Carbon Monoxide Safety and Furnaces: What Ontario Homeowners Must Know

What Is Carbon Monoxide and Why Is It Dangerous

Carbon monoxide (CO) is an odourless, colourless, tasteless gas produced when fuel burns incompletely. You cannot see it, smell it, or taste it — making it impossible to detect without a functioning CO detector. Every year in Canada, approximately 300 people are hospitalized and 50 die from carbon monoxide poisoning, many of them from residential heating equipment during the winter months.

How CO affects the body

When inhaled, carbon monoxide binds to haemoglobin in the blood 200-250 times more readily than oxygen does. This means even small concentrations of CO in the air can progressively saturate your blood's ability to carry oxygen to your brain, heart, and muscles. The effects are cumulative — low-level exposure over hours produces the same result as higher-level exposure over minutes. Initial symptoms mimic the flu (headache, nausea, fatigue), which is why CO poisoning is frequently misdiagnosed or ignored until it reaches dangerous levels. Because CO accumulates in the bloodstream, sleeping occupants are at greatest risk — they may lose consciousness without ever waking.

Why furnaces are the primary residential source

Natural gas furnaces are the most common source of residential CO exposure in Ontario because they operate for thousands of hours per heating season, they burn fuel continuously during operation, and they are connected to every room in the house through the duct system. A furnace with a cracked heat exchanger does not just leak CO into the mechanical room — it distributes CO-contaminated air through every supply register in the home, affecting every occupied room simultaneously. Other combustion appliances (water heaters, gas stoves, fireplaces) produce CO in a single location, but a furnace with a compromised heat exchanger turns the entire duct system into a CO distribution network.

Ontario's long heating season compounds the risk. Furnaces in southern Ontario run from October through April — roughly 2,500-3,000 hours of operation annually. In northern Ontario, the heating season extends from September through May, pushing annual operating hours to 3,500-5,000. This sustained use accelerates wear on the heat exchanger, burner components, and venting system — all the components whose failure leads to CO exposure. By comparison, a furnace in a milder climate that runs 1,000 hours per year experiences far less thermal stress. This is why furnace maintenance and heat exchanger inspection are the most critical CO prevention measures in any Ontario home with gas heating, and why they should never be skipped or deferred.

How Furnaces Produce Carbon Monoxide

Understanding the specific ways a furnace can produce and leak CO helps you recognize warning signs and understand why certain maintenance tasks are critical to your family's safety.

Cracked heat exchanger

The heat exchanger is the barrier between the combustion gases (containing CO) and the household air that circulates through your ductwork. Heat exchangers are metal chambers that expand and contract with every heating cycle — thousands of times per year. Over 15-20 years, this thermal stress causes metal fatigue, and eventually cracks develop. A crack allows combustion gases to leak from the combustion side into the airstream flowing to your living space. The crack may be microscopic initially, producing CO levels too low to trigger a detector but high enough to cause chronic symptoms (persistent headaches, fatigue, difficulty concentrating). As the crack worsens under continued thermal cycling, CO levels increase until they become immediately dangerous.

Warning signs of a cracked heat exchanger: soot or carbon buildup around the burner area, visible cracks or corrosion on the heat exchanger surface (visible during professional inspection), water pooling at the base of the furnace (not from the condensate drain), a yellow or flickering burner flame instead of a steady blue flame, and the blower motor pushing the burner flame (visible when the blower starts — the flame should not move when airflow begins if the heat exchanger is sealed). If your furnace is 15-20+ years old, heat exchanger inspection should be the top priority during every annual tune-up.

Blocked or damaged venting

The exhaust flue carries combustion gases (including CO) from the furnace to the outdoors. When the flue is blocked, partially blocked, or damaged, CO cannot escape and backs up into the home. Standard-efficiency furnaces (80% AFUE) vent through a metal chimney that relies on the natural buoyancy of hot exhaust gases to draw them upward and out. This system is vulnerable to backdrafting — wind, negative pressure in the home, or a competing exhaust fan can reverse the flow and pull CO back down the chimney into the home. High-efficiency furnaces (90%+ AFUE) use sealed PVC vent pipes with a power inducer that forces exhaust gases out, making them less susceptible to backdrafting but vulnerable to different problems: ice buildup at the outdoor termination during extreme cold, bird or insect nests blocking the pipe, and damaged or disconnected pipe sections in the attic or walls.

