season.

Stoves, Ovens, and Gas Ranges

Gas stoves and ovens can produce CO, especially if the flame is not burning correctly or ventilation is poor. Using a gas oven to heat a room is unsafe. Prioritize ventilation and proper appliance maintenance.

Watch for burner flames that are yellow or orange instead of blue, as this can indicate incomplete combustion. If the flame lifts away from the burner or goes out frequently, the appliance may need adjustment. Carbon monoxide from gas stove use increases when burners are clogged with food debris or when the kitchen lacks adequate ventilation — always use a range hood or open a window when cooking with gas.

Fireplaces, Wood Stoves, and Chimneys

Both wood-burning and gas fireplaces can be CO sources if the chimney flue is blocked, the damper is closed, or exhaust is back-drafting into the room. Soot, smoke smell, or poor draft are warning signs that require professional evaluation.

Warning signs include smoke entering the room instead of going up the chimney, a strong burning smell when the fireplace is in use, visible soot buildup around the fireplace opening, or difficulty starting a fire due to poor draft. If you notice any of these, stop using the fireplace and have a chimney sweep or qualified technician inspect the flue and damper.

Space Heaters and Unvented Heaters

Kerosene, propane, and other unvented or poorly vented heaters can create dangerous CO levels. Follow manufacturer instructions and never use fuel-burning heaters in unventilated rooms or while sleeping.

Clothes Dryers and Other Fuel-Burning Appliances

Gas dryers and other fuel-burning equipment can contribute to CO if exhaust is not properly vented outside.

Attached Garage and Vehicles

Running a vehicle in an attached garage — even with the garage door open — can allow exhaust to accumulate and infiltrate living areas. This includes "warming up" a car, idling near the garage, or operating gasoline-powered tools. Treat an attached garage as a high-risk zone and never idle a vehicle indoors.

More on vehicle scenarios: Carbon monoxide in your car.

Generators, Grills, and Outdoor Devices

Portable generators are a leading source of fatal CO poisonings during storms and power outages. Never operate a generator inside a home, garage, basement, crawlspace, or near open windows or doors. Use generators outdoors only, far away from buildings, with exhaust directed away.

Charcoal grills and fuel-burning camping devices can also produce deadly CO. Never use grills, camp stoves, or charcoal indoors (including tents, garages, or enclosed porches), even for a short time.

A common misconception is that using a grill or fuel-burning equipment outdoors is always safe. Grills placed on covered patios, under overhangs, or near open windows and doors can still send CO into living spaces — especially if wind pushes exhaust toward the building. Semi-enclosed areas like screened porches, carports, and breezeways can trap enough CO to create risk. Always position grills and outdoor fuel-burning equipment well away from any building opening, and never bring them closer during rain or bad weather.

Travel and Seasonal Scenarios

CO incidents also occur in rentals, hotels, cabins, RVs, boats, and older homes — especially during winter or after storms when heating systems run continuously or when generators are used. If you travel frequently, consider a portable CO alarm for extra protection and always locate CO alarms in the property upon arrival.

Warning Signs a CO Source May Be Malfunctioning

CO itself has no warning odor, but some situations suggest a combustion or venting problem:

• Soot or staining around an appliance, vent, or fireplace.
• A pilot light that frequently goes out or a flame that looks unusual.
• Excess condensation on windows or walls near fuel-burning appliances.
• Stale or stuffy air when heaters or fireplaces are running.
• People or pets feeling sick at the same time, especially when indoors.

These signs are not definitive, but they justify caution and professional inspection — especially if a CO alarm has sounded or symptoms are present.

To narrow down which appliance may be the problem: for furnaces and boilers, listen for unusual cycling patterns (turning on and off frequently) and check the area around the unit for soot or a burning smell. For water heaters, look at the vent pipe connections — gaps, rust, or disconnections can allow carbon monoxide from furnace and water heater exhaust to spill indoors. For fireplaces, hold a lit match near the damper opening when cold — if the smoke drifts into the room instead of up the chimney, you may have a draft or blockage issue. In every case, a qualified technician with calibrated equipment should confirm and resolve the problem.

What to Do If You Suspect a CO Source

If your CO alarm sounds or you notice warning signs near a fuel-burning appliance, leave the building to fresh air first and call for professional guidance. Stop using the suspected appliance if it is safe to do so, but do not attempt DIY repairs on combustion equipment.

After the immediate risk is addressed, arrange a qualified inspection of the appliance and venting before using it again.

When arranging inspection, the right professional depends on the suspected source: call an HVAC technician for furnace, boiler, or air conditioning issues; contact your gas utility company if you suspect a gas leak or smell gas; hire a certified chimney sweep for fireplace or wood stove problems; and reach out to a licensed plumber for water heater venting concerns. For carbon monoxide from generator situations, the solution is usually repositioning — generators must always be operated outdoors, far from any building opening.

Emergency response steps: What to do if you suspect a carbon monoxide leak. Detection guide: How to detect a carbon monoxide leak. Prevention checklist.

Quick Source Reference

Sources & References

• US CPSC — Carbon Monoxide Information Center
• CDC — Carbon Monoxide Poisoning
• US EPA — Carbon Monoxide's Impact on Indoor Air Quality
• US CPSC — Generator Safety
• NFPA — Carbon Monoxide Safety

Last updated: February 15, 2026