When a commercial kitchen canopy underperforms, the instinct is to blame the canopy or the fan. Most of the time the real culprit is invisible: there isn't enough fresh air getting back into the kitchen to replace what the exhaust pulls out. This is the make-up air problem, and it's the single most misunderstood part of commercial kitchen ventilation.
This guide explains what make-up air is, why your kitchen needs it, what AS 1668.2 requires, and how to diagnose and fix the negative-pressure problems that come from getting it wrong. If you've got slamming doors, lifting burner flames, or a heating bill that doesn't make sense, this is almost certainly why.
Every cubic metre of air your exhaust canopy pulls out of the kitchen has to be replaced by a cubic metre of air coming in. That replacement air is called make-up air. It's a basic law of physics — air can't be created or destroyed, only moved. If your fan removes 1,500 litres of air per second, then 1,500 litres per second has to enter the kitchen from somewhere.
In a well-designed kitchen, most of that replacement air comes from a dedicated supply system that brings in tempered outside air on purpose. In a poorly designed kitchen, it gets pulled in through whatever gaps it can find — under doors, through windows, down the flue, through the dining room — at whatever temperature the outside happens to be.
The difference between those two scenarios is the difference between a kitchen that works comfortably and one that fights itself every service.
AS 1668.2:2024 sets the make-up air balance for commercial kitchens. The supply air system must deliver between 65% and 85% of the total exhaust flow rate. The remaining 15–35% is drawn from adjacent spaces as transfer air — for example, conditioned air migrating in from a dining room through a service hatch or doorway.
The reason it isn't 100% is deliberate. Kitchens are designed to run at slightly negative pressure relative to the dining room so that cooking smells, smoke, and grease-laden air don't drift out into the customer space. By supplying 65–85% directly and pulling the remainder from the dining side, the kitchen stays just negative enough to contain odours without going so negative that it causes problems.
The worked relationship looks like this:
| Exhaust flow rate | Make-up air at 65% | Make-up air at 85% |
|---|---|---|
| 600 L/s | 390 L/s | 510 L/s |
| 1,000 L/s | 650 L/s | 850 L/s |
| 1,500 L/s | 975 L/s | 1,275 L/s |
| 2,000 L/s | 1,300 L/s | 1,700 L/s |
Your building surveyor checks this balance as part of the AS 1668.2 compliance assessment during the building permit process. Our canopy sizing guide walks through how the exhaust flow rate itself is calculated from your cooking process types.
When there isn't enough make-up air, the kitchen goes too far into negative pressure. The exhaust fan keeps trying to pull its rated airflow, but there's not enough replacement air entering, so the pressure inside the kitchen drops below the pressure outside. The consequences cascade:
The classic symptom. Negative pressure means the outside air is effectively pushing the doors closed. Staff have to pull hard to open an external door, and it slams behind them. This is the most obvious sign of a make-up air deficit and the one operators notice first.
Negative pressure disrupts the air supply to gas burners. Flames lift off the burner, burn yellow instead of blue, or go out entirely. This is a genuine safety concern — incomplete combustion produces carbon monoxide. Under AS/NZS 5601.1:2022 (the gas installation standard), proper ventilation is part of safe gas appliance operation.
Counterintuitively, too little make-up air makes the canopy worse at its job. Without enough replacement air, the fan can't actually move its rated volume, so capture velocity at the canopy drops. Smoke and grease that should be drawn up into the filters instead spills out the sides into the kitchen.
In severe cases, negative pressure can reverse the draught in a water heater flue or other natural-draught appliance, pulling combustion gases back into the building. This is dangerous and is one of the reasons AS 1668.2 takes make-up air seriously.
When replacement air enters through random gaps instead of a controlled supply, it comes in at outside temperature. In Melbourne winter that's cold air your heaters have to warm; in summer it's hot air your cooling has to fight. A controlled, tempered make-up air supply is far more energy-efficient than air leaking in through doors and windows. We covered the seasonal side of this in our pre-winter ventilation checklist.
There are several ways to deliver make-up air, ranging from simple to sophisticated. The right choice depends on your exhaust volume, building, climate, and budget.
A supply fan draws outside air through a filter and delivers it directly into the kitchen, usually near the canopy. This is the standard solution for medium and large kitchens. It gives precise control over how much air enters and where. On a tempered MAU, the incoming air is heated (and sometimes cooled) before it enters the kitchen, which solves the seasonal comfort and energy problem.
