Quick answer
Size the hood by the appliance lineup first: length equals the cooking equipment plus overhang at the open edges. Exhaust volume then follows the capture requirement in your adopted mechanical code, not a rule of thumb. Make-up air must replace what the hood removes, tempered and distributed away from the hood, or the kitchen will not work.
How long should a commercial kitchen hood be?
Hood length is geometry before it is airflow. The canopy has to cover every appliance that produces grease, smoke, steam or heat, plus an overhang at each open edge so that the plume is captured before it spreads.
The sequence is:
- List the appliances that need capture. Ranges, griddles, fryers, broilers, wok burners, combi ovens with a grease-producing mode, and any appliance that generates steam in volume.
- Group them by type. Grease-producing appliances and steam-producing appliances behave differently and are often separated, because a single hood serving both has to handle two different plumes.
- Measure the lineup. Add the appliance widths, plus clearance between units, and add the overhang at the open ends.
- Check the geometry against the room. A canopy that overlaps a doorway, a window or a supply diffuser will not capture reliably, however well it is sized on paper.
| Appliance group | Hood type that fits | Why |
|---|---|---|
| Ranges, griddles, fryers, broilers | Canopy hood with grease filters | Captures the thermal plume rising from a wide surface |
| Wok range, high-intensity burners | Canopy with deeper capture or a dedicated wok hood | Plume velocity is higher and the plume is narrower |
| Combi ovens, steamers, boiling pans | Condensate hood or a canopy with a condensate channel | Steam collapses into water; the hood has to drain it |
| Dishwashing area | Condensate hood | Humidity control, not grease capture |
Two geometry rules save expensive mistakes. First, the hood must be wider than the appliance lineup at the open ends, not exactly the same length; the plume spreads as it rises. Second, the hood’s lower edge must be low enough to capture before the plume escapes the canopy, but high enough for the tallest appliance and for the cook to work. Where those two constraints conflict, the answer is usually a deeper canopy rather than a lower one.
What determines the exhaust air volume?
Exhaust volume is set by the capture requirement: the air velocity at the hood’s open face has to be sufficient to pull the plume in against the room’s air movement. That requirement is written in the mechanical code your project is permitted under, expressed per unit of hood face area or per unit length of appliance, and it is not a figure to be taken from a supplier’s brochure.
What that means in practice is that the number is a project input, not a design opinion. Ask the mechanical engineer for the figure the code requires at your hood face, then let the fan and duct be selected against it.
Several factors reduce actual capture at a given exhaust volume, and each one is worth checking before the fan is ordered:
| Factor | Effect on capture | Practical fix |
|---|---|---|
| Cross-drafts from doors, windows, diffusers | Deflects or dilutes the plume | Relocate supply diffusers; add a vestibule or air curtain |
| Insufficient overhang at open ends | Plume escapes at the edges | Extend the canopy; add side skirts |
| Filter loading | Restricts airflow over time | Specify a cleaning schedule, and a filter type staff will actually clean |
| Appliance pushed forward of the hood edge | Plume rises outside the canopy | Align the appliance with the hood; add a rear stand-off |
| Tall canopy above a high plume | Plume spreads before capture | Reduce mounting height or add a deeper capture volume |
| Exhaust imbalance between adjacent hoods | One hood starves | Balance the system after installation, not on paper alone |
The last point is the one that most often fails in service. Two hoods on one fan, sized independently, will not share air the way the drawing assumes. A commissioning balance with measurements at each hood is what converts a designed system into a working one.
How much make-up air does the kitchen need?
A hood removes air from the kitchen, and that