Combustion problems

Even with an intact heat exchanger and clear venting, a furnace can produce excessive CO through combustion problems. Dirty or misaligned burners create uneven combustion, producing CO instead of CO2 in areas where the flame burns inefficiently. Incorrect gas pressure — too high or too low — affects the air-to-fuel ratio and can dramatically increase CO production. A failing flame sensor causes repeated ignition attempts that flood the combustion chamber with unburned gas, which then ignites in an uncontrolled manner. Insufficient combustion air supply (common in tightly sealed homes without adequate makeup air) starves the burner of oxygen, shifting combustion toward CO-producing conditions. These problems are detectable during professional combustion analysis — a test that measures CO, CO2, and O2 in the flue gas — which is why annual professional tune-ups are not optional for safety.

Ontario's Carbon Monoxide Detector Laws

Ontario has some of the strongest CO detector legislation in North America, enacted after several preventable CO deaths in the province. Understanding the law ensures compliance and — more importantly — protects your family.

The Hawkins-Gignac Act

The Hawkins-Gignac Act (Ontario Regulation 194/14) is named after the Hawkins family of Woodstock, Ontario — Laurie Hawkins and John Gignac's niece and her family — who died from CO poisoning from a blocked furnace vent in 2008. The law requires CO detectors on every storey of a residential dwelling and adjacent to each sleeping area if the home contains any fuel-burning appliance (natural gas, propane, oil, wood) or has an attached garage. This applies to all residential property types: single-family homes, townhouses, condominiums, apartments, and rental units. The law applies year-round, not just during heating season — gas water heaters, stoves, and fireplaces operate outside the heating season and produce CO.

Landlord and tenant responsibilities

Landlords must install and maintain CO detectors in all rental units that require them under the regulation. Tenants must not remove or disable detectors, must notify the landlord if a detector is malfunctioning, and must replace batteries if the unit is battery-operated (unless the lease specifies otherwise). Landlords who fail to provide required CO detectors can be fined up to $50,000 for individuals and $100,000 for corporations under the Fire Protection and Prevention Act. Tenants who discover missing or non-functional CO detectors should notify their landlord in writing and, if the situation is not corrected, contact their local fire department for enforcement.

Penalties and enforcement

Local fire departments are responsible for enforcing CO detector requirements during inspections and complaint investigations. Fire inspectors can issue compliance orders requiring installation of CO detectors, and if the order is not complied with within the specified timeframe, they can proceed to fines under the Provincial Offences Act. Fines for non-compliance can reach $50,000 for individuals and $100,000 for corporations — penalties that reflect the seriousness with which Ontario treats CO safety after the preventable deaths that prompted the legislation.

Insurance implications add another layer of consequence. If a CO incident occurs in a home without required CO detectors, the homeowner's or landlord's insurance claim may be denied or reduced on the grounds of regulatory non-compliance — the insurer can argue that the damage was preventable had the required safety devices been installed. Similarly, if a CO incident occurs in a home where the furnace was not maintained and inspection records cannot be produced, the insurer may question whether the homeowner met their duty of care. The cost of compliance — $30-$60 per detector, $100-$200 for an annual tune-up — is negligible compared to the legal, financial, and human cost of a CO incident in a home without proper detection and maintained equipment.

CO Detector Placement and Types

Proper placement is as important as having detectors at all — a CO detector in the wrong location may not alarm in time to protect sleeping occupants, while one placed correctly provides early warning that enables safe evacuation.

Where to install CO detectors

Install CO detectors in these locations: on every floor of the home including the basement (required by Ontario law), within 5 metres of each sleeping area (required by Ontario law), near the furnace and water heater area (recommended — this is where CO is most likely to originate), and in any room containing a fuel-burning appliance (gas fireplace, gas stove). Mount detectors at plug height or on the wall 5 feet above the floor — CO mixes with air and distributes evenly throughout a room, so detector height is less critical than for smoke detectors. Avoid placing detectors directly above or within 5 feet of a fuel-burning appliance (this can cause nuisance alarms from normal trace CO production), in dead air spaces (corners where two walls meet the ceiling), or in humid areas like bathrooms (humidity can damage the sensor).

