Gas furnaces heat homes by burning natural gas or propane. When operating correctly, combustion products are safely vented outdoors and carbon monoxide (CO) levels remain minimal. CO is odorless, colorless, and highly dangerous in enclosed spaces, so a malfunctioning furnace can release CO into living areas. Understanding how furnaces can generate CO, identifying warning signs, and applying preventive measures helps reduce risk. Regular maintenance, correct installation, and reliable CO detectors are essential components of furnace safety. This article explains how gas furnaces may produce carbon monoxide and what actions safeguard occupants.

How Gas Furnaces May Generate Carbon Monoxide

CO is produced during combustion when fuel does not burn completely. Several conditions can disrupt proper combustion and allow CO to enter indoor spaces. In addition, faults in venting or furnace components can prevent exhaust from venting outside, increasing CO exposure risk.

Key mechanisms by which gas furnaces can generate or release carbon monoxide include:

Incomplete combustion due to insufficient oxygen or improper fuel-air ratios.
Faulty burners or clogged jets that do not burn fuel cleanly.
Damaged or cracked heat exchangers that allow CO to seep into the living space.
Blocked or restricted flues and vent pipes that trap exhaust inside the home.
Backdrafting caused by negative pressure in the home, often from exhaust fans, furnaces, or fireplaces drawing air inward instead of outward.
Malfunctioning induced-draft or exhaust components that fail to remove combustion byproducts.

Even when a furnace appears to operate normally, hidden issues can create CO risks. Routine maintenance helps detect these problems before CO accumulates to hazardous levels.

Common Causes Of CO Buildup In Gas Furnaces

Understanding frequent culprits helps homeowners prioritize inspection and repair. The following causes are among the most common sources of CO buildup in gas furnaces:

Cracked or corroded heat exchanger: A damaged heat exchanger can allow CO to mix with indoor air, especially if gaps widen over time.
Venting obstructions: Snow, debris, nests, or soot can block chimneys or exhaust pipes, forcing CO back into living spaces.
Improper venting installation: Incorrect vent slope, undersized flues, or misdirected exhaust can impede proper exhaust flow.
Backdrafting from other appliances: When multiple fuel-burning devices share a vent or space, pressure differences can pull exhaust back indoors.
Restricted combustion air: In tightly sealed homes, insufficient fresh air can cause incomplete combustion and CO production.
Malfunctioning inducer or draft motor: Faulty components may fail to pull combustion gases through the venting system.
Improper furnace maintenance: Dirty filters, clogged burners, or neglected calibrations can degrade combustion efficiency and raise CO risk.

Signs And Symptoms Of CO Exposure

CO exposure can be difficult to detect without sensors. Early signs are often mistaken for flu or fatigue, delaying a critical response. Recognizing indicators helps prevent serious harm.

Household symptoms: Headache, dizziness, weakness, shortness of breath, chest pain, confusion, nausea, and fainting at higher exposures.
In homes with CO sources: Even mild exposure may cause irritation of the eyes or throat.
For pets: Lethargy, vomiting, or malaise may appear in animals before occupants notice symptoms.
Alarmed detectors: A CO detector alert is a clear sign that indoor CO levels are rising and immediate action is needed.

Any suspicious combination of symptoms in residents or an alarm indicating elevated CO warrants immediate action and professional assessment.

CO Detectors And Safety Equipment

CO detectors are essential safety devices in homes with gas furnaces. They complement smoke alarms and provide early warning of dangerous CO levels. Proper installation and maintenance maximize effectiveness.

Placement: Install detectors on every living level, near sleeping areas, and outside major spaces like kitchens and utility rooms.
Installation guidelines: Follow manufacturer instructions; avoid mounting detectors directly above furnaces, stoves, or humid areas where readings can be skewed.
Testing and maintenance: Test detectors monthly, replace batteries as recommended, and replace the unit according to the manufacturer’s schedule.
Alarm levels: Most residential CO alarms are set to alert around 70 ppm within 1–4 hours, with higher levels triggering sooner. Use multiple detectors for comprehensive coverage.

In addition to detectors, regular furnace maintenance remains essential. Detectors provide warning, while technicians diagnose and fix underlying issues.

Maintenance And Prevention

Regular maintenance reduces the likelihood of CO production and leakage. A proactive maintenance plan focuses on inspection, cleaning, calibration, and venting integrity.

Annual professional inspection: A licensed HVAC technician should inspect burners, heat exchanger integrity, venting, combustion air supply, and safety controls.
Filter and airflow checks: Replace air filters regularly to maintain proper airflow and combustion efficiency.
Ventilation assessment: Ensure adequate combustion air in tight homes and verify that flues are clear and correctly pitched.
Inducer and draft system: Inspect and test the inducer motor and draft hood to confirm proper exhaust flow.
System upgrades: Consider upgrading aging or inefficient equipment to models with improved safety features and better combustion control.

Note: Household DIY repairs of gas furnaces are not recommended. Handling gas appliances without proper training can create additional CO risks or other hazards.

What To Do If CO Is Detected

When carbon monoxide is detected or symptoms of exposure appear, immediate action is required to protect occupants and contact professionals.

Evacuate: Move to fresh air away from the home and call emergency services if anyone shows symptoms or detectors alarm persistently.
Do not re-enter: Do not re-enter until authorities declare the environment safe.
Shut off gas if feasible and safe: If the situation allows, turn off the gas supply to the furnace from the exterior shutoff.
Ventilate after clearance: Once cleared by professionals, ensure proper ventilation and have the furnace inspected before restoring operation.

After a CO incident, a licensed HVAC technician should perform a comprehensive assessment, identify the source, and implement corrective measures before the furnace is recommissioned.

Regulations And Industry Standards

Several codes and standards govern safe furnace operation and CO protection. Adherence helps ensure consistent safety practices across installations and maintenance.

Code requirements: National and local codes typically mandate CO detectors on each living level and near sleeping areas in homes with fuel-burning appliances.
NFPA and fuel gas codes: Standards such as the National Fire Protection Association’s NFPA 54 (National Fuel Gas Code) address combustion safety, venting, and appliance installation.
Manufacturer guidelines: Furnaces should be installed and serviced according to the appliance manufacturer’s instructions, with periodic certifications for safety.

Homeowners should consult local regulations and engage licensed professionals for installation, inspection, and repair to ensure compliance and safety.

Costs And Maintenance Considerations

Owning a gas furnace includes ongoing maintenance costs, but proactive steps can reduce risk and long-term expenses. Annual inspections help avert expensive repairs and CO-related hazards.

Annual inspection cost: Typical HVAC inspections range from moderate to higher fees depending on region and service scope, but the investment reduces risk and can extend equipment life.
Detector expenditures: CO detectors are affordable, with maintenance typically requiring battery replacements and periodic replacement every several years.
Repair vs. replacement: Addressing small issues early is often far cheaper than managing extensive damage from CO exposure or failed heat exchangers.
Energy efficiency benefits: Regular maintenance improves furnace efficiency, reduces fuel use, and minimizes emissions beyond safety considerations.

Overall, the cost of prevention—through detectors and annual professional service—pays off by enhancing safety, comfort, and long-term energy savings.