Carbon Monoxide From Furnaces: Causes, Symptoms, Detection, and Prevention

Meta Description: Learn how carbon monoxide from furnaces happens, symptoms, alarm placement, testing, and prevention tips to keep your home safe. Signs, fixes, and code guidance.

Carbon monoxide from a furnace is a serious safety risk that is preventable with the right knowledge and maintenance. This guide explains how a furnace can produce CO, how to spot warning signs, where to place CO alarms, and what steps to take to protect a home. It provides U.S.-specific safety guidelines and practical tips that reduce risk without overcomplicating maintenance.

Why Furnaces Can Produce Carbon Monoxide

Carbon monoxide (CO) is a colorless, odorless gas produced by incomplete combustion. A properly tuned furnace vents exhaust outside. When combustion or venting goes wrong, carbon monoxide from the furnace can accumulate indoors.

• Incomplete Combustion: Dirty burners, incorrect gas pressure, or misaligned flames cause poor fuel-air mixing and elevate CO.
• Cracked Heat Exchanger: Separates flue gases from indoor air. Cracks can allow exhaust to leak into supply air.
• Blocked Or Damaged Venting: Bird nests, snow, rust, or a disconnected vent can force gases back into rooms.
• Backdrafting/Negative Pressure: Powerful exhaust fans, dryers, or tight homes pull combustion gases from the flue into the house.
• Insufficient Combustion Air: A sealed house or a confined furnace room starves the flame of oxygen, increasing CO.
• Failed Inducer Or Fan: On induced-draft or condensing furnaces, a broken inducer or pressure switch can impair safe venting.
• Improper Installation: Wrong flue size, too many elbows, or long horizontal runs can cause spillage and CO.
• High Altitude Issues: Less oxygen at elevation requires manufacturer-rated adjustments; otherwise CO rises.

Gas, Oil, And Electric: What Differs

Natural gas and propane furnaces can produce CO if combustion is faulty. Oil furnaces also pose CO risks and often show soot when out of tune. Electric furnaces do not create CO, but a shared vented appliance, attached garage, or generator can still elevate CO in the home.

Venting Types And Risk Levels

Older atmospheric furnaces (Category I) use room air for combustion and vent into a chimney, so they are more susceptible to backdrafting. Sealed combustion/direct-vent furnaces (Category IV) draw outdoor air and vent through sealed piping, reducing indoor CO risk when installed correctly.

Health Risks And Exposure Limits

CO binds to hemoglobin more readily than oxygen, reducing oxygen delivery to tissues. Symptoms of carbon monoxide poisoning from a furnace can be subtle and mistaken for flu, especially in winter.

• Early: Headache, dizziness, nausea, fatigue.
• Worsening: Confusion, chest pain, shortness of breath, vomiting.
• Severe: Loss of consciousness, seizures, cardiac arrest, death.

Infants, pregnant people, older adults, those with heart or lung disease, and pets are more susceptible. If symptoms improve outside but return at home, suspect furnace carbon monoxide or another CO source.

Warning Signs Of A Furnace CO Problem

CO itself has no smell, taste, or color. Look for indirect clues that combustion or venting is compromised.

• Alarm Activity: Any measurable reading or alarm on a CO monitor, especially near the furnace or bedrooms.
• Soot Or Streaking: Black soot or scorch marks around burners, vent connectors, or appliance draft hood.
• Yellow Or Flickering Flames: Healthy natural gas flames are mostly blue; persistent yellow indicates incomplete combustion.
• Condensation Or Rust: Excess moisture on windows near the furnace, rusted flue pipes, or corrosion on the heat exchanger.
• Backdraft Evidence: Warm air spilling from the draft hood, smoke test failing, or exhaust smell when nearby appliances run.
• Frequent Pilot Or Burner Outages: Nuisance shutdowns can indicate venting or pressure problems.
• Household Symptoms: Headaches or nausea that worsen at home, improve outdoors, and affect multiple people or pets.