Types of CO detectors

Electrochemical sensor detectors are the most common and most reliable type for residential use. They contain a chemical solution that generates a small electrical current when CO molecules contact the sensor — the current strength corresponds to CO concentration. These detectors have a 5-7 year lifespan and cost $30-$60 each. Biomimetic sensor detectors use a gel that changes colour when exposed to CO. They are less expensive but less accurate and slower to respond at low concentrations. Metal oxide semiconductor (MOS) detectors are more common in commercial and industrial applications and less common in residential units. For home use, choose electrochemical sensor detectors from recognized manufacturers (Kidde, First Alert, Nest) that are CSA-certified for use in Canada.

Smart and connected detectors

Modern connected CO detectors offer significant safety advantages over standalone units. Interconnected systems (wired or wireless) trigger all detectors in the home when any single detector senses CO — ensuring occupants in every room hear the alarm even if the CO source is on a different floor. Smart detectors connect to your phone via Wi-Fi and send alerts when CO is detected, when the battery is low, or when the sensor is approaching end-of-life — providing notification even when you are away from home. Some smart detectors also display real-time CO levels in parts per million, allowing you to monitor trends and identify intermittent low-level CO sources that would not trigger the alarm but may indicate a developing problem. Combination smoke and CO detectors reduce the number of devices on your ceiling while providing both types of protection in a single unit.

Carbon Monoxide Poisoning Symptoms

Recognizing CO poisoning symptoms saves lives — especially when detectors malfunction, are absent, or the CO level is below the alarm threshold but high enough to cause harm over extended exposure.

Low-level exposure symptoms

At CO concentrations of 50-100 ppm — levels that may not immediately trigger a detector — symptoms develop over hours and closely mimic common illnesses. Headache (often described as a dull, persistent pressure headache that does not respond to pain medication), mild nausea, fatigue and difficulty concentrating, and mild dizziness. These symptoms are frequently attributed to a cold, flu, stress, or poor sleep. The critical diagnostic clue is that multiple household members develop the same symptoms simultaneously, and symptoms improve when leaving the home and return when coming back. Pets, particularly birds and small animals, are more susceptible and may show symptoms before humans — a lethargic or ill pet combined with flu-like symptoms in family members is a warning pattern.

Moderate to severe exposure symptoms

At 100-400 ppm, symptoms escalate rapidly: severe headache that does not respond to medication, significant dizziness and disorientation, visual disturbances (blurred vision, difficulty focusing), persistent vomiting, confusion and impaired judgment (which critically reduces the victim's ability to recognize the danger and take action to evacuate), chest pain and shortness of breath especially in those with heart conditions, and muscle weakness that can prevent walking or even standing.

At these concentrations, the most vulnerable household members — sleeping occupants, the elderly, infants and young children, pregnant women (CO crosses the placenta and affects the fetus at lower levels than the mother), and anyone with pre-existing heart or respiratory conditions — face severe and potentially fatal risk. Alcohol consumption increases susceptibility because alcohol impairs the body's ability to compensate for reduced oxygen transport. The insidious nature of moderate CO exposure is that the cognitive impairment it causes reduces the victim's ability to recognize what is happening — people may feel too confused to leave the home or too weak to reach the door, which is why functioning CO detectors that alarm before concentrations reach these levels are essential.

Life-threatening exposure

Above 400 ppm, CO poisoning becomes immediately life-threatening. Loss of consciousness can occur within minutes, followed by convulsions, brain damage, and death. At 1,600+ ppm, death can occur within 20 minutes. Survivors of severe CO poisoning may suffer permanent neurological damage including memory loss, cognitive impairment, personality changes, and motor function problems. These long-term effects can appear weeks after the acute exposure and may be permanent. The urgency of evacuation at any CO alarm cannot be overstated — the difference between mild symptoms and fatal exposure is often minutes, and CO impairs the very cognitive functions (judgment, decision-making, motor coordination) needed to evacuate safely.

Preventing Carbon Monoxide from Your Furnace

CO prevention is not a single action — it is a layered approach combining equipment maintenance, monitoring, ventilation, and detection that collectively reduce CO risk to near zero.

