Carbon monoxide (CO) is a colorless, odorless gas that can be produced when fossil fuels burn in heating equipment like furnaces. Modern gas and oil furnaces are designed to minimize CO emissions, but failures in combustion, venting, or heat exchange can allow CO to enter living spaces. This article explains how furnaces produce CO, the risks across furnace types, practical prevention steps, the role of CO detectors, and what to do if exposure is suspected. Understanding these factors helps homeowners protect occupants and comply with safety guidelines.

How Furnaces Generate Carbon Monoxide

Furnaces generate carbon monoxide when fuel combustion is incomplete. This often happens if oxygen supply is insufficient, the fuel–air mixture is off, or the exhaust path is blocked. Common failures include dirty burners, a cracked heat exchanger, a blocked flue or vent, or improper venting relative to the appliance’s design. Incomplete combustion yields CO instead of the expected carbon dioxide, and poorly vented systems can allow CO to seep into living spaces rather than being vented outdoors.

Incomplete combustion due to improper fuel-air mix or dirty components.
Blocked or restricted vents that prevent exhaust from escaping.
Heat exchanger damage allowing CO to bypass the exhaust path.
Backdrafting where exhaust re-enters the home.
Malfunctioning safety controls that fail to shut down the furnace when needed.

Even with high-efficiency models, CO can be produced if a system is not correctly installed or maintained. In homes with combustion appliances, a venting issue or fault can create CO entry routes independent of outdoor conditions such as wind or snow buildup.

Types Of Furnaces And CO Risk

CO risk varies by furnace type. Gas, oil, and propane furnaces burn fossil fuels and can emit CO if combustion is incomplete or venting fails. Electric furnaces, in contrast, do not burn fuel and do not produce CO during operation; any CO detected in the home would come from other sources or from electrical faults or appliances nearby.

Gas furnaces can produce CO if burners are dirty, the heat exchanger cracks, or venting is blocked.
Oil furnaces carry similar risks; routine maintenance is essential to prevent CO formation.
Propane furnaces rely on clean combustion; leaks or improper venting increase CO potential.
Electric furnaces generally do not emit CO, but associated equipment or nearby combustion sources can still create CO in the home.

High-efficiency sealed-combustion models reduce ambient CO risk by drawing combustion air from outside and venting directly outdoors, but they are not immune to faults. Regular service remains critical across all furnace types to prevent CO buildup.

Signs Of CO Exposure And Symptoms

CO poisoning can develop slowly or rapidly, and symptoms often resemble flu or food poisoning, complicating detection. Early signs include headache, dizziness, confusion, shortness of breath, nausea, vomiting, and fatigue. Prolonged exposure can cause chest pain in people with heart disease, impaired thinking, blurred vision, fainting, and, in severe cases, coma or death. Infants, pregnant individuals, older adults, and people with chronic heart or lung conditions are particularly vulnerable to CO’s effects.

Early symptoms mirror common illnesses, making detection difficult without a CO detector.
Severe exposure requires immediate emergency intervention.
Persistent symptoms after leaving the area may indicate lingering CO presence and warrant testing.

Because CO is odorless and colorless, detectors and safe ventilation practices are essential to identify and mitigate exposure before symptoms develop.

Preventing CO Production: Maintenance, Venting, And Combustion Efficiency

Preventing CO production hinges on proper installation, regular maintenance, and robust venting. Homeowners should schedule annual inspections with licensed HVAC professionals who can assess combustion efficiency, test for CO, and verify venting integrity. Routine tasks like filter changes, burner cleaning, and checking the heat exchanger for cracks reduce CO risk. Ensuring that the furnace and its venting system are appropriately sized, installed, and sealed minimizes backdrafts and leakage.

Annual professional inspection of burners, heat exchanger, venting, and safety controls.
Regular burner cleaning to maintain a proper fuel-air mix.
Vent and flue inspection to confirm no blockages, corrosion, or leaks.
Heat exchanger integrity to prevent CO from entering living spaces.
Proper furnace sizing to avoid excessive cycling and incomplete combustion.

Homeowners should also ensure good ventilation for gas or propane appliances, especially in tightly sealed homes. If a furnace is old or performing poorly, upgrading to a newer, sealed-combustion model can significantly reduce CO risk while improving efficiency and comfort.

CO Detectors And Safety Systems

CO detectors are a key defense against exposure. Install detectors on every living area floor, especially outside bedrooms or sleeping zones, and in or near rooms with fuel-burning appliances. Choose a combination of hardwired detectors with battery backup or battery-powered units if wiring isn’t feasible. Test detectors monthly, replace batteries as needed, and replace units every 5 to 7 years according to manufacturer guidance.

Detector types electrochemical sensors offer reliable response to CO levels.
Placement near sleeping areas and on each level of the home.
Maintenance regular testing and timely replacement.
Integrated safety use of detectors alongside automatic furnace shutoffs in some systems.

Detectors should not be placed too close to fuel-burning appliances, in garages, or behind furniture where airflow is blocked. They should be mounted high enough in case CO rises with warm indoor air, but also follow the specific product guidance.

What To Do If You Suspect CO Poisoning

If CO exposure is suspected, act quickly and safely. Move to fresh air immediately, then call emergency services or your local poison control center. Do not re-enter a building with a confirmed or suspected CO incident until responders declare it safe. If you can do so safely, turn off the furnace and any other fuel-burning appliances from a safe location. Seek medical evaluation, especially if symptoms are present or if there are vulnerable individuals in the home.

Move to fresh air right away if symptoms appear or a detector alarms.
Call emergency services for suspected poisoning or alarm activation.
Shut off the furnace only if it is safe to do so from a non-contaminated area.
Seek medical help even if symptoms fade after leaving the area.

After a CO event, have a qualified technician inspect the system and identify the cause before returning to use. Detectors should be tested, and any faulty equipment should be repaired or replaced to prevent recurrence.

Special Considerations For Homes With Attached Garages And Old Furnaces

Homes with attached garages face a heightened CO risk because car exhaust can backdraft into living spaces, especially if doors or vents are shared with the garage. When a car runs, CO can accumulate if the garage is not properly sealed or vented. Additionally, older furnaces may have degraded heat exchangers or venting problems that increase CO production. In these scenarios, upgrades to a sealed-combustion furnace and enhanced venting can dramatically reduce CO risk and improve overall safety.

Garage-related risks car exhaust can infiltrate the home during door openings or poor sealing.
Older equipment is more prone to cracks or corrosion in the heat exchanger.
Upgrades to sealed combustion and direct venting reduce ambient CO levels and improve efficiency.

For homes with high CO exposure risk, it is prudent to install multiple detectors, ensure professional maintenance, and consider a furnace replacement if the system is beyond its service life. Keeping areas around furnaces clean and unobstructed helps prevent accidental blockages that could impair venting.