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Indoor Air · VentilationRange Hood Make-Up Air for Homes
Range hood make-up air for homes replaces the air a large kitchen hood throws outside, so the house does not go negative and pull flue gas back down a natural-draft water heater or fireplace. Under the 2024 Ontario Building Code, Article 9.32.3.8, the trigger is the appliances and not a hood size: if the house has a fuel-fired furnace, boiler or water heater that is neither direct-vent nor mechanically vented, exhaust devices beyond the principal ventilation fan need make-up air matched to their capacity. The make-up air fan must be interlocked with the exhaust and the air tempered to at least 12°C.
A kitchen hood is the largest exhaust fan in most houses. An ordinary hood moves a few hundred cubic feet of air a minute and a pro-style hood over a gas range can move a thousand or more, and every cubic foot has to come back in from somewhere. In a leaky old house it comes through the walls. In a house with new windows, spray foam and weatherstripping it comes down the easiest opening left, and that is often a chimney or a B-vent.
The appliances at risk are the ones that rely on warm flue gas rising unaided: a tank water heater with a draft hood, an older furnace or boiler tied into a chimney, a wood-burning or open gas fireplace. In 1970s–90s subdivisions across Burlington, Oakville and Mississauga the water heater is usually the last of these left, sitting beside a sealed two-pipe furnace. Run the hood on high and that flue can reverse, spilling combustion products, carbon monoxide among them, into the basement (signs a water heater is backdrafting).
The Building Code deals with this in Article 9.32.3.8, protection against depressurisation. The current edition took effect on January 1, 2025 and no longer sorts houses into types; the question now is whether the dwelling contains a fuel-fired space- or water-heating appliance that is neither direct-vent nor mechanically vented. If it does, exhaust devices beyond the principal ventilation fan need make-up air at least equal to their capacity, delivered by an interlocked fan and tempered to 12°C. A depressurisation test to CAN/CGSB-51.71 is the alternative way to show the house is safe.
The code applies to new houses and to renovations under permit, where the municipal building department reviews the mechanical design. An existing kitchen with a big hood and an old water heater is not exempt from the physics. Residential make-up air is part of our indoor air quality and ventilation work; restaurant and commercial kitchen systems are on the separate make-up air units page.
Have the house checked when
- A hood rated well above a standard builder's hood is going into a house with a draft-hood water heater or a chimney-vented furnace
- The fireplace smokes into the room, or smells of soot, when the hood is running
- There are scorch marks or melted plastic rings on top of the water heater around the draft hood
- Doors whistle or pull shut, and the hood is loud but clears steam poorly
- A carbon monoxide alarm has sounded while cooking: leave the house, then call from outside
How it works
We list every fuel-burning appliance and how it vents, read the hood's rated airflow from its plate or spec sheet, and note the other exhaust fans: dryer, bathroom fans, a central vacuum vented outdoors.
With the hood and other exhausts running and the water heater firing, we check for spillage at the draft hood and measure house pressure against outdoors, the same instruments used on a carbon monoxide testing visit. The result decides between an interlocked damper, a powered and tempered make-up air fan, or changing the appliance to a sealed type, each priced in writing.
The duct, damper or fan, heater and controls go in, wired so make-up air starts whenever the hood does. The spillage test is repeated at full exhaust before the job is closed. Ask for an assessment before the hood is ordered if you can.
Common questions
How many CFM before a range hood needs make-up air in Ontario?
The current Ontario Building Code does not set a CFM number. Article 9.32.3.8 asks whether the house has a fuel-fired heating or water-heating appliance that is not direct-vent or mechanically vented; if it does, exhaust devices beyond the principal ventilation fan need matched make-up air, whatever their size. If every appliance is sealed, that trigger falls away, though a very large hood may still need replacement air to work properly.
Can I avoid make-up air by changing my water heater?
Often, yes. Replacing a natural-draft tank with a power-vented water heater, a direct-vent tank or a tankless unit, alongside a sealed two-pipe furnace, removes the appliances that can spill. That is frequently the cleaner answer in a kitchen renovation. An open wood fireplace remains a backdraft risk in practice and needs its own look before a big hood goes in.
Why does make-up air have to be heated?
Because a large hood on high pulls in a great deal of outdoor air. At −15°C that air would chill the kitchen floor, frost the duct and tempt people to switch the system off. The code calls for tempering to at least 12°C before the air reaches the house or its ducts, usually with an electric duct heater on its own circuit, installed by a licensed electrical contractor under an ESA notification.
Is a passive make-up air damper enough?
For a modest hood in a moderately tight house it can be. A motorised damper on a duct to outdoors opens when the hood runs and lets air in under the house's own suction. It only flows once the house is already negative, so with a high-capacity hood it may not hold pressure in a safe range. Under a permit the prescriptive route is a fan; a passive damper has to be justified by the depressurisation test.
What does residential make-up air cost?
There is no single price, and we do not publish one. Cost follows the hood's airflow, the length and route of the duct to an outside wall, whether the air needs an electric heater and a new circuit, and the interlock wiring to the hood. A passive damper is the small end; a powered, tempered system costs considerably more. The assessment ends in a written quote.