Annual professional inspection

The single most effective CO prevention measure is an annual furnace maintenance visit by a TSSA-certified technician. During this visit, the technician performs combustion analysis — the only definitive test for CO production — measuring CO levels in the flue gas and comparing them to manufacturer specifications and safety thresholds. A healthy furnace produces under 50 ppm of CO in the undiluted flue gas when measured at steady-state operation. CO levels between 50-100 ppm indicate a combustion problem that needs attention — typically dirty burners, incorrect gas pressure, or insufficient combustion air. Levels above 100 ppm in the flue gas (ambient air adjusted) indicate a serious combustion problem that requires immediate correction before the furnace is operated further.

Beyond combustion analysis, the technician inspects the heat exchanger for cracks and corrosion using visual inspection, flame observation, and where necessary, camera inspection of the internal surfaces. They verify the integrity of all flue connections from the furnace to the outdoor termination, test every safety control (pressure switches, flame rollout switches, high-limit switches), verify the combustion air supply is adequate, and measure temperature rise across the heat exchanger to confirm proper airflow. This comprehensive inspection catches problems while they are still correctable — a burner producing 70 ppm of CO today can be cleaned and adjusted back to 20 ppm, preventing the gradual escalation to dangerous levels that inevitably occurs when the problem goes undetected and uncorrected through an entire heating season.

Proper venting maintenance

Inspect your furnace venting regularly — what you can see without tools. For standard-efficiency furnaces with metal chimneys: verify the chimney cap is in place (preventing bird nests and rain entry), check for rust or deterioration on visible chimney sections, and look for disconnections or sags in the horizontal connector pipe between the furnace and chimney. For high-efficiency furnaces with PVC vent pipes: inspect the exterior termination regularly (monthly during winter) for ice buildup, snow blockage, or debris. Ice can form over the exhaust termination during sustained extreme cold, gradually restricting and eventually blocking the exhaust — triggering a safety lockout on the furnace. Clear any ice or snow from the vent terminations and ensure the intake and exhaust pipes have adequate clearance from each other and from the ground (per the installation code requirements).

Combustion air supply

Every gas furnace needs an adequate supply of air for combustion. In older homes with natural air leakage, this air enters through cracks and gaps in the building envelope. In newer, tightly sealed homes or homes that have been retrofitted with improved air sealing, the natural air supply may be insufficient — especially when exhaust fans (range hood, bathroom fans, dryer) compete for air and create negative pressure that can backdraft combustion appliances. A dedicated combustion air duct from the outdoors to the furnace room ensures adequate air supply regardless of home air-tightness. If your home has been air-sealed, insulated, or had windows replaced, have an HVAC technician verify that the furnace combustion air supply is still adequate — improvements to the building envelope can change the pressure dynamics enough to create CO backdrafting conditions that did not exist before the upgrade.

Other Carbon Monoxide Sources in Your Home

While the furnace is typically the primary CO concern, other fuel-burning appliances and activities produce CO and should be part of your safety awareness.

Gas water heaters and fireplaces

Natural gas water heaters with atmospheric venting (the traditional metal chimney-type vent) are susceptible to the same backdrafting problems as standard-efficiency furnaces. A water heater that backdrafts spills combustion gases — including CO — into the mechanical room or utility area. Gas fireplaces, both vented and ventless, produce CO during operation. Vented gas fireplaces exhaust CO outdoors through a chimney or direct-vent system, but damaged venting or improper installation can allow CO into the living space. Ventless (vent-free) gas fireplaces are designed to operate without a chimney, relying on the room's air volume to dilute CO production — they include an oxygen depletion sensor (ODS) that shuts the unit off when oxygen levels drop. However, ventless gas fireplaces remain controversial from a safety standpoint and are banned in some jurisdictions for good reason.

Attached garages

An attached garage is one of the most common and least recognized CO sources in residential settings. A car running in an attached garage produces CO concentrations that can reach dangerous levels within minutes, and the gas migrates through air leaks, shared walls, and door gaps into the living space — even when the garage door is open. Never warm up a vehicle in an attached garage, even with the garage door open — CO accumulates faster than natural ventilation can dissipate it. Carbon monoxide from an attached garage is the reason Ontario's CO detector law includes homes with attached garages regardless of whether the home has fuel-burning appliances. Seal all penetrations between the garage and living space (electrical, plumbing, ductwork, the wall-ceiling junction) and ensure the door between the garage and house is weatherstripped and closes tightly.

