Our Barrhaven home has two fireplaces sharing one flue — is that legal under Ontario fire code?

Two fireplaces sharing a single flue is not permitted under the Ontario Building Code, and it creates serious safety and performance problems that make it a genuine hazard in your Barrhaven home.

Here is why this matters: The Ontario Building Code requires each fireplace or heating appliance to have its own dedicated flue — there are no exceptions for shared chimneys. When two fireplaces use the same flue, you risk dangerous backdrafting (where exhaust gases and smoke from one fireplace reverse direction and enter the room instead of exiting), carbon monoxide accumulation in the home, loss of heating efficiency as gases from one fireplace cool down before exiting the top, and uneven draft conditions that cause one fireplace to smoke while the other functions normally. In Ottawa's climate, where cold outdoor air can create persistent downdraft conditions, the risk of backdrafting from a shared flue becomes even more pronounced during winter when both units might be in use or when wind patterns shift. This is especially problematic if one fireplace is used more frequently than the other — the less-used flue can become a pathway for gases from the active fireplace to back into the house.

The practical solution depends on your fireplace types and chimney structure. If both are wood-burning fireplaces, you need to separate them into two distinct flues. This typically means installing a chimney partition wall inside the existing chimney structure (dividing it vertically with a masonry or insulated wall) or, more commonly, installing a secondary flue liner alongside the existing one. The cost for chimney partition or secondary liner installation in Barrhaven runs $3,000 to $6,000 depending on chimney height, accessibility, and whether the partition is full-height or partial-height. If both are gas fireplaces or inserts, the solution is often simpler — you may be able to install a secondary vent pipe for one unit, which costs $1,500 to $3,000, though this depends on the fireplace locations and roof configuration. If one is wood and one is gas, you must absolutely separate them, as they have different venting requirements and mixing them creates serious safety hazards.

Before proceeding with any fix, you need a professional assessment. A WETT-certified chimney inspector can evaluate your existing chimney structure, determine whether it is currently safe to use, and recommend the appropriate separation method. This inspection (Level 2 or Level 3) will cost $350 to $1,000 but will give you a clear picture of what you are working with and what permits you will need. The City of Ottawa requires a building permit for chimney partition or secondary flue installation — this is not a grey area or a project to attempt without proper authorization. Your home insurance may also require proof of code-compliant separation before they will insure the fireplaces, so addressing this now is critical for both safety and insurability.

I would also mention that if this is an older home where the two-fireplace-one-flue setup dates back decades, it may have been done before current code was established — but that does not make it legal or safe today. The same applies if the previous owner simply never had it inspected.

A WETT-certified chimney professional in the Barrhaven area can assess your specific situation and recommend the right fix. You can browse fireplace and chimney contractors through the Ottawa Construction Network directory at justynrookcontracting.com/directory to find experienced professionals who handle this type of remedial chimney work.

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