Range Hood Makeup Air: Why a Big Hood Can’t Just Pull Air Out of Nowhere
A 1,200-CFM hood doesn’t create air — it removes it. If nothing replaces what it pulls out, the whole house goes negative, and that’s where the trouble starts.
Makeup air is outdoor air brought in deliberately to replace what a kitchen exhaust hood blows out. A hood can only remove air that something else lets back in — so a powerful hood, running in a house that can’t supply the replacement, drives the indoor pressure negative. That’s why building code requires makeup air for any hood over 400 CFM: a negative house can pull combustion gases back down a flue, won’t let the hood reach its rated capture, and drags in unfiltered, humid outdoor air through every remaining crack.
- A hood exhausts air; that air has to be replaced from somewhere, or the house goes negative.
- IRC Section M1503 requires makeup air for any kitchen hood that can exhaust more than 400 CFM, at a rate roughly equal to the exhaust.
- The makeup air damper must open automatically whenever the hood runs.
- A strongly negative house can backdraft a water heater, furnace, or fireplace — pulling carbon monoxide indoors.
- Two 1,200-CFM hoods running together pull 2,400 CFM — far more than any house leaks naturally.
You can’t exhaust air that isn’t replaced
This is the part homeowners rarely hear when they spec a restaurant-style hood: a hood is a one-way pump. It throws conditioned indoor air outside, and the only way the room can give up that air is if an equal amount comes back in somewhere else. In a leaky old house, that replacement air sneaks in through gaps and the house barely notices. In a tight, well-built modern house, there’s almost nowhere for it to come from — so the pressure inside drops, the hood fights its own vacuum, and the house starts pulling air down any path it can find, including paths you very much don’t want.
The 400-CFM line in the code
This isn’t a matter of opinion — it’s in the building code. Section M1503 of the International Residential Code (numbered M1503.4 in older editions, M1503.6 in the 2021 and 2024 code) requires that any kitchen exhaust hood capable of moving more than 400 CFM be provided with makeup air at a rate approximately equal to the exhaust rate. The makeup air can be brought in mechanically or passively, but it has to have at least one damper that opens automatically whenever the hood runs — so the replacement air shows up exactly when the exhaust is pulling.
Four hundred CFM is a low bar. Plenty of mid-range hoods exceed it, and the large hoods going into kitchens today blow well past it. The current code focuses the requirement on homes that have an atmospheric (non-direct-vent) fuel-burning appliance inside the building, because that’s where depressurization turns dangerous — but the physics of a negative house applies to every home.
The worst case: two 1,200-CFM hoods
High-end kitchens increasingly have two hoods — one over the range, one over a separate cooktop or island. Run them together and you’re exhausting 2,400 CFM. To put that in perspective, the whole-house ventilation a tight home needs is on the order of 90 CFM. This is more than twenty-five times that, pulled out of the house all at once.
| Air being removed | Where 2,400 CFM of replacement air comes from |
|---|---|
| Leaky older house | Some sneaks in through gaps — but at this volume, still nowhere near enough. |
| Tight modern house | Almost none. The house goes sharply negative. |
| With a dedicated makeup air system | Roughly 2,400 CFM supplied on purpose, damper opening with the hoods. |
No house envelope leaks at 2,400 CFM. So without a dedicated makeup air system, those two hoods will depressurize the home hard — backdrafting risk, doors that resist opening, and a hood that never actually achieves its rated capture because it’s starving. The replacement air has to be designed in, not hoped for.
Why this lands on the load calculation
Here’s the connection people miss: makeup air is outdoor air, and outdoor air is a load. If that 2,400 CFM is tempered before it enters the house, it adds a large sensible and — in a hot, humid climate — a very large latent load while the hoods run. If it’s dumped in raw to save cost, the house pays in comfort and humidity instead. Either way, a big hood is not just a kitchen decision; it’s a whole-house pressure and load decision.
It also ties directly to airtight homes and ventilation: the tighter and better-built the house, the more it needs a dedicated makeup air system, because it has no accidental leakage to fall back on. A proper load calculation is where the makeup air, the ventilation, and the equipment get reconciled, so the house stays balanced, safe, and comfortable when everything is running at once. We do this work nationwide and residentially, with select light commercial such as small offices and recreation centers.
Frequently asked questions
What is makeup air for a range hood?
Makeup air is outdoor air brought into the home to replace the air a kitchen exhaust hood blows outside. Without it, the hood pulls the house into negative pressure because the exhausted air has nowhere to be replaced from.
At what CFM is makeup air required by code?
Section M1503 of the International Residential Code requires makeup air for any kitchen exhaust hood capable of exhausting more than 400 CFM, supplied at a rate approximately equal to the exhaust rate.
Why is a depressurized house dangerous?
A strongly negative house can reverse the draft on an atmospheric water heater, furnace, or fireplace, pulling combustion gases including carbon monoxide back into the living space instead of up the flue.
How much makeup air do two 1,200-CFM hoods need?
Running together they exhaust about 2,400 CFM, so they need roughly 2,400 CFM of makeup air. No house leaks that fast on its own, so the replacement air must be supplied by a dedicated system with an automatic damper that opens when the hoods run.
Does makeup air affect my heating and cooling load?
Yes, if the makeup air is tempered. Outdoor air brought in to replace hood exhaust adds a sensible load year-round and a significant latent load in humid climates while the hoods are running, which a load calculation should account for.
Do you handle makeup air nationwide?
Yes. We account for kitchen exhaust and makeup air as part of the whole-house pressure and load picture for residential projects across the country.
Sizing a big hood? Get the whole-house picture first
We reconcile kitchen exhaust, makeup air, ventilation, and equipment in one load calculation, so the house stays balanced and safe with everything running. We work with builders, contractors, and homeowners nationwide.
See pricing & start your project →