Portable generators and outdoor equipment

Portable generators produce extremely high CO concentrations and must never be operated indoors, in a garage, in a basement, or near open windows or doors. During power outages — when furnaces and CO detectors may not be functioning — homeowners sometimes bring generators inside with catastrophic results. Generator exhaust should discharge at least 20 feet from any window, door, or vent opening. The same applies to charcoal grills, propane heaters, camp stoves, and any combustion-powered equipment designed for outdoor use — these produce CO levels that are immediately dangerous in enclosed spaces. During winter power outages when the furnace is not operating, the temptation to use these devices indoors is strong — resist it. Use only ULC-certified indoor-rated heaters and follow all manufacturer instructions for ventilation.

What to Do If Your CO Alarm Sounds

A CO alarm is a life-safety device — not a nuisance alarm to be silenced and ignored. Treat every CO alarm activation as a genuine emergency until it is confirmed safe by trained personnel with professional testing equipment.

Immediate evacuation steps

When the CO alarm sounds: stop what you are doing immediately and alert everyone in the home. Do not waste time looking for the source of CO — get everyone out first. Leave doors and windows open as you exit to ventilate (if easily done without delay — do not go out of your way if it would slow evacuation). Account for all family members and pets once outside. Move at least 50 feet away from the home to fresh air. Call 911 from outside — do not call from inside the home. Do not re-enter the home for any reason until emergency services have tested the air and confirmed it is safe.

If anyone is showing symptoms of CO poisoning (headache, dizziness, nausea, confusion), inform the 911 operator and request medical evaluation. Even if symptoms seem mild, medical assessment is important because CO exposure effects can worsen or develop delayed neurological symptoms hours after the initial exposure. Emergency responders will test the air inside your home with professional-grade CO monitors that measure exact concentrations and identify the source — information your residential CO detector cannot provide.

After the emergency

Do not re-enter the home until the fire department or emergency services have cleared it. If a CO source was identified, do not use the appliance until it has been inspected and repaired by a qualified TSSA-certified technician. If the source was your furnace, do not attempt to restart it yourself — have the technician verify it is safe before reactivating it. Contact your utility company (Enbridge Gas or your local gas utility) if you suspect a gas leak in addition to CO — utility companies provide emergency gas response 24/7 at no charge.

After the immediate crisis is resolved, schedule a comprehensive inspection of all fuel-burning appliances in your home. The source that triggered the alarm may not be the only appliance producing CO — other appliances may be producing lower but still problematic levels. Have the technician perform combustion analysis on every gas appliance: furnace, water heater, gas fireplace, gas dryer, and gas stove. Document the inspection results for your records and insurance purposes.

When the alarm sounds but no one feels symptoms

A CO alarm activation without noticeable symptoms does not mean it is a false alarm. Low-level CO exposure produces subtle symptoms that are easy to overlook or attribute to other causes. The detector is designed to alarm before symptoms become severe — this is its purpose. Follow the same evacuation protocol regardless of symptoms. False alarms from CO detectors are rare — unlike smoke detectors, which can be triggered by cooking, steam, or dust, CO detectors have very few non-CO triggers. Possible causes of a genuine alarm with no obvious symptoms: intermittent CO from a backdrafting appliance (the CO level spiked and then dropped), low-level CO that has not yet caused noticeable symptoms, or a CO source in an area of the home not currently occupied (the detector near the source alarmed first).

How Furnace Maintenance Prevents CO Exposure

Annual furnace maintenance is not just about efficiency and equipment longevity — it is the primary defence against CO exposure in your home. Understanding exactly what the technician checks and why gives you confidence that the investment is protecting your family's safety.

Combustion analysis: the definitive CO test

Professional combustion analysis uses a digital analyser inserted into the furnace exhaust flue to measure CO, CO2, O2, and flue gas temperature simultaneously. A healthy furnace produces under 50 ppm of CO in the undiluted flue gas. Levels between 50-100 ppm indicate a combustion problem that needs adjustment. Levels above 100 ppm require immediate investigation and correction before the furnace is operated further. The combustion analyser also calculates combustion efficiency and identifies whether the air-to-fuel ratio is properly calibrated. This is the only test that definitively answers whether your furnace is producing dangerous CO levels — a visual inspection alone cannot determine this. An emergency service call for a CO alarm often reveals combustion problems that would have been caught and corrected during a routine annual tune-up.

