People shop hard for the exhaust hood and forget the other half: the air that replaces what the fan throws outside. Every cubic foot your exhaust fan pulls out has to be replaced by a cubic foot coming in — that replacement air is called make-up air (sometimes MUA, or tempered supply air). If you don't supply it deliberately, the building will pull it in through whatever it can find: door gaps, the dining room, even down the water-heater flue. A hood without matched make-up air is a common, expensive design mistake.
When the fan removes more air than comes in, the kitchen goes into negative pressure. The symptoms are easy to recognize once you know them:
Ironically, a starved hood also makes the fan work harder while capturing less; industry guidance notes capture can drop meaningfully when make-up air is inadequate.
A hood does not have to replace 100% of the exhaust through the make-up air unit. The common design target is to introduce roughly 80–90% of the exhaust volume as dedicated make-up air, letting the remaining 10–20% come from transfer air — conditioned air moving in from the dining room and adjacent spaces through the building's HVAC. That small deliberate imbalance keeps the kitchen slightly negative relative to the dining room, so odors and heat stay out of the customer area.
| Concept | Rule of thumb | Why it matters |
|---|---|---|
| Dedicated make-up air | ~80–90% of exhaust CFM | Replaces most of what the fan removes |
| Transfer air | ~10–20% of exhaust CFM | Keeps kitchen slightly negative vs dining room |
| Direct/short-circuit supply at the hood | Keep low (often ≤10%) | Too much dumped at the hood ruins capture |
Use our hood CFM calculator to estimate exhaust volume first — make-up air is sized from that number, not the other way around.
In mild climates a simple untempered supply fan may be allowed. In cold climates, dumping outdoor air straight onto the line is miserable and can freeze pipes, so make-up air units are usually tempered — heated (and sometimes evaporatively cooled) before delivery. Tempering is the single biggest cost and energy driver of the ventilation system, which is why many energy codes push for demand-controlled ventilation that ramps the fans up and down with actual cooking load instead of running full blast all day.
Tip: Ask any hood quote whether make-up air is included and how it's tempered. A cheap hood package that leaves you to solve make-up air later almost always costs more in the end — and can fail inspection. See requirements by location for local mechanical-code specifics.