Does Your Range Hood Need Makeup Air? The 400 CFM Line and What Sits Behind It

Kitchen ventilation is sold on one number. Every hood is a cfm figure, higher is understood to be better, and the professional-style ranges that anchor most kitchen remodels are usually paired with a hood chosen to match their appearance rather than their exhaust requirement.

There is a line in the residential code at 400 cfm, and crossing it changes the job. Not the hood — the house. Above that figure, the building has to be given a way to replace the air the hood removes, and that provision is a duct, a damper, an interlock and a place for the air to come in.

It is one of the most reliably discovered-rather-than-budgeted items on a kitchen remodel, and it is discovered at exactly the wrong moment: after the cabinetry is drawn and the appliance is bought.

What the code actually says

The provision is short:

Each exhaust hood system capable of exhausting in excess of 400 cubic feet per minute shall be provided with makeup air at a rate approximately equal to the exhaust air rate.

It is numbered M1503.4 in the 2012 through 2018 editions and M1503.6 in the 2021 and 2024 editions. The substance is unchanged across that renumbering, which is worth stating rather than silently picking one, because a citation found online is evidence of what some edition says rather than of what the adopted one does.

Two phrases in that sentence carry the whole requirement, and both are commonly misreported.

"Capable of exhausting"

The trigger is capability, not usage.

A 600 cfm hood triggers the requirement whether or not anybody ever runs it above the first speed. The code is describing what the installed equipment can do, not what the current occupant intends to do with it — and that is the right way round, because occupants change, and because the failure this provision prevents happens on the one evening the hood runs flat out with the windows shut.

The practical consequence is that the number to check is the hood's rating as installed, including whatever blower option was specified. Many hoods are sold as a canopy with a choice of internal, in-line or remote blowers at different ratings, and the canopy's marketing page is not the rating.

"At a rate approximately equal to the exhaust air rate"

This is the detail that is misreported most often, including by sources that are otherwise careful: that the requirement is for makeup air equal to the amount in excess of 400 cfm.

The code text does not say that. It says approximately equal to the exhaust air rate.

Why the requirement exists

Not for comfort, and not to stop a door being hard to open. The reason is combustion safety.

A fan that removes several hundred cubic feet of air per minute from a building has to be replaced from somewhere. In a reasonably tight house there is no large deliberate opening, so the air comes in through whatever path offers least resistance — and in a house with a naturally drafting fuel appliance, that path is frequently the appliance's own vent, running backwards.

A furnace or water heater with a draft hood relies on buoyancy alone. It has the weakest pressure connection to the outdoors of anything in the building. It will reverse before a window seal does, before a door sweep does, and before the envelope leaks enough to satisfy the hood. When it reverses, combustion products come into the house instead of leaving it.

This is the same physics that makes an appliance closet a design problem in the first place, and it is worth being explicit about the difference between the two requirements, because the fix for one is not the fix for the other:

Does your furnace or water heater have enough combustion air covers the first column in full. The point worth carrying across is that a closet that passes the combustion-air arithmetic can still be put into backdraft by a hood in the kitchen. They are different requirements, in different chapters, triggered by different things, and satisfying one does not dispose of the other.

The severity also depends on what else is in the building. Direct-vent and mechanically drafted appliances — sealed combustion, fan-assisted flues — are far less vulnerable, because their air supply and flue path do not depend on the pressure in the room. An all-electric house, or one with only direct-vent appliances, faces a comfort and capture-efficiency problem rather than a combustion-safety one. The code requirement does not change; the consequence of ignoring it does.

The damper, and why an ordinary backdraft flap will not do

A makeup air opening is a hole in the building envelope. Left open, it is a permanent thermal and comfort penalty, so the code requires a means of closure.

In the current editions that is stated as a damper requirement. Each damper is to be a gravity damper or an electrically operated damper that automatically opens when the exhaust system operates. And there is a condition that rules out the obvious cheap solution:

Gravity or barometric dampers shall not be used in passive makeup air systems except where the dampers are rated to provide the design makeup airflow at a pressure differential of 0.01 inch water column or less.

That is a demanding specification. An ordinary sheet-metal backdraft flap needs considerably more pressure than that to open meaningfully, which means a passive makeup air system built from generic parts will not deliver its design airflow until the house is already depressurised well past the point where the appliance vent has reversed. The damper has to be a rated one, or the system has to be motorised and interlocked with the hood.

Dampers are also required to be positioned so they are accessible for inspection, service, repair and replacement without removing permanent construction — which matters, because a damper buried above a soffit is a damper nobody will ever confirm still works.

The design decision nobody frames as one

The most useful thing in this article is not the requirement. It is that the requirement is avoidable by choice, and the choice is made months before anybody costs it.

A hood at or below 400 cfm does not trigger the provision. For a great many kitchens that is not a compromise: capture efficiency depends much more on hood geometry than on raw airflow. How far the canopy projects over the front burners, how deep it is, and how close it sits to the cooking surface determine whether the plume is captured at all. A well-proportioned 350 cfm hood over a standard range routinely outperforms a badly proportioned 900 cfm one, because the second is pulling hard on air that has already escaped past its edge.

So the real question at the design stage is not "how many cfm can I get." It is:

1. What is the hood's rated cfm as installed, with the blower actually specified?
2. Does that exceed 400?
3. If it does — where is the makeup air coming from, and is it in this price?

Three questions, answerable in a sentence each, asked while the kitchen is still on paper. Asked then, the answer is a design choice. Asked at plan review, it is a change order against cabinetry that has already been ordered.

Which edition, and which amendments

The figures above are the model code's. Adoption is not uniform: jurisdictions adopt different editions and several amend the mechanical provisions, and some energy codes impose their own requirements on large exhaust appliances independently of the residential code.

The numbering alone makes the point — M1503.4 in the 2012 through 2018 editions, M1503.6 in the 2021 and 2024 ones. Which text governs a specific address is the authority having jurisdiction's answer, settled at permit rather than argued at inspection.

The pattern is the same one that runs through where a bathroom exhaust fan has to vent: an exhaust device is selected on a headline number, installed into conditions nobody checked, and the consequence lands somewhere other than the room it was installed in.

Seamless Home owns the design, permit and inspection layer on the work it runs, which is where a hood selection stops being an appliance decision and becomes a mechanical one. Coverage is confirmed per service area rather than promised as blanket availability. If a kitchen remodel is in front of you, tell us what you are planning and we will tell you what the scope actually contains.