Can I convert my old masonry chimney to vent a new high-efficiency furnace in my Toronto home?

In most cases, you cannot vent a new high-efficiency furnace directly through an existing masonry chimney — and doing so without proper modification can create serious safety hazards including carbon monoxide poisoning. The reason comes down to a fundamental difference in how conventional and high-efficiency furnaces produce exhaust.

Conventional furnaces (80% efficiency or less) produce hot exhaust gases that rise naturally through a masonry chimney by convection. The exhaust temperature is high enough — typically 150 to 250 degrees Celsius — to keep the flue warm and dry, preventing condensation inside the chimney. High-efficiency condensing furnaces (90% efficiency and above), which are now standard in Ontario for new installations, extract so much heat from the combustion gases that the exhaust temperature drops to only 40 to 60 degrees Celsius. At these low temperatures, the exhaust contains significant water vapour that condenses inside the chimney flue, producing acidic condensate that attacks masonry, deteriorates mortar joints from the inside, and can eventually cause the chimney liner to crumble — sending moisture, flue gases, and potentially carbon monoxide into your home's walls and living spaces.

The proper solution is one of three approaches. The most common for GTA homes upgrading to a high-efficiency furnace is to abandon the masonry chimney for furnace venting entirely and instead vent the new furnace through a sidewall using PVC or CPVC pipe, which is the standard method recommended by furnace manufacturers. The acidic condensate from a high-efficiency furnace is designed to drain through a condensate line to a floor drain — it never enters the chimney at all. This sidewall venting approach is typically included in the furnace installation cost and does not require any chimney modification. Your HVAC contractor handles this as part of the furnace installation.

The second option, if sidewall venting is not practical due to lot line setbacks, window clearances, or municipal bylaws, is to reline the existing masonry chimney with a stainless steel liner rated for condensing appliances. The stainless steel liner (typically AL29-4C alloy, which resists acidic condensate) is inserted down the full length of the chimney, sealed at top and bottom, and connected to the furnace exhaust. This protects the masonry from condensate damage while using the existing chimney structure. Relining costs $1,500 to $4,000 in the GTA depending on chimney height and accessibility.

The third option is a flexible aluminium liner, which is less expensive ($800 to $2,000) but has a shorter lifespan and is not rated for all condensing appliance types. Discuss liner material options with your HVAC contractor and ensure whatever is installed meets the furnace manufacturer's specifications and Ontario Building Code requirements.

If the masonry chimney is no longer being used for any appliance venting after the furnace upgrade — and you do not have a gas fireplace, wood-burning fireplace, or gas water heater still venting through it — the chimney flue should be properly capped at the top to prevent rain, snow, and animal entry, and the clean-out at the bottom should be sealed. The chimney structure itself can remain in place as a structural and architectural element. Many Toronto homeowners in this situation choose to have the chimney repointed and the crown repaired at this time, since the chimney still needs to be maintained to prevent water infiltration even if it is no longer actively venting.

Important regulatory requirements: Under the Ontario Building Code, any change to a furnace venting system requires a building permit and inspection. Gas furnace installation must be performed by a TSSA-licensed gas technician (Technical Standards and Safety Authority). If any electrical work is involved — such as adding a dedicated circuit or relocating an outlet for the new furnace — an ESA-licensed electrical contractor and ESA inspection are required. Ensure your HVAC contractor pulls all necessary permits and arranges inspections. All contractors working on the project must carry WSIB coverage.

If you still use the chimney for a gas or wood-burning fireplace, the fireplace can continue to vent through the masonry chimney while the furnace vents separately through the sidewall — this is the most common configuration in GTA homes that have upgraded their furnace but kept a working fireplace.