What size exhaust duct do I need for a large range hood in my Toronto kitchen?

The exhaust duct size for your range hood depends directly on the hood's CFM rating, and getting this wrong will choke performance and create excessive noise. Most range hoods in the 400-600 CFM range require a minimum 6-inch round duct, while hoods rated above 600 CFM typically need an 8-inch duct. High-performance commercial-style hoods pushing 900-1,200 CFM often require 10-inch ductwork. Your hood's installation manual will specify the required duct size — never go smaller than what the manufacturer recommends.

The reason duct sizing matters so much is airflow velocity and static pressure. When you push a large volume of air through an undersized duct, the velocity increases dramatically, which creates two problems: the air resistance (static pressure) skyrockets, forcing the blower to work harder and move less air than its rating suggests, and the high velocity generates a loud rushing or whistling noise that makes the hood unbearable to use. A 600 CFM hood connected to a 4-inch duct — something I've seen in older Toronto kitchens where homeowners installed a new hood on an existing vent — will deliver maybe 250-300 CFM in practice and sound like a jet engine doing it.

Duct material matters just as much as size in the GTA. Use rigid or semi-rigid metal duct for your range hood exhaust — galvanized steel or aluminum. Never use flex duct for range hood exhaust runs. Flex duct's corrugated interior creates far more friction than smooth metal, effectively reducing your duct's capacity by 25-40%. A 6-inch flex duct performs more like a 5-inch smooth duct in terms of actual airflow delivery. The Ontario Building Code also requires that kitchen exhaust duct materials meet specific flame spread and smoke development ratings, which rules out standard plastic flex duct entirely.

Keep your duct run as short and straight as possible. Every 90-degree elbow in the run adds the equivalent of 5-10 feet of straight duct in terms of airflow resistance. If your kitchen is on an interior wall and you need to run the duct through the ceiling, across the attic, and out through the roof or a gable wall, you could be looking at 20-30 equivalent feet of duct by the time you account for the elbows. In that scenario, upsizing to the next larger duct diameter is smart insurance. A 6-inch duct on a long, winding run should be bumped to 7 or 8 inches to compensate.

There is one critical issue many Toronto homeowners overlook: if your range hood is rated above approximately 400 CFM, the Ontario Building Code requires a make-up air system to replace the air being exhausted. The threshold is 75 litres per second, which works out to roughly 159 CFM — but in practice, make-up air requirements are triggered by hoods in the 400+ CFM range once you account for the OBC's calculation methods. A make-up air system adds $2,000-$5,000 to your project but is non-negotiable for code compliance and combustion safety, especially if you have a gas furnace or water heater. Installing the range hood exhaust duct itself typically costs $500-$2,500 in the GTA depending on run length and routing complexity, plus the cost of the hood itself. A building permit is required for new kitchen exhaust installations and runs around $200-$500 through the City of Toronto. Have a qualified contractor assess your kitchen layout before purchasing the hood — they can advise on the optimal duct route and whether make-up air will be required for your specific setup.