Do Home Furnaces Produce Carbon Monoxide and How to Prevent It

The article explains whether home furnaces produce carbon monoxide (CO), the causes, symptoms of CO exposure, detection methods, maintenance tips, and safety measures to reduce risk. It focuses on natural gas, propane, oil, and wood-burning systems used in U.S. homes and provides actionable guidance for homeowners and renters.

Key Point	Summary
Do Furnaces Produce CO?	Yes—Incomplete combustion in fuel-burning furnaces can produce CO.
Main Causes	Blocked vents, cracked heat exchangers, poor maintenance, improper installation.
Detection	CO alarms, annual inspections, visual checks for soot or yellow burners.
Prevention	Regular maintenance, proper ventilation, installed and working CO detectors.

How Furnaces Can Produce Carbon Monoxide

Furnaces that burn fossil fuels—natural gas, propane, heating oil—or solid fuels such as wood produce combustion gases. When combustion is complete, carbon converts primarily to carbon dioxide (CO2). Incomplete combustion creates carbon monoxide, a colorless, odorless gas that is toxic at low concentrations.

Common furnace-related causes of CO production include blocked flues or chimneys, cracked heat exchangers, incorrect gas pressure or burner adjustment, and insufficient ventilation. Any condition that prevents adequate oxygen from mixing with fuel during combustion increases CO risk.

Types Of Home Heating Systems And CO Risk

Natural Gas Furnaces

Natural gas furnaces are common in U.S. homes and are efficient when properly maintained. Risks arise from blocked flues, malfunctioning burners, or a cracked heat exchanger that allows CO to enter the living space.

Propane Furnaces

Propane furnaces behave similarly to natural gas models: proper ventilation and burner tuning are essential. Propane combustion can produce CO if the air/fuel mixture is wrong or exhaust is obstructed.

Oil Furnaces

Oil-fired furnaces have separate risks due to soot buildup and burner inefficiencies. Poorly adjusted burners and clogged nozzles can lead to incomplete combustion and elevated CO levels.

Wood-Burning Furnaces And Stoves

Wood stoves and furnaces can generate significant CO, especially during startup or smoldering fires. Creosote buildup and blocked chimneys greatly increase the chance that CO or smoke will spill into the home.

How Carbon Monoxide Enters The Home

CO typically travels from the appliance’s combustion chamber into the conditioned space through air leaks, cracked heat exchangers, or backdrafting when exhaust gases fail to exit through the flue. Backdrafting can occur when outdoor wind pressure or indoor negative pressure reverses the exhaust flow.

Shared venting systems, improperly connected ducts, and chimneys serving multiple appliances increase the complexity and risk of CO migration between spaces.

Symptoms Of Carbon Monoxide Exposure

CO binds to hemoglobin, reducing the blood’s capacity to carry oxygen. Symptoms vary with concentration and exposure time. Early signs include headache, dizziness, nausea, and fatigue.

At higher levels or prolonged exposure, symptoms escalate to confusion, loss of consciousness, and death. Children, pregnant people, and those with heart or respiratory conditions are more vulnerable.

How CO Is Measured And Health Thresholds

CO concentrations are measured in parts per million (ppm). The U.S. Environmental Protection Agency and occupational standards provide exposure thresholds:

• 35 ppm is the NAAQS 1-hour recommendation for indoor short-term exposure (varies by jurisdiction).
• 200 ppm can cause headache and other symptoms within a few hours.
• 400–800 ppm may lead to life-threatening conditions and death within hours.

Home CO alarms typically trigger at lower thresholds to provide an early warning and allow occupants to ventilate or evacuate.

Detection: Alarms, Indicators, And Professional Testing

CO alarms are the primary defense for homeowners. They should be installed outside sleeping areas and on every level of the home, per manufacturer and local code guidance. Replace CO detectors every 5–7 years or per the device instructions.

Signs of a furnace producing CO include soot streaks, a yellow or irregular burner flame instead of a steady blue flame, pilot light outages, and excessive condensation near the furnace or flue connection.

For suspected CO problems, a qualified HVAC technician can perform combustion analysis with a flue gas analyzer and pressure tests to identify backdrafting or leaks. Local utilities sometimes offer carbon monoxide or combustion safety inspections.

