Can a Furnace Leak Carbon Monoxide When Turned Off? Causes, Risks, and Prevention

The possibility that a furnace can leak carbon monoxide (CO) when turned off raises important safety concerns for homeowners. This article explains how CO can be produced or migrate from a furnace even when the system is not operating, outlines common causes, identifies risks and symptoms, and provides prevention and detection strategies to reduce hazards.

Topic	Key Points
Can A Furnace Leak CO When Off	Yes — through backdrafting, cracked heat exchangers, shared venting, pilot light or nearby appliance leaks, and lingering residual gas
Main Causes	Heat exchanger cracks, improper venting, negative pressure, flue blockages, and improper appliance installation
Detection	CO alarms, professional combustion testing, flue gas analyzers, and visual vent inspections
Prevention	Annual maintenance, CO alarms, proper venting, sealing combustion air, and immediate repairs

How Carbon Monoxide Is Produced And Why It Matters

Carbon monoxide is an odorless, colorless, and potentially deadly gas formed by incomplete combustion of carbon-based fuels such as natural gas, propane, oil, and wood. Furnaces that burn these fuels produce CO during normal operation if combustion is incomplete or venting is impaired.

Health risks include headache, dizziness, nausea, confusion, loss of consciousness, and death at high concentrations. Chronic low-level exposure can cause long-term neurological effects.

Can A Furnace Leak Carbon Monoxide When Turned Off?

Yes. While a furnace typically produces CO when burning fuel, CO can still enter living spaces when the furnace is off through several mechanisms. Understanding these mechanisms clarifies why a turned-off furnace is not always risk-free.

Backdrafting And Negative Pressure

Backdrafting occurs when pressure in the home pulls flue gases back down the vent instead of allowing them to rise outdoors. This can happen when exhaust fans, range hoods, or clothes dryers create negative pressure, or when wind and stack effect alter vent flow. Even with the furnace off, residual CO in the flue or nearby combustion appliances can be drawn into the house.

Cracked Or Corroded Heat Exchanger

A damaged heat exchanger can allow combustion gases to mix with the furnace’s airflow. If a furnace was recently running, CO can remain in the appliance or ducts and migrate into the home when pressure changes occur, even after the burner shuts off.

Shared Or Improper Venting

Closely spaced or shared venting for multiple appliances can allow CO from one appliance to enter another appliance’s draft hood or flue. When the furnace is off, a nearby operating water heater or boiler could cause CO to enter the furnace vent system and then the living space.

Pilot Lights, Standing Pilots, And Ignition Systems

Older furnaces with standing pilot lights or other pilot systems constantly burn a small flame. Malfunctioning pilots or faulty components can emit CO continuously, independent of the main burner. Even modern ignition systems can leak if seals or valves fail.

Residual Gas Accumulation

After a furnace cycles off, pockets of CO-rich air can remain in the flue, basements, or crawl spaces. House pressure changes or door openings can allow those pockets to enter living spaces. Motorized dampers and zone controls can also trap gases that later escape.

Common Causes And Failure Scenarios

1. Cracked Heat Exchanger

A cracked heat exchanger is one of the most dangerous faults. Cracks can develop from corrosion, age, or thermal stress. This allows combustion gases to bypass the flue and enter the blower or plenum, posing a CO risk even when the burner is off if gases linger.

2. Blocked Flue Or Chimney

Bird nests, debris, snow, or creosote buildup can partially block a flue. Intermittent venting leads to gas accumulation and backdrafting. Blockages can trap CO that later enters the home during pressure fluctuations.

3. Improper Installation Or Vent Configuration

Poorly installed venting, incorrect slope, or undersized flues reduce natural draft efficiency. Shared chimneys and too many vented appliances on one flue increase the chance that one appliance’s emissions will enter another’s vent path.

4. Negative House Pressure

Homes made tightly energy-efficient without adequate make-up air can develop negative pressure. Exhaust fans, range hoods, fireplace use, or running ventilation systems can create suction that pulls flue gases into the living area.

5. Malfunctioning Valves Or Seals

Gas valves, draft hoods, and gasket seals degrade over time. Leaks in these components allow gas to escape into the furnace cabinet or adjacent spaces, potentially entering the home when the system cycles or when pressure changes occur.