Heat exchanger inspection

The technician inspects the heat exchanger using a combination of visual examination (looking for cracks, rust, and corrosion), flame observation (watching for flame disturbance when the blower starts, which indicates a crack), and in some cases, a camera or borescope inspection to see inside the heat exchanger cells. On high-efficiency furnaces with secondary heat exchangers, the secondary section is examined for corrosion from acidic condensate — these sections are the most vulnerable to cracking in condensing furnaces. Heat exchanger inspection is particularly critical for furnaces over 10 years old, when thermal fatigue makes cracking increasingly likely. If a crack is found, the technician should immediately tag the furnace as unsafe and shut it down until the heat exchanger is replaced or the furnace is replaced entirely.

Venting system verification

The technician inspects the entire flue system from the furnace to the outdoor termination. For standard-efficiency furnaces: verifying the chimney liner is intact, the chimney cap is in place, the horizontal connector pipe has proper pitch (rising toward the chimney), all joints are securely fastened and sealed, and there are no obstructions inside the chimney. For high-efficiency furnaces: verifying PVC pipe joints are properly cemented, the vent system has proper slope for condensate drainage, the outdoor termination is clear and properly located (away from windows, doors, and dryer vents), and the intake and exhaust pipes are correctly configured per the manufacturer's installation manual. Vent system problems are the second most common cause of residential CO exposure after heat exchanger failures.

Seasonal CO Risk Factors in Ontario

CO risk in Ontario is not constant throughout the year — specific seasonal conditions increase the likelihood of CO exposure, and awareness of these risk periods helps you take targeted precautions.

Early heating season (October-November)

The first weeks of furnace operation each fall represent a peak CO risk period. A furnace that developed problems during the off-season — a cracked heat exchanger that worsened, a vent that became blocked by a bird nest, a condensate drain that clogged — begins operating with these undetected defects. This is why the instructions to prepare your HVAC for winter emphasize scheduling a professional tune-up before you need the furnace daily. The first time you run the furnace each fall, test your CO detectors and monitor for symptoms during the first few hours of operation. A burning smell during initial startup is normal (dust burning off the heat exchanger), but any persistent smell beyond 30 minutes warrants investigation.

Extreme cold events (January-February)

During sustained extreme cold (-20°C to -35°C), furnaces run near-continuously, sometimes cycling off for only a few minutes per hour. This sustained operation stresses every component — the heat exchanger cycles through more thermal expansion events, the blower motor runs longer, gas pressure may fluctuate under high demand across the utility network, and the venting system must handle continuous exhaust flow. Problems that would remain dormant under intermittent operation can manifest suddenly under continuous-run conditions.

Extreme cold also creates secondary CO risks from multiple directions. Ice blocking furnace exhaust vents is a frequent problem with high-efficiency furnaces — the low-temperature exhaust (100-150°F) contains significant moisture that freezes at the outdoor termination point during sustained extreme cold, gradually restricting and eventually blocking the vent entirely. Homeowners respond to extreme cold by sealing their homes more tightly against drafts, closing windows completely, stuffing towels under doors, and covering drafty areas — all of which reduce natural ventilation that would otherwise dilute any CO present in the home. Many homeowners also increase their use of supplemental heating sources during cold snaps: gas fireplaces running for extended periods, portable propane or kerosene heaters brought indoors, and in the worst cases, generators and outdoor heating devices operated in garages or basements. Each of these adds another potential CO source to an already tightly sealed environment.

Spring shoulder season and power outages

Spring brings its own CO risks as heating needs become intermittent and homeowners reduce their vigilance. The furnace may cycle on for cold mornings and off during warmer afternoons — this intermittent operation can cause condensation inside the flue pipe that corrodes metal venting from the inside. Spring storms bring power outages that lead to generator use, and the warmer weather creates a false sense of safety that makes people complacent about CO detector maintenance. Use the transition to spring as a reminder to test CO detectors, verify that all detectors have current batteries, and check the expiry date on each unit. Replace any detector that is within 6 months of its expiry date rather than waiting — an expired detector during a late-season cold snap provides zero protection.

Frequently Asked Questions