How the two differ in kind
A commercial kitchen hood is sized to capture and remove the heat, grease, smoke and moisture that cooking produces, and the exhaust rates involved are large. That air has to be replaced.
MAKE-UP AIR is the deliberate replacement: outdoor air brought in, tempered where the climate requires it, and introduced in a way that does not disturb the hood's capture. Provided properly, the kitchen sits close to neutral pressure, the hood achieves its design flow, and the building behaves.
Without it, the building goes NEGATIVE — the exhaust fan is removing air and the only way replacement can arrive is through whatever openings exist — and two things follow, one unsafe and one merely expensive.
The unsafe one first. A building held negative draws air through any path it can find, and a commercial kitchen commonly contains atmospherically vented gas appliances whose flues rely on a weak, buoyancy-driven draught. A sufficiently negative space can overcome that draught and pull combustion products back down the flue into the room — which is a carbon monoxide hazard, and it scales with the exhaust rate. The same mechanism draws air from adjoining spaces, from a crawl space or basement, and from outside through every door, which is why kitchen doors become hard to open and why the building whistles.
The expensive one: a fan working against a building it cannot draw air into does not achieve its rated flow. The hood's capture depends on that flow, so an under-supplied kitchen has a hood that does not capture — smoke and grease escaping into the space, which is the failure the hood exists to prevent.
And the make-up air's INTRODUCTION matters as much as its quantity: a supply blowing across the hood's face destroys the capture it was installed to enable.
The factors that actually differ
Show
| Hood exhaust rate | Make-up air | |
|---|---|---|
| What it does | Removes heat, grease, smoke and moisture from the cooking line. | Replaces the air removed, so the building does not go negative. |
| What sets the quantity | The appliance line-up, the hood type and its overhang, and the capture velocity required. | The exhaust rate, less whatever the building's other supply provides — most of it, deliberately. |
| Safety consequence if absent | Not applicable. | Backdrafting of atmospherically vented appliances, which is a carbon monoxide hazard. |
| Performance consequence if absent | The fan cannot achieve its rated flow, so the hood stops capturing. | The kitchen is smoky and greasy despite having a hood. |
| How it is introduced | Through the hood, at a designed capture velocity. | Low velocity, away from the hood's face — a supply blowing across it destroys the capture. |
| Tempering | Not applicable. | Required in cold climates, or the kitchen is unworkable — which is why untempered systems get switched off. |
| Energy | Fan power, plus the conditioned air being thrown away. | The dominant cost — heating or cooling a large volume of outdoor air continuously. |
| Demand control | Variable-speed hoods reduce the exhaust when the line is idle, which is the main saving available. | Must track the exhaust, or the balance is lost whenever the hood modulates. |
| Interlock | Should not run without its make-up air running. | Interlocked with the exhaust, so the two start and stop together. |
| Who checks it | Commissioned by measurement, not assumed from the fan's nameplate. | The same, and the pressure differential across the kitchen is the check that matters. |
Which one, and when
Choose hood exhaust rate when…
- Sizing the hood for the appliance line-up beneath it — the starting point.
- Where capture is failing and the exhaust rate or the hood's geometry is the suspect.
- Where appliances change, since a heavier duty line-up needs more exhaust.
- Specifying demand control, which is where the running cost can genuinely be reduced.
Choose make-up air when…
- Always, alongside the exhaust — it is not an optional addition.
- Where there is any atmospherically vented appliance in or connected to the space.
- Where doors are hard to open, the kitchen whistles, or pilot lights blow out — the symptoms of negative pressure.
- Where the hood does not capture despite a correctly sized fan, which is the performance symptom.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Why is negative pressure dangerous in a kitchen?
- Because of what is connected to the space. An atmospherically vented gas appliance — an open-flue water heater, a boiler, some cooking equipment — relies on a weak draught created by hot gases rising in its flue, and that draught can be overcome by a room held sufficiently below outdoor pressure. When it is, the flow reverses and combustion products enter the room instead of leaving, which is a carbon monoxide hazard rather than an efficiency one. A commercial kitchen exhaust is large enough to do this comfortably in a building with no make-up air. The lesser symptoms point the same way: doors that are hard to open, air whistling through gaps, and pilot lights blowing out. Sealed-combustion and direct-vent appliances take their air from outside and are not affected, which is one reason they have become standard where they are available.
- Why does the hood stop capturing without make-up air?
