How We Size a Kitchen Exhaust Hood for Your Cooking Line

Sizing a kitchen exhaust hood correctly is the single most important decision in any commercial kitchen ventilation project. Get it wrong and you end up with a system that either can't capture grease-laden fumes — leaving your kitchen smoky, smelly and non-compliant — or one that's massively over-engineered and burns electricity needlessly. When a client asks us to design a hood for their cooking line, we don't just measure the equipment and add a number. We look at the whole picture: what's being cooked, how hot it gets, where the kitchen sits in the building, and what the relevant authorities require.

What exactly does "sizing" a hood mean?

Hood sizing covers two related things: the physical dimensions of the canopy itself, and the airflow volume — measured in cubic metres per hour (m³/h) or litres per second (l/s) — that the exhaust system needs to move through it. Both have to be right. A hood can be the correct size but still fail if the fan behind it doesn't pull enough air. Equally, a powerful fan can't compensate for a hood that's too small to cover the cooking plume in the first place.

How do we assess the cooking appliances beneath the hood?

The first thing we do on any site survey is make a proper list of every appliance that will sit under the hood — wok burners, char-grills, deep fryers, flat-top griddles, combi ovens, steamers. Each one has a different heat output and grease-generation rate, and that directly affects the thermal plume rising off it.

• High-intensity appliances like wok burners and char-grills produce large, fast-moving plumes. They need higher capture velocities and more generous hood dimensions.
• Lower-intensity appliances like steamers or holding equipment generate gentler plumes — though they still need adequate extraction to prevent condensation and odour migration.
• Mixed cooking lines are common in Singapore's food service industry, and we always design the hood around the most demanding appliance cluster, not an average.

In our experience, the biggest mistake operators make is specifying a hood based on the cooking line they have today, without considering what they'll add in six months. We always ask about expansion plans before we finalise anything.

What is capture velocity and why does it matter?

Capture velocity is the air speed at the open face of the hood — the speed at which air is drawn inward. It needs to be high enough to intercept the rising thermal plume before it escapes into the kitchen. If it's too low, smoke and grease-laden air spill out sideways. If it's unnecessarily high, you create draughts that disturb cooking flames, increase noise, and waste energy.

For a typical Singapore commercial kitchen with wok cooking, we generally work within a capture velocity range that accounts for the intense heat load and open cooking style common here. For lighter-duty lines — bakeries, cold prep areas with some heating — the requirement is lower. We always confirm the exact airflow figures against current NEA and SCDF guidelines before we commit to a design.

How much overhang does a hood need on each side?

A hood must extend beyond the cooking equipment on every open side. This overhang ensures the rising plume — which spreads outward as it rises — stays within the capture zone of the hood. The required overhang depends on the mounting height of the hood above the cooking surface and the intensity of the appliances below.

• For a hood mounted at a standard height above a wok range, we typically build in a meaningful overhang on the front and both ends.
• Where a cooking line sits against a wall, the wall itself acts as a backstop, so the rear overhang requirement changes — but the other three sides still need to be correct.
• Island cooking lines — those accessible from all four sides — need overhang on all four sides and are sized more conservatively to account for cross-draughts from movement around the line.

On one kitchen we serviced, a previous contractor had installed a hood that was flush with the front edge of the wok range and had no front overhang at all. The result was that the entire front half of the cooking plume escaped into the kitchen. We rebuilt the hood with the correct geometry, and the difference was immediately noticeable.

How does duct sizing connect to hood sizing?

The hood and the ductwork are one system. A hood sized correctly for the cooking load still needs ductwork that can carry the required air volume without excessive resistance. Duct velocity, bends, length of run, and the location of the exhaust discharge point all affect how hard the fan has to work.

We fabricate our own ductwork in-house, which means we can design the duct cross-section, the grease baffles, and the access panels as a single coordinated package. We also account for the grease trap and filter arrangement at the hood face — baffles need to be sized and spaced so they extract grease efficiently without creating so much pressure drop that the fan can't maintain the required airflow.

Where a client's kitchen is on an upper floor with a long duct run to the roof discharge, we factor in the additional static pressure and, where appropriate, recommend a variable speed drive (VSD) on the fan motor. We stock and install VSDs ourselves, so there's no waiting on external suppliers.

What about make-up air?

One point that's frequently overlooked: for every cubic metre of air your exhaust hood removes, that air has to come from somewhere. In a sealed kitchen, negative pressure builds up quickly, making doors hard to open, causing backdraught into gas appliances, and actually reducing the exhaust system's effectiveness.

A properly sized hood design always considers the make-up air supply — whether that's a dedicated fresh air system, transfer air from adjacent areas, or a combination. We assess this as part of every kitchen ventilation project, not as an afterthought.

Does hood type affect how we size it?

Yes. We work with several hood configurations depending on the kitchen layout:

• Wall-mounted canopy hoods — the most common type in Singapore kitchens, positioned against a wall above the cooking line.
• Island canopy hoods — suspended from the ceiling above a central cooking island, requiring larger dimensions due to four open sides.
• Low-proximity hoods — mounted closer to the cooking surface, used where ceiling heights are limited. These can achieve effective capture at lower airflow volumes if designed correctly.
• Backshelf hoods — compact hoods positioned at the rear of a cooking line, often used for lighter-duty applications.

Each type has a different sizing methodology. We always confirm hood type with the client before any calculations are finalised, because changing the type later means starting the duct design again.

Frequently Asked Questions

Can we just size our exhaust hood based on the cooking line length?

Length is a starting point, but it's not sufficient on its own. We also need to know the appliance types, heat output, mounting height, and kitchen layout before we can confirm dimensions and airflow. A long line of steamers needs a very different hood to the same length of char-grills. Always get a proper engineering assessment done rather than relying on rules of thumb.

What happens if our hood is undersized?

An undersized hood can't fully capture the cooking plume. Grease and smoke escape into the kitchen, depositing grease on surfaces and increasing fire risk. It also means your kitchen will struggle to pass NEA inspections, and SCDF may flag the system if it doesn't meet suppression system coordination requirements. We've been called in to rectify undersized systems more times than we can count — it's always cheaper to size correctly from the start.

How often should a properly sized hood be cleaned?

That depends on the cooking intensity and operating hours. A high-volume wok kitchen may need hood and duct cleaning every one to three months. A lighter-duty kitchen might go six months between full cleans. We assess grease load as part of our maintenance planning and recommend a schedule based on actual usage, not a generic interval. Our own BC Air chemical series is formulated specifically for the kind of heavy carbon and grease build-up common in Singapore's cooking environments.

Do we need separate hoods for different sections of our cooking line?

Sometimes, yes. If part of your line produces heavy smoke and another section is low-heat, a single hood covering both can compromise performance — either the low-heat section gets too much airflow and creates draughts, or the high-heat section gets too little. We assess zoning requirements on every project and will recommend separate hood sections with individual or combined extraction where it makes sense.

Can you retrofit a new hood into our existing kitchen without disrupting operations?

We do this regularly. We plan the installation sequence carefully to minimise downtime, and our 24/7 standby capability means we can schedule critical work during off-peak hours or overnight. We'll survey the existing ductwork and fan setup first to confirm what can be retained and what needs to be replaced, so there are no surprises once we're on site.

If you're planning a new kitchen, expanding your cooking line, or suspect your current hood isn't performing as it should, get in touch with us for a site survey and quotation. We'll give you a straightforward assessment based on what we actually find — and our 24/7 standby team is always available if you have an urgent situation that can't wait.