CO Alarms: Types, Placement, And Maintenance

Types Of CO Alarms And Monitors

• UL 2034 Residential CO Alarms: Designed to alert at life-threatening levels and reduce nuisance alarms; may not display low-level readings.
• Low-Level CO Monitors: Some non-UL-2034 devices display lower ppm; useful for sensitive populations. Choose reputable brands.
• Combination Smoke/CO Alarms: Save space and enable interconnection; confirm compliance with UL 217 (smoke) and UL 2034 (CO).
• System-Connected Detectors: Hardwired to security or fire systems; many conform to UL 2075.

Best Practices For Placement

• Every Level: Install CO alarms on each floor, including the basement, and outside each sleeping area. Many codes also require inside bedrooms.
• Location Height: CO mixes with air; follow manufacturer instructions. Wall or ceiling mounting is typically acceptable.
• Avoid Dead Zones: Keep away from corners, behind curtains, near HVAC supply registers, or within 5 feet of cooking appliances or bathrooms.
• Near Attached Garage: Place an alarm near the door from an attached garage to living spaces.
• Interconnect: Interconnected alarms (wired or wireless) sound together so sleeping occupants hear alerts.

Testing, Lifespan, And Reliability

• Test Monthly: Use the test button; replace batteries at least annually unless using sealed 10-year models.
• Replace On Schedule: Most CO alarms last 5–7 years; check the manufacture date and end-of-life chirps.
• Do Not Ignore Chirps: Low-battery or end-of-life signals require immediate action to maintain protection.

For guidance, see the U.S. Consumer Product Safety Commission: CPSC CO Safety, and the CDC: CDC Carbon Monoxide.

Preventing Carbon Monoxide From Furnaces

Homeowner Safety Checklist

• Schedule Annual Service: Have a qualified HVAC technician inspect and tune the furnace before heating season.
• Keep Vents Clear: After storms, clear snow, ice, and debris from sidewall or roof vents and intake pipes.
• Change Filters Regularly: A clogged filter can overheat the heat exchanger and affect combustion air.
• Protect Combustion Air: Keep the furnace room unobstructed; do not store chemicals or gasoline nearby.
• Watch Flame Quality: Report persistent yellow flames or soot to a professional immediately.
• Avoid Negative Pressure: Do not run powerful exhaust fans continuously in tight homes without makeup air.
• Install And Maintain CO Alarms: Place alarms correctly and test them often.

What A Professional Tune-Up Should Include

A qualified technician does more than “clean and check.” Ask for measurable results. Objective testing is the best defense against carbon monoxide from a furnace.

Ask for the combustion analyzer printout or readings. A good contractor will document CO, O₂, stack temperature, and draft before and after service.

Combustion Air And Venting Best Practices

• Provide Makeup Air: In tight homes or confined spaces, add dedicated combustion air per IRC and manufacturer instructions.
• Line Masonry Chimneys: Older chimneys may need liners sized for the appliance to avoid condensation and spillage.
• Follow Venting Category Rules: Use approved materials (e.g., PVC/CPVC/PP for condensing furnaces) and maintain slope toward the drain.
• Maintain Clearances: Keep terminations away from windows, doors, and grade per the installation manual.
• Avoid Shared Vent Problems: When multiple appliances share a flue, ensure sizing and sequencing prevent backdrafting.

What To Do If A CO Alarm Sounds Or A Leak Is Suspected

1. Move To Fresh Air Immediately: Evacuate everyone, including pets, to outdoors or by an open door or window.
2. Call 911 Or Your Fire Department: Do not re-enter until responders say it is safe.
3. Shut Off The Furnace If You Can Do So Quickly And Safely: Do not linger in a contaminated area.
4. Seek Medical Care: Tell clinicians you may have CO exposure; blood COHb testing may be needed.
5. Do Not Ventilate Before Testing If You’re Already Outside: Opening windows can make it harder for responders to find the source. Safety comes first.
6. Schedule A Qualified HVAC Inspection: Correct the cause before restarting the furnace.

After an incident, document repairs, keep service records, and consider adding a low-level CO monitor in addition to code-required alarms.

Common Furnace CO Problems, Fixes, And Costs

Costs vary by region and equipment. Always verify repairs with combustion testing.

Codes, Standards, And U.S. Guidance

Most U.S. jurisdictions follow versions of the International Residential Code and require CO alarms outside sleeping areas, and sometimes inside bedrooms, especially where fuel-burning appliances or attached garages exist. Requirements vary by state and city.