Signs And Symptoms Of Carbon Monoxide Exposure

Recognizing CO exposure helps prompt action. Symptoms are often described as flu-like and may escalate quickly at higher concentrations.

• Headache, dizziness, and weakness
• Nausea, vomiting, and shortness of breath
• Confusion, blurred vision, and difficulty thinking
• Loss of consciousness or sudden collapse
• Multiple household members experiencing similar symptoms

Important: Symptoms that improve when leaving the home and return upon re-entering strongly suggest CO in the living space.

Detection Methods For Carbon Monoxide

CO Alarms And Placement

UL-listed residential CO alarms are essential. Place alarms near sleeping areas, on each level of the home, and near attached garages. Replace alarms per manufacturer guidance and test them monthly.

Professional Combustion Testing

HVAC technicians use combustion analyzers to measure CO levels in the flue and at the appliance. A certified technician can perform draft tests, measure CO under normal and worst-case conditions, and inspect heat exchangers with proper tools.

Visual Inspection And Smoke Testing

Visual inspection for soot, staining, rust, or corrosion around the furnace, flue, or vent connectors can reveal combustion or venting issues. Smoke testing helps identify backdrafting or leakage paths during diagnostic checks.

Immediate Actions If Carbon Monoxide Is Suspected

If CO exposure is suspected, act immediately to reduce harm and determine the source.

1. Evacuate everyone from the home to fresh air and call 911 for severe symptoms or multiple illnesses.
2. Contact local emergency services if CO alarms sound or symptoms are present.
3. Do not re-enter the building until it has been ventilated and inspected by professionals.
4. Call a qualified HVAC or gas appliance technician to perform combustion testing and repairs before using the furnace again.

Prevention And Maintenance Strategies

Regular maintenance and good household practices reduce the risk of CO leaks from a furnace when it is off or on.

Annual Professional Inspections

Schedule yearly furnace inspections by a licensed HVAC technician. Inspections should include combustion analysis, heat exchanger inspection, vent system checks, and verification of proper draft under normal and worst-case conditions.

Install And Maintain CO Alarms

Install multiple CO alarms per manufacturer and local codes. Replace units according to the expiration date and replace batteries as needed to ensure continuous protection.

Ensure Proper Venting And Chimney Care

Keep vents and chimneys clear of obstructions such as bird nests and debris. Ensure proper flue sizing and separation of appliance vents. Consider annual chimney cleaning if applicable.

Address Negative Pressure And Provide Makeup Air

Balance exhaust appliances and provide makeup air for tightly sealed homes. Use powered make-up air when heavy exhaust equipment operates, and avoid running multiple large exhaust devices simultaneously in older homes without adequate ventilation.

Repair Or Replace Faulty Equipment

Repair cracked heat exchangers, malfunctioning valves, and deteriorated seals immediately. Replace aging furnaces when repairs are insufficient or when heat exchanger damage compromises safety.

Regulations, Standards, And Professional Guidance

Building codes and standards guide safe installation and operation of fuel-burning appliances. Local codes often require CO alarms and proper venting. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), National Fire Protection Association (NFPA), and manufacturers publish guidelines for safe combustion appliance operation.

Homeowners should follow local code requirements and rely on licensed professionals for inspections, repairs, and verification testing.

When To Replace A Furnace

Replacement should be considered when repairs are frequent, the heat exchanger is cracked, or the unit is old and inefficient. Replacing an unsafe furnace prevents recurring CO risks and offers improved efficiency and reliability.

Signs that replacement may be necessary include persistent CO readings during inspections, visible corrosion, frequent burner issues, and rising repair costs relative to replacement.

Final Practical Tips For Homeowners

• Place CO alarms near bedrooms and on each living level.
• Schedule annual maintenance before heating season.
• Keep vents and chimneys clear year-round.
• Avoid indoor use of generators, grills, or combustion devices near openings.
• If multiple people become ill in a house, treat it as a potential CO incident until proven otherwise.

Key takeaway: While furnaces primarily produce carbon monoxide when operating, CO can still enter a home when a furnace is turned off through mechanisms like backdrafting, cracked heat exchangers, blocked vents, and pressure changes. Regular maintenance, CO alarms, professional testing, and prompt repairs are essential to reduce the risk.

For specific concerns about a furnace or suspected CO leak, consult a licensed HVAC technician or local emergency services immediately.