Maintenance And Prevention Strategies

Regular maintenance significantly reduces the risk of CO production. Key tasks include annual professional furnace inspections, cleaning burners, checking and sealing the heat exchanger, and keeping vents and chimneys clear.

• Annual Tune-Up: Have a qualified technician inspect combustion, heat exchanger integrity, and venting systems yearly.
• Replace Filters: Change air filters regularly to maintain airflow and efficient combustion.
• Clear Vents: Keep outdoor intake and exhaust vents free of debris, snow, and nests.
• Chimney Maintenance: Sweep chimneys and flues to remove creosote and blockages for oil or wood systems.

What To Do If A CO Alarm Sounds

If a CO alarm activates, evacuate the home immediately and call 911 or your local emergency number from outside. Do not re-enter until first responders or a qualified technician confirm it is safe.

If only low-level CO is suspected, ventilate the home by opening windows and doors, shut off fuel-burning appliances if safe to do so, and contact a professional for inspection.

Installation And Placement Of CO Alarms

Install alarms in hallways near bedrooms and on each level, including basements. Avoid placing alarms directly above combustion appliances or in very humid or dusty areas that can cause false readings.

Combination smoke/CO units are convenient but verify they meet UL standards for both hazards. Test alarms monthly and replace batteries per manufacturer guidelines, using long-life sealed batteries if possible.

When To Call A Professional

Call a licensed HVAC technician if there are signs of combustion problems, persistent CO alarm activations, soot buildup, fuel smell, or visible corrosion or cracks on the furnace. Professionals can perform a combustion efficiency test, heat exchanger inspection, and flue draft assessment.

In urgent situations where occupants feel ill or multiple alarms sound, occupants should leave and alert emergency services before waiting for a technician.

Codes, Regulations, And Insurance Considerations

Building codes and local ordinances often mandate CO alarm installation in new or renovated homes. Many states require CO alarms in rental properties and near sleeping areas. Homeowners should consult local code enforcement for exact requirements.

Failing to maintain heating equipment may affect home insurance claims if CO exposure or damage occurs. Document maintenance records and professional inspections to support compliance and claims if necessary.

Common Myths And Misconceptions

Myth: “A furnace that is on cannot produce CO.” Reality: Even running furnaces can produce CO if combustion is incomplete due to malfunction or venting issues.

Myth: “CO has a smell.” Reality: CO is odorless and colorless; detecting it relies on alarms and indicators, not smell.

Myth: “A single CO alarm is enough for any home.” Reality: Multiple alarms on each level and near sleeping areas provide better protection and earlier warning.

Practical Checklist For Homeowners

• Install CO alarms outside sleeping areas and on every level.
• Schedule annual furnace inspection and tune-up by a licensed technician.
• Keep vents, flues, and chimneys clear of debris and nests.
• Change air filters regularly to maintain airflow and efficiency.
• Watch for signs like soot, yellow flames, and pilot outages; call a pro if observed.
• Have combustion appliances tested if relocating a furnace or changing vent configurations.
• Keep a copy of maintenance records and technician reports for reference and insurance.

When Remodeling Or Replacing A Furnace

When upgrading a furnace or modifying venting, ensure a licensed installer follows manufacturer instructions and local codes. Proper sizing, exhaust type (direct vent vs. atmospheric vent), and correct vent termination are critical to preventing CO issues.

Consider high-efficiency sealed combustion furnaces that draw air from outdoors and vent directly outside, which reduce the risk of indoor CO entry compared with atmospheric vent appliances.

Resources And Further Reading

Reliable sources for more information include the U.S. Consumer Product Safety Commission, Centers for Disease Control and Prevention, U.S. Environmental Protection Agency, and the National Fire Protection Association. Local utilities and state public health departments also provide guidance and inspection programs.

For immediate safety questions or to report a CO incident, contact local emergency services, a licensed HVAC contractor, or the gas utility company.

Keywords: Do Home Furnaces Produce Carbon Monoxide, furnace carbon monoxide, CO alarms, furnace maintenance, heat exchanger, backdrafting, combustion analysis.