• UL 2034: Standard for residential CO alarms; explains response times and limits.
• UL 2075: Standard for system-connected CO detectors.
• NFPA 72: National Fire Alarm and Signaling Code now includes CO detection provisions.
• NFPA 54 (National Fuel Gas Code): Installation requirements for gas-burning appliances and venting.
• EPA And CPSC Guidance: Practical steps for preventing CO exposure at home.

Landlords in many states must provide CO alarms in rental units. Homeowners selling a property may need to certify that alarms are installed. Check local code officials or state websites for specifics.

Key resources: EPA CO Indoors, CPSC CO Center, CDC CO Poisoning, and NFPA.

Technical Notes For Better Furnace Safety

Combustion Targets And Readings

When technicians measure combustion, they often report “air-free” CO, O₂, CO₂, stack temperature, and draft. Lower CO is better, and many residential gas furnaces should operate below roughly 100 ppm CO air-free. Some modern units achieve under 25 ppm. Always follow the manufacturer’s specifications.

Oil appliances are tuned to a smoke number of zero or trace with CO kept low. Draft should be stable and meet appliance requirements under worst-case conditions, including all exhaust fans operating.

Backdraft Diagnostics

Professionals may perform a “worst-case depressurization” test of the combustion appliance zone (CAZ). If house pressure is too negative when exhaust devices run, the flue can reverse. Solutions include adding dedicated makeup air, sealing leaky return ducts, and balancing ventilation.

Altitude And Orifices

At higher elevations, less oxygen means furnaces often need derating or different orifices. Follow the installation manual for altitude adjustments and ensure the gas valve pressure is set correctly for local conditions and fuel type.

FAQs About Carbon Monoxide And Furnaces

Does A New High-Efficiency Furnace Eliminate CO Risk? No. Condensing, sealed-combustion furnaces greatly reduce risk but still require correct installation, venting, and annual professional service with combustion testing.

Can I Smell Carbon Monoxide From A Furnace? No. CO has no smell. You might notice combustion odors or aldehydes from an appliance that is malfunctioning, but a CO alarm is the only reliable alert.

Where Should I Put A CO Alarm If My Furnace Is In The Basement? Place one on every level, including the basement, and outside each sleeping area. Consider an alarm in each bedroom for best protection and code compliance.

My CO Alarm Briefly Showed 10–20 Ppm. Is That Dangerous? Short, low-level spikes can occur, but sustained readings should be investigated. If symptoms occur or readings persist, call a professional. Ensure all combustion appliances are tuned and vented properly.

Will An Electric Space Heater Cause CO? No. Electric heaters do not burn fuel. Any fuel-burning device (furnace, boiler, fireplace, generator, vehicle in an attached garage) can produce CO.

Should I Open Windows If The Alarm Sounds? Get everyone outside first and call 911. If you must remain inside momentarily to exit, opening a door or window for fresh air can help, but do not stay to ventilate. Let responders test and clear the home.

How Often Should A Furnace Be Serviced? At least annually. Schedule maintenance before heating season and request a written report with combustion numbers and draft measurements.

Can A Cracked Heat Exchanger Be Repaired? No. It requires exchanger replacement or a new furnace. Operating with a cracked exchanger is unsafe and can cause a carbon monoxide leak from the furnace.

Practical Tips To Reduce CO Risk This Winter

• After Heavy Snow: Clear sidewall intake and exhaust pipes, and check chimney caps.
• Before Parties Or Large Gatherings: Avoid running big range hoods or multiple exhaust fans for long periods without makeup air.
• During Power Outages: Never use generators or grills in garages, near open windows, or indoors; CO can infiltrate a home and set off alarms.
• After Remodeling: Air sealing can change house pressure. Re-test combustion safety and confirm adequate makeup air.
• When Buying A Home: Ask for a heating inspection with combustion analysis and confirm code-compliant CO alarms are installed.

Staying safe from carbon monoxide from furnaces centers on three pillars: reliable CO alarms, annual professional service with measurable testing, and homeowner vigilance around venting, airflow, and warning signs. Following these practices keeps the heating system efficient and the household safe.