The Range Hood Rule Is Not About How Powerful Your Hood Is

The two conditions in Article 9.32.4.1

The article sits in Subsection 9.32.4, Additional Protection Against Depressurization. Depressurization means the air pressure inside the house drops below the air pressure outside. That happens when a fan pushes more air out of the building than is coming back in.

Sentence (1) reads: additional make-up air for the actual appliance exhaust rate shall be provided for any appliance that discharges air to the exterior at an installed rate exceeding 0.5 air change per hour when it is located within a dwelling unit that contains a vented appliance that is subject to back drafting (Naturally Aspirating Fuel-Fired Vented Appliance).

Read the sentence as two tests joined by the word when. The first test is about the fan: does it move air out faster than 0.5 air change per hour. The second test is about the house: does the dwelling unit contain an appliance that burns fuel and relies on natural draft to get rid of its exhaust. Both have to be true before the requirement lands.

Makeup air is replacement air. It is outdoor air brought in on purpose to replace what the exhaust fan removed. The article calls for it only where the combination of a strong fan and a natural-draft appliance creates a real hazard.

What a naturally aspirating fuel-fired vented appliance is

The phrase in the brackets is doing the heavy lifting, so take it one word at a time. Fuel-fired means the appliance burns something: natural gas, propane, oil, or wood. Vented means its exhaust leaves the house through a chimney or a flue. Naturally aspirating means no fan pushes that exhaust out. Hot gas rises on its own, and that rising motion is the only thing carrying the fumes up and away.

Older gas furnaces work this way. So do older gas water heaters with a draft hood, which is the metal collar sitting on top of the tank where the flue pipe starts. A wood-burning fireplace is the same idea with a larger opening.

The opposite is a sealed direct-vent appliance. Direct vent means the appliance draws its combustion air from outside through one pipe and pushes its exhaust outside through another, with a fan doing the pushing. The burner is sealed off from the room. A house air pressure change does not reverse that flow the way it can reverse a natural draft.

So the second condition is a question about the rest of your house. If every fuel-burning appliance you own is sealed and fan-vented, or if you are all electric, the dwelling unit does not contain a vented appliance subject to back drafting, and Article 9.32.4.1 does not apply no matter how strong the hood is.

• Older gas furnace venting into a masonry or metal chimney with no fan on the exhaust.
• Gas storage water heater with a draft hood on top of the tank.
• Wood-burning fireplace or a wood stove venting up a chimney.
• Older gas fireplace with an open front, where the room air feeds the flame.
• Sealed direct-vent furnaces, sealed direct-vent tankless water heaters, heat pumps, and electric appliances sit outside this group, because a fan controls their airflow.

Why back drafting is the hazard the article prevents

Picture the kitchen hood running at full speed in a house with good air sealing. The fan removes a large volume of air every minute. That air has to be replaced, because a house cannot hold a vacuum. Replacement air comes in wherever the path of least resistance happens to be.

In a leaky old house, that path is the walls, the windows, and a hundred small gaps. In an air-sealed house, those gaps are closed, and the largest remaining opening to the outdoors is often the chimney of a natural-draft appliance.

When air flows down that chimney instead of up it, the exhaust from the water heater or furnace comes with it. That exhaust contains carbon monoxide, a gas you cannot see or smell that is dangerous at low levels. Air flowing the wrong way down a flue is what back drafting means.

This is why the appendix to Section 9.32 makes the point that builders can no longer rely on uncontrolled ventilation through the building envelope. As air barriers got better, the accidental leaks that used to supply replacement air went away. Mechanical systems now have to provide ventilation in most buildings on purpose, and depressurization has to be designed for rather than absorbed by a leaky wall.

Why 0.5 air change per hour is not a CFM number

An air change per hour counts how many times the entire volume of air inside the dwelling unit is swapped out in one hour. One air change per hour means the fan moved a volume of air equal to the whole interior volume of the home. So 0.5 air change per hour means half that volume in an hour.

CFM means cubic feet per minute, and it measures how much air a fan moves. A CFM rating describes the fan alone. An air change rate describes the fan in relation to the house it sits in.

That relationship is why the same hood gives two different answers in two different homes. Put it in a small condo with a low interior volume and the exhaust rate crosses 0.5 air change per hour. Put the identical hood in a large open home with high ceilings and it may sit under the threshold, because there is more air inside to move.

A designer works this out from the measured interior volume of the dwelling unit and the installed exhaust rate of the appliance. The Code says the installed rate, so the number that counts is what the hood will actually move once it is ducted, capped, and mounted, rather than the figure on the box.

What the Code asks for once makeup air is required

Sentence (2) sets the size of the system: where additional make-up air is required for appliances described in Sentence (1), it shall be provided by a supply fan rated to deliver outdoor air at the rate of the installed exhaust appliance. A supply fan is a fan that pushes outdoor air in, and it has to match the hood it serves.

Sentence (3) sets the control: the supply fan as required in Sentence (2) shall be interconnected with the exhaust fan for which make-up air is required. Interconnected means wired together so the supply fan starts when the hood starts. A homeowner cannot leave it off, and the system cannot depend on anyone remembering to switch it on.

Sentence (4) sets the temperature of the incoming air. The outdoor air shall be tempered to at least 1°C before being introduced to a normally unoccupied area of the dwelling unit, or tempered to at least 12°C before being introduced to occupied areas either by passive transfer grille or directly from outside. Tempered means heated before it reaches the room.