- Because the fan cannot move air the building will not let in. A fan's flow depends on the pressure it works against, and a building that cannot supply replacement air presents a rising resistance as it goes negative — so the exhaust falls below its design rate. Hood capture depends on that rate: the design assumes a face velocity across the hood's opening sufficient to draw the thermal plume from the appliances up into the hood, and below it the plume escapes at the edges and spills into the kitchen. The result is a kitchen with a correctly sized hood, a correctly sized fan and smoke rolling out from under it — which gets diagnosed as a hood problem when it is an air supply problem, and is not fixed by a bigger fan.
- How should make-up air be introduced?
- Gently, and away from the hood's face — because the capture depends on a stable thermal plume rising into the hood, and any cross-draught disturbs it. Air supplied at high velocity toward the cooking line blows the plume sideways and out from under the hood, which destroys the capture the hood was designed for; the same happens with an open door in the wrong place, or a fan cooling the cooks. Good practice introduces make-up air at low velocity through large diffusers or perforated faces some distance from the line, or through short-circuit arrangements built into the hood itself which supply air directly into the capture zone. The design also has to consider where the air goes afterwards, since a kitchen supplied only at one end has a draught along it.
- Does make-up air need heating?
- In any cold climate, yes, and untempered systems are the ones that get switched off. A commercial kitchen's exhaust rate is large, so the make-up air volume is large, and introducing that volume at outdoor winter temperature makes the kitchen unworkable at the point where the air enters — cold enough that staff block the supply, tape over the diffusers, or turn the unit off, at which point the building is negative again and the safety problem is back. Tempering is therefore part of the system rather than a refinement, and it is also the dominant operating cost, which is why demand-controlled ventilation has become standard on new installations: reducing the exhaust when the cooking line is idle reduces the make-up air and the energy to temper it in the same proportion.
- What is demand-controlled kitchen ventilation?
- A control strategy that varies the exhaust rate with what the cooking line is actually doing, rather than running at design flow whenever the kitchen is open. Sensors — temperature in the hood, and often optical sensors detecting smoke and vapour — detect cooking activity, and the exhaust and make-up air fans modulate together, dropping to a low rate when the appliances are idle and rising as cooking starts. The saving is substantial because a commercial kitchen spends much of its open hours below full load, and every unit of exhaust avoided is also a unit of make-up air that does not have to be tempered. The requirement is that the make-up air TRACKS the exhaust: if it does not, every modulation unbalances the building, which reintroduces the problem the make-up air was installed to solve.
- How is the exhaust rate determined?
- From the appliances under the hood and from the hood's own geometry, rather than from the kitchen's size. The rate needed depends on the duty of the appliances — a charbroiler produces a far more energetic plume than a steamer — on the hood type and whether it has side panels, and on the overhang beyond the appliance edges, since a hood that overhangs generously captures a wider plume at a lower flow. Codes and standards classify appliances by duty and give rates per unit length of hood for each combination. The practical consequences are that changing the appliances changes the required exhaust, and that improving the hood's geometry — more overhang, side panels — reduces the flow required, which is the cheapest saving available on an existing kitchen.
- How is the balance verified?
- By measuring, at commissioning and periodically afterwards, rather than by assuming the fans do what their nameplates say. The measurements that matter are the actual exhaust and supply flows, and the pressure differential between the kitchen and the adjoining spaces and outdoors — which is the number that tells you whether the building is negative. Where atmospherically vented appliances are present, a worst-case depressurisation test is the specific check: every exhaust device is run simultaneously with the building in its most unfavourable configuration, and the appliances are tested for correct draught under that condition. That is a defined procedure rather than an informal check, and it is what establishes that the kitchen is safe in the state it will actually be operated in.
- Does any of this apply to a domestic kitchen?
- In milder form, and increasingly, because domestic extract rates have risen while houses have become tighter. A large domestic range hood can move a substantial volume, and in a well-sealed house that is enough to depressurise it noticeably — with the same consequences in kind: backdrafting risk where there is an open-flue appliance or a solid-fuel stove, make-up air drawn from a garage or a crawl space, and a hood that does not capture because the fan cannot achieve its flow. Several codes now require make-up air for domestic hoods above a threshold extract rate for exactly this reason. The domestic remedies are the same: a dedicated make-up air path interlocked with the hood, and sealed-combustion appliances rather than open-flue ones.