Untempered make-up air enters at outside temperature — fine in mild conditions, uncomfortable in a Melbourne July or a February heatwave. Tempered make-up air is heated or cooled to a comfortable supply temperature first. Tempered systems cost more upfront and to run, but they keep staff comfortable and stop your main HVAC from fighting a constant stream of unconditioned air. For most Melbourne commercial kitchens, tempered supply is worth it.
The 15–35% of make-up air not supplied directly is drawn from adjacent conditioned spaces — the dining room, a corridor, or back-of-house areas. This happens naturally through doorways, service hatches, and pass-throughs. It's free air that's already conditioned, which is why AS 1668.2 builds it into the balance rather than requiring 100% direct supply.
Very small kitchens with low exhaust volumes can sometimes get away with passive make-up air through dedicated wall vents or louvres rather than a powered supply fan. This only works when the exhaust volume is modest and the building isn't tightly sealed. As exhaust volume climbs, passive supply stops being able to keep up and a powered MAU becomes necessary.
Got slamming doors or lifting flames? Let's check your make-up air balance.
You don't need instruments to get a strong indication. Run these simple checks with the exhaust fan on its highest setting:
A proper diagnosis uses airflow measurement to compare actual exhaust against actual supply, then checks the result against the AS 1668.2 65–85% rule. That measurement is part of any professional ventilation assessment.
The fix depends on what's causing the deficit:
In many Melbourne fit-outs the cleanest solution combines a tempered make-up air unit with a VSD on the exhaust, so both sides of the balance modulate together with cooking demand. That keeps the kitchen comfortable, compliant, and as energy-efficient as possible.
In a new build or full fit-out, make-up air is designed in from the start — the supply and exhaust are sized together, the ductwork is planned, and the balance is correct on day one. This is always cheaper and cleaner than retrofitting.
Retrofitting make-up air into an existing kitchen is harder because the building wasn't designed for it. Finding a path for supply ductwork, locating the MAU, and getting outside air to the right place all become constraints. It's still very achievable — it just needs proper assessment first. If you're planning a kitchen upgrade, getting the make-up air designed alongside the canopy from the outset avoids the expensive retrofit later. Our guide to switching to a new exhaust system covers the wider upgrade picture.
Make-up air is the fresh air that replaces the air your exhaust canopy removes from the kitchen. Because air can't be created or destroyed, every litre the fan exhausts must be replaced by a litre entering the kitchen. A make-up air system supplies that replacement air on purpose, in a controlled and ideally tempered way, rather than letting it leak in through doors, windows, and gaps.
AS 1668.2 requires make-up air supply equal to 65–85% of the total exhaust flow rate. The remaining 15–35% is drawn from adjacent spaces as transfer air. For example, a kitchen exhausting 1,500 L/s needs a make-up air supply of roughly 975–1,275 L/s, with the balance coming from the dining room or back-of-house. Keeping the kitchen slightly negative relative to the dining room stops cooking odours drifting into the customer area.
Negative pressure. The exhaust fan is pulling more air out of the kitchen than make-up air can replace, so the pressure inside drops below the pressure outside. The pressure difference effectively pushes external doors closed, making them hard to open and prone to slamming. It's the most common and most obvious sign of a make-up air deficit.
Yes. Severe negative pressure can disrupt gas burner combustion (producing carbon monoxide from incomplete burning), and in extreme cases can reverse the draught in natural-draught flues, pulling combustion gases back into the building. Proper ventilation and make-up air are part of safe gas appliance operation under AS/NZS 5601.1:2022. If your gas flames lift or burn yellow when the canopy runs, get it assessed.
A properly designed tempered make-up air system actually reduces total energy cost compared to having no supply system. Without one, replacement air leaks in through gaps at outside temperature, which your heating and cooling then have to fight. A controlled tempered supply conditions the incoming air efficiently and stops the random infiltration. Pairing the MAU with a VSD on the exhaust so both modulate with cooking demand reduces the cost further.
Yes. Retrofitting make-up air into an existing kitchen is common and achievable — it just needs proper assessment because the building wasn't originally designed for it. The main constraints are finding a path for supply ductwork and locating the make-up air unit. It's always cheaper to design make-up air in from the start during a new fit-out, so if you're planning a kitchen upgrade, include it from the outset rather than retrofitting later.