That tempering clause is where the budget moves. Heating a large volume of incoming outdoor air needs a heater, and a heater needs its own electrical circuit and its own space. On a winter night in North Vancouver the outdoor air arriving behind that fan is cold, and the system has to lift it to 12°C before it enters a kitchen where people are standing.

The ducting, the grilles, and the inlet outside

Sentence (4) offers two ways to get tempered air into an occupied area: by passive transfer grille or directly from outside. A passive transfer grille is a vent in a wall or door that lets air move between rooms with no fan behind it. That option means the air can be brought into one space and allowed to move toward the kitchen through a grille rather than ducted to the kitchen itself.

Air moving between rooms is governed elsewhere in the same section. Article 9.32.3.10 requires interior doors to be undercut by a minimum of 12 mm above the finished floor, or the rooms to be provided with an air-transfer grille with an unobstructed vent area of not less than 100 cm2. A closed bedroom door with new thick flooring under it can wipe out that undercut, which is a small detail that changes how the whole system behaves.

The outdoor end is covered by Article 9.32.3.9. Outdoor air inlets and exhaust outlets have to be shielded from weather, birds, and rodents, using hoods that include a screen made of corrosion-resistant material with openings of 6 to 12 mm. On the North Shore that screening earns its keep, because a large open inlet on a wet wall is an invitation.

One rule that catches mechanical layouts: Sentence 9.32.3.8(8) states that supply ducts for a mechanical ventilation system shall not be used to provide combustion or dilution air to fuel-burning appliances. The makeup air duct serves the room, and the fuel-burning appliance gets its air by its own means.

• Wall or roof penetration for the supply inlet, placed away from the exhaust outlet and from any driveway.
• Screened hood on the inlet, corrosion-resistant, openings of 6 to 12 mm.
• Duct run from inlet to heater to the room, planned before cabinets are drawn.
• Door undercuts of at least 12 mm, or transfer grilles of at least 100 cm2, where air has to cross between rooms.
• Control wiring between the hood and the supply fan so the two start together.

How this sits beside the ventilation your house already needs

Article 9.32.4.1 sits on top of a system that is required anyway. Appendix note A-9.32.3 explains that every dwelling unit must include a principal ventilation system under Article 9.32.3.3, and that this system is the combination of an exhaust fan and a supply fan, with a passive supply allowed in some instances under Sentence 9.32.3.4.(6).

The principal ventilation system exhaust fan is a separate requirement from the fan in every bathroom and every kitchen. One fan can do two jobs: the appendix says a bathroom fan may satisfy both the principal ventilation exhaust fan requirement and the bathroom fan requirement. The condition is that the requirements for each are met, and where a fan serves both functions it needs controls conforming to Sentences 9.32.3.5.(3) and (4).

For a kitchen renovation that matters because the hood, the principal ventilation system, and any makeup air system all share the same house and the same walls. Designing them one at a time produces three separate wall penetrations and a set of fans that fight each other.

The people who get this right treat ventilation as a single design item at the start of the project, with one drawing showing every inlet, every outlet, and every fan. That drawing goes to the building department with the permit application rather than after the cabinets are ordered.

The cheaper answer is often the other appliance

Because the trigger needs both conditions, there are two ways out of it. Reduce the exhaust rate, or remove the natural-draft appliance from the house.

The second route is the one most North Shore homeowners overlook, and it is often the one already on their list. Replacing an old draft-hood gas water heater with a sealed direct-vent unit or an electric heat pump water heater removes an appliance subject to back drafting. Doing the same with an old natural-draft furnace does the same thing. Once no natural-draft vented appliance remains in the dwelling unit, the second condition of Sentence (1) is no longer met.

A heat pump retrofit or an energy upgrade usually replaces those appliances anyway. If that work is already planned for the next few years, pulling it forward to run alongside the kitchen can be cheaper than building a tempered makeup air system with its own heater, its own circuit, and its own hole in the wall.

A wood-burning fireplace is the harder case, because it is often the feature the homeowner wants to keep. That decision belongs on the table at design stage with the hood selection, since the two are linked by this article.

• Replace a draft-hood gas water heater with a sealed direct-vent or heat pump water heater.
• Replace an old natural-draft furnace with a sealed direct-vent unit or a heat pump.
• Choose a hood with a lower installed exhaust rate so the first condition is not met.
• Decide what happens to an existing wood-burning fireplace before the hood is ordered.
• Run the air change calculation on the real interior volume before any of the above is priced.

Where this belongs in a renovation schedule

The makeup air decision is a design-stage decision, and it touches four things that are hard to change later. The electrical load, because a tempering heater needs a circuit and the panel has to have room for it. The duct routing, because a supply duct has to get from an outside wall to the room. The wall penetration, because the inlet has to sit somewhere that meets Article 9.32.3.9 and still looks acceptable from the street. The cabinet layout, because a duct and a heater take up space that somebody was planning to use for storage.

Every one of those gets expensive once the cabinets are ordered. A supply duct discovered during installation means moving cabinets, opening finished walls, or dropping a bulkhead over a run that nobody drew.

The sequence we use is simple. Confirm which fuel-burning appliances are in the house and how each one vents, before a hood is selected. Get the interior volume and the installed exhaust rate from a designer, and get the air change calculation in writing. Take that to the building department along with the rest of the permit package.

A CFM number from a showroom is not that calculation. The showroom does not know the volume of your house, and it does not know what is venting into your chimney.