What are the carbon monoxide risks from leaky ductwork near a Toronto home furnace?

Leaky ductwork near your furnace creates a real carbon monoxide risk through a mechanism called backdrafting, and this is one of the most serious safety concerns in older Toronto homes with natural-draft gas furnaces. Understanding how this happens and what to look for could literally save your family's life.

Here is how the danger works. Your furnace, water heater, and any other gas appliances produce combustion gases — including carbon monoxide — that are designed to exit your home through the chimney or flue. These appliances rely on a pressure difference to draw combustion gases up and out. When you have significant air leaks in the return duct system near the furnace, particularly in the furnace room, those leaks create a negative pressure zone around the furnace. The return side of the duct system acts like a vacuum, pulling air from the surrounding space. If the return ducts are leaky enough, they can pull more air out of the furnace room than the house can replace, creating negative pressure that overwhelms the natural draft of the chimney. When that happens, combustion gases — including carbon monoxide — are pulled back down the chimney and into the home instead of exhausting outside. This is backdrafting, and it kills people in Canadian homes every winter.

Older Toronto homes are particularly vulnerable because many have natural-draft furnaces with open combustion chambers, leaky return duct plenums made from panned floor joists, and inadequate combustion air supply. Post-war homes in Scarborough, North York, and Etobicoke commonly use floor joist cavities as return air pathways — sheet metal nailed to the bottom of joists creating a crude return plenum. These joist-cavity returns are notoriously leaky, often pulling air from crawlspaces, wall cavities, and the furnace room itself. When combined with a natural-draft furnace, this creates the perfect conditions for backdrafting.

The risk increases when other exhaust appliances run simultaneously. A large kitchen range hood pulling 400 to 600 CFM, a bathroom exhaust fan, and a clothes dryer can collectively pull significant volumes of air out of the home. If the home is relatively airtight — especially after window or door replacements — there is not enough make-up air entering to replace what is being exhausted, and the furnace chimney becomes the path of least resistance for replacement air. Instead of combustion gases going up the chimney, outdoor air comes down the chimney and combustion gases spill into the furnace room.

What you should do immediately. First, ensure you have working carbon monoxide detectors on every level of your home and within 5 metres of every sleeping area — this is required by Ontario law. Second, have your furnace and chimney inspected annually by a TSSA-registered technician who will check for proper draft and combustion safety. Third, have a qualified contractor inspect your return ductwork near the furnace for air leaks. Professional duct sealing with mastic on all accessible return duct joints in the furnace room area typically costs $300 to $800 and can dramatically reduce the backdrafting risk. For a whole-house duct sealing project, expect $1,500 to $4,000.

The permanent solution for homes with natural-draft furnaces and leaky ductwork is upgrading to a sealed-combustion (direct-vent) high-efficiency furnace. These units draw combustion air directly from outside through a dedicated PVC pipe and exhaust through another — they are completely isolated from household air pressure. This eliminates the backdrafting risk entirely. A high-efficiency furnace replacement in the GTA runs $4,500 to $8,000 installed, but the safety benefit alone makes it worthwhile. Any gas-related work must be performed by a TSSA-registered contractor — this is non-negotiable under Ontario law.