Carbon Monoxide and Your Furnace: What Ontario Homeowners Need to Know This Fall

Carbon monoxide is colourless, odourless, and potentially deadly, and every fall, as Ontario homeowners fire up their furnaces for the first time in months, the risk of CO exposure quietly climbs. If you have a gas furnace, understanding the connection between your heating system and carbon monoxide isn’t optional. It could save your family’s life.

What Is Carbon Monoxide and Why Does It Come From Furnaces?

Carbon monoxide (CO) is a byproduct of incomplete combustion. When a gas furnace burns natural gas properly, it produces mostly carbon dioxide and water vapour. But when combustion is incomplete, due to a cracked heat exchanger, blocked flue, dirty burner, or poor ventilation, CO is produced instead.

The danger is that you can’t detect it without a proper alarm. By the time symptoms appear, headache, dizziness, nausea, confusion, the situation can already be serious.

According to Health Canada, CO poisoning causes approximately 50 deaths per year in Canada, with hundreds more hospitalized. A significant portion of residential CO incidents involve heating appliances.

Ontario Law: Are CO Detectors Required?

Yes, and the rules in Ontario are clear.

Under Ontario’s Carbon Monoxide and Smoke Detector Act, every home that contains a fuel-burning appliance (including a gas furnace), a fireplace, or an attached garage is required by law to have a working carbon monoxide detector. The detector must be installed on every storey of the home that contains a sleeping area.

This law applies to all existing homes, not just new construction. If you currently don’t have a CO detector and you have a gas furnace, you are out of compliance, and far more importantly, your family is at risk.

How Your Furnace Can Leak Carbon Monoxide

Several common furnace issues can result in CO entering your home:

Cracked heat exchanger, This is the most serious and most common cause. The heat exchanger is the component that separates combustion gases from the air that circulates through your home. When it develops a crack (a common issue in older furnaces), CO can leak directly into your living space with every heating cycle.

Blocked or damaged flue pipe, Your furnace’s exhaust is supposed to vent outdoors through a flue or chimney. If the flue becomes blocked by a bird nest, debris, or ice in winter, exhaust gases, including CO, can back up into the home.

Dirty or malfunctioning burners, Burners that are clogged with debris or misaligned can cause incomplete combustion, increasing CO production.

Backdrafting, Negative air pressure inside the home (common in well-sealed modern houses) can cause exhaust to be drawn back inside instead of venting out.

Signs You May Have a CO Problem

Don’t wait for an alarm. These signs may indicate a CO issue in your home:

• Yellow or orange pilot light flame instead of blue
• Soot or black marks around the furnace
• Increased condensation on windows
• Musty or unusual smell when the furnace runs
• Household members experiencing unexplained headaches, especially when at home

If your CO detector sounds an alarm: leave the home immediately, call 9-1-1, and do not re-enter until emergency services have cleared it.

Choosing and Placing CO Detectors

Not all CO detectors are equal. Here’s what to look for:

• CSA or UL certification, Only purchase detectors certified to Canadian or North American standards.
• Combination alarms, Smoke/CO combination alarms are permitted under Ontario law and convenient for most rooms.
• Placement, Install CO detectors near sleeping areas on every floor. For maximum protection, also place one near the furnace room.
• Replacement, Most CO detectors have a lifespan of 5–7 years. Check the back of your unit for the replacement date.

Plug-in models with battery backup are recommended over battery-only units, as battery failure can leave you unprotected.

Annual Furnace Inspection: Your Best Defence

The single most effective way to prevent CO exposure from your furnace is an annual professional inspection. A licensed HVAC technician will:

• Inspect and test the heat exchanger for cracks
• Check and clean burners for complete combustion
• Inspect the flue and venting system
• Test for CO in the exhaust stream
• Verify proper airflow and draft

It’s also worth reading Do You Really Need an Annual HVAC Tune-Up? understanding what a proper inspection covers helps you avoid the risk of running a furnace that hasn’t been checked since last spring.

What About Air Quality More Broadly?

Carbon monoxide is just one component of indoor air quality during the heating season. Fall is also when dust, allergens, and particulates recirculate through your ductwork after months of inactivity. For a broader look at how your HVAC system affects the air inside your home, see Heat Pumps and Indoor Air Quality: What You Should Know.

Products We Recommend

If you’re looking to upgrade your heating system to a safer, more efficient model, browse our furnace products for current options. For heat exchanger failures in older units, replacement is often the most cost-effective and safest choice.

If your furnace needs repair, our furnace repair service covers the GTA and surrounding communities.

Carbon monoxide risk from a gas furnace is real, but entirely preventable with the right precautions. As we head into fall heating season, take two actions this week: confirm you have working CO detectors on every floor with sleeping areas, and book your annual furnace inspection.

Don’t wait for a warning sign. By the time CO symptoms appear, the situation is already dangerous.