Does a make-up air unit need its own ductwork or can it tie into existing ducts in Toronto?

A make-up air unit can be connected to your existing duct system in some configurations, but many installations work better with dedicated ductwork — and which approach is right depends on your home's layout, furnace capacity, and the volume of make-up air required. There is no single correct answer, but understanding the options will help you have a productive conversation with your contractor.

The most common approach in GTA homes is to connect the make-up air unit to the return air plenum of the existing furnace. The return plenum is the large sheet metal box on the intake side of your furnace, where air from your home is drawn back to be filtered, heated or cooled, and redistributed through the supply ducts. When the make-up air unit introduces tempered outdoor air into this plenum, the furnace blower mixes it with recirculated air and distributes it evenly throughout the home via the existing supply ducts. This approach works well because it uses your existing ductwork for distribution, avoids dumping a concentrated stream of outdoor air into one room, and allows the furnace to further condition the air if needed. However, it requires the furnace blower to be running whenever the range hood operates, and the furnace system must have enough capacity to handle the additional air volume without exceeding its static pressure limits.

The alternative is dedicated ductwork from the make-up air unit to a discharge point, typically a ceiling or wall diffuser in or near the kitchen. This approach is more common in custom homes or major kitchen renovations where the make-up air volume is large (matching a 600 to 1,200 CFM range hood) and the furnace system cannot handle the additional load. Dedicated ductwork gives the installer more control over where the air enters the home and avoids any impact on the existing HVAC system's airflow balance. The downside is additional ductwork cost — running a new insulated duct from the make-up air unit to the kitchen can add $500 to $1,500 to the project depending on the route and length.

In many older Toronto homes — particularly post-war bungalows and two-storey houses in Scarborough, North York, and Etobicoke — the existing duct system is already undersized for modern heating and cooling loads. Adding make-up air volume to an already strained system can push static pressure too high, reduce airflow to bedrooms, and cause the furnace blower to work harder than it should. In these situations, a contractor may recommend dedicated ductwork or suggest upgrading the return air capacity before tying make-up air into the existing system.

Regardless of which approach is used, the make-up air duct must be insulated where it passes through unconditioned spaces to prevent condensation in Toronto's humid summers and heat loss in winter. The connection to the exterior must include a weatherproof hood, insect screen, and backdraft damper. All duct joints must be sealed with mastic or UL 181-rated foil tape. A building permit is required for make-up air installations in Toronto, and any gas connections for gas-fired tempering must be done by a TSSA-registered contractor. Toronto Ductwork can match you with a qualified contractor through the Toronto Construction Network to evaluate the best ductwork approach for your specific home.