Can I vent multiple bathroom exhaust fans into one duct in my Toronto home?

Connecting multiple bathroom exhaust fans to a single shared duct is technically possible but creates significant performance and moisture problems, and in most residential applications across the GTA it is better to run a dedicated duct for each fan. If you do combine fans onto a shared duct, specific design requirements must be met to prevent backdrafting, moisture damage, and code compliance issues.

The fundamental problem with connecting two or more exhaust fans to one duct is backdrafting. When one fan runs and the other is off, exhaust air takes the path of least resistance. If the duct path to the exterior has more friction than the path backward through the inactive fan's duct connection, humid exhaust air will flow backward into the second bathroom rather than exiting the building. This pushes moist air into a bathroom that may not even be in use, causing condensation on mirrors, walls, and ceilings. In Toronto's cold winters, this backdrafted moisture can condense on cold surfaces and promote mould growth — exactly the problem bathroom exhaust is supposed to prevent.

To prevent backdrafting in a shared duct configuration, each fan connection to the common duct must have a backdraft damper installed immediately at the junction point. These dampers close when the fan is not running, preventing reverse airflow. The shared trunk duct must also be sized larger than each individual branch to handle the combined airflow when both fans run simultaneously. If each fan is rated at 80 CFM, the common duct needs to handle 160 CFM — typically requiring a 6-inch round duct rather than the standard 4-inch. Undersizing the common duct restricts airflow from both fans and makes condensation pooling worse.

In practice, the cost of properly designing a shared duct system — with oversized common duct, backdraft dampers at each junction, and careful slope management — often approaches or exceeds the cost of simply running separate dedicated ducts. A dedicated 4-inch duct from each bathroom fan to its own exterior termination point is simpler, more reliable, avoids all backdraft issues, and is easier to maintain. Each installation runs $300 to $800 in the GTA including the duct run, insulation in unconditioned spaces, and exterior vent cap.

If building constraints genuinely prevent separate duct runs — which sometimes happens in Toronto homes with limited exterior wall access or complex attic framing — a multi-port exhaust fan is a better solution than connecting separate fans to one duct. Panasonic, Fantech, and other manufacturers make inline multi-port exhaust fans designed specifically for this purpose. A single powerful inline fan is installed in the attic or mechanical space, with individual intake ports ducted to each bathroom. The fan pulls air from all connected bathrooms simultaneously through properly sized ductwork and exhausts through a single exterior termination. These systems cost $800 to $2,000 installed in the GTA, including the fan unit, ductwork, and controls, but they eliminate backdrafting entirely and are specifically engineered for multi-bathroom ventilation.

Whichever approach you choose, all exhaust ductwork must terminate at the building exterior per the Ontario Building Code — never into the attic, soffit, or crawlspace. Any electrical work must be done by an ESA-Licensed Electrical Contractor. A ductwork professional can evaluate your home's layout and recommend the most practical solution. Browse HVAC and ductwork contractors through the Toronto Construction Network directory at torontoconstructionnetwork.com.