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Range Hood Makeup Air in Ontario: Big Hoods in Tight Houses, Backdrafting Gas Appliances, the 400 cfm Habit, Makeup Air Options and What It Costs
Heating, Cooling & Ventilation
A Big Range Hood in a Tight House Is a Fan Trying to Empty a Sealed Jar, and the Air It Gets Back May Come Down the Water Heater Chimney
Every hood sold has a cfm number on the box, and the number goes up every year because a bigger number sells. In a 1970s house full of drafts that never mattered; the air came back in around the windows. In a new house, an ICF house, or an old house that has just been air sealed, there is nowhere easy for 600 cfm to come back from, and the easiest path left may be backward down the vent of a gas appliance. That is a carbon monoxide problem, not a ventilation nuisance. This page explains what happens, what your inspector will ask, the makeup air choices, and what they cost.
What Happens When the Hood Goes On High
Air leaving a house has to be replaced by the same amount of air coming in. A 600 cfm hood pulls 600 cubic feet a minute out through the wall. In a tight house the cracks cannot supply that, so the house goes negative, like a juice box with the straw still in it. The air then comes in wherever it can, and a natural-draft water heater, an older furnace or a fireplace chimney is a large, open hole straight to the outdoors. The flow reverses, and the flue gases that were supposed to go up come down into the basement instead. That is backdrafting.
Sealed combustion appliances, the direct-vent kind with two pipes through the wall, take their combustion air from outdoors and are much harder to backdraft. Much harder is not never, and a wood stove or a gas fireplace in the same house changes the picture again. The point is not to panic about the hood. It is to have the venting looked at by someone qualified before the hood goes in.
The non-negotiables: a CO alarm on every floor, near the bedrooms. Gas venting and combustion air checked by a TSSA-registered contractor with a certified gas technician whenever the house is tightened or a big hood goes in. Nobody opens a sealed combustion appliance, disables a spill switch or touches the gas pressure to make a hood behave; the fix is makeup air, not a bypass.
The Code, the Inspector and the 400 cfm Habit
The rule is in Part 9 ventilation and your inspector will ask about makeup air on a large hood. I am not going to quote you a clause number or a cfm trigger from the Ontario code, because the right answer for your house depends on what else is in it and your inspector has the final word. Ask the building department before you buy the hood, not after it is on the wall.
The 400 cfm number you hear in every kitchen showroom is a habit, borrowed largely from building rules south of the border, where hoods over 400 cfm need makeup air. It is a fair warning sign in an Ontario house as well: above about 400 cfm in a tight house, assume you need makeup air and a venting check until somebody qualified tells you otherwise. The longer list of what the code wants from heating and ventilation is on the Ontario Building Code HVAC page.
How Big a Hood Do You Actually Need?
Most kitchens get a hood twice as big as the cooking needs, because the hood was chosen in the showroom and the house was not. These are trade rules of thumb, clearly that, not code:
- Gas range, rule of thumb: about 100 cfm per 10,000 BTU/h of burner. A 36 in. range whose burners add up to about 60,000 BTU/h comes out near 600 cfm by this habit, which is exactly why gas kitchens end up in makeup air territory.
- Electric or induction, wall hood, rule of thumb: about 100 cfm per linear foot of cooktop. A 30 in. induction top is about 250 cfm. Induction puts out less waste heat and smoke than gas, and the hood can be smaller.
- Capture beats horsepower. A hood at least as wide as the cooktop, a few inches wider if you can, mounted at the height the manufacturer says, with a deep enough canopy to hold the smoke, catches more than a bigger fan on a shallow hood. An island hood needs more air than a wall hood because there is no wall to help.
- The duct is part of the hood. Smooth metal, the diameter the hood calls for, short, with a proper backdraft damper and a wall cap. A 600 cfm fan on a crushed 6 in. duct is a 300 cfm hood with a lot of noise.
A well-shaped hood near 300 cfm over induction may need no makeup air at all; whether yours does is the inspector’s call, and choosing the smaller hood is often the cheapest makeup air system there is.
Makeup Air Options and What They Cost
| Item | What it is | Cost |
|---|---|---|
| Makeup air for a large hood | An outdoor air duct with a motorized damper wired to open when the hood runs, sometimes with a heater to temper the air | $1,000 to $3,000 (allowance) |
| Venting and combustion check on the gas appliances | A gas technician’s service visit; the annual service allowance | $150 to $250 |
| HRV rebalancing after the kitchen changes | Flow hood readings on the HRV, adjusted | $200 to $400 |
The makeup air figure is an allowance, not a quote. The low end is a short passive duct with an interlocked damper near the range; the high end is a long run with a heater so January air does not land on the cook’s ankles at minus 25 C.
Why Ontario prices are all over the map: the contractors and price guides we checked for this page mostly did not price makeup air as its own line at all, which is part of the problem; it gets buried in the kitchen quote or forgotten. The ductwork side swings the most. One home-services marketplace put HRV ducts alone at $1,300 to $6,600, and makeup air ducts vary for the same reasons: run length, finished ceilings, a heater or not, and whether the electrician is in. Budget $2,000, the middle of our allowance, and get two or three quotes on the same written scope.
Opening a window when the hood is on high does work, in July. In January it is a plan that lasts one supper. An HRV is not makeup air either; it is balanced, it brings in what it takes out and nothing more.
A Kitchen Renovation Priced
A 2015 house near Barrie that tested tight, getting a 36 in. gas range and a 600 cfm wall hood. Natural-draft gas water heater, sealed combustion furnace, an HRV. Makeup air with an interlocked damper and a heater, $2,000 from the middle of the allowance. A gas technician’s visit to check the water heater and furnace venting with the hood on high, $200. The HRV rebalanced after the new grilles, $300. Subtotal $2,500, HST $325, total $2,825. The interlock wiring is done by an ESA licensed electrical contractor under a permit, and that line belongs on the electrician’s quote. If the technician finds the water heater spilling with the hood on, the next conversation is about that water heater, and it is a better conversation to have now than after the CO alarm goes off.
The same kitchen with a 30 in. induction top and a well-shaped hood near 250 cfm may skip the $2,000 entirely. Keep the $200 venting check either way. The rest of the ventilation section is on the ventilation and ductwork hub, and if the house was just air sealed, read can a house be too airtight before you buy the hood.
Questions people ask
Do I need makeup air for my range hood in Ontario?
On a large hood in a tight house, very likely. The rule is in Part 9 ventilation and your inspector will ask about makeup air on a large hood, so ask the building department before you buy. Above about 400 cfm, treat makeup air as needed until someone qualified says otherwise.
Can a range hood cause carbon monoxide?
It can, indirectly. A big hood in a tight house can depressurize it enough to pull flue gases backward down the vent of a natural-draft water heater, furnace or fireplace. Keep a CO alarm on every floor and have a TSSA-registered gas contractor check the venting with the hood running.
How much does range hood makeup air cost?
Allow $1,000 to $3,000 before HST, depending on run length, finished ceilings and whether the air is heated. Add the electrician for the damper interlock and a gas technician’s venting check, about $150 to $250.
How many cfm does my range hood need?
As rules of thumb, not code: about 100 cfm per 10,000 BTU/h of burner for gas, and about 100 cfm per linear foot of cooktop for an electric or induction wall hood. Capture area and a good duct matter more than the number on the box.
Does my HRV provide makeup air for the hood?
No. An HRV is balanced; it brings in about what it exhausts. It does nothing to replace 600 cfm leaving through the hood.
Put the Kitchen Inside the Mechanical Plan
The cost comparison calculator prices the whole system five ways, so a big hood and its makeup air can be weighed against the heating and ventilation it affects. If the kitchen quote and the HVAC quote do not mention each other, the $99 review reads up to three quotes and tells you where the makeup air fell through the crack.
Sources and further reading
- Ontario’s Building Code: the official page with the current regulation; heating, ventilation and air conditioning in houses are Part 9, sections 9.32 (ventilation) and 9.33 (heating and air conditioning).
- TSSA: hire registered and certified fuels professionals: Technical Standards and Safety Authority: why gas furnace, boiler and water heater work must be done by a registered contractor with a certified technician, and how to check.
- Electrical Safety Authority: Ontario: licensed electrical contractors, permits for heat pump and water heater circuits, and how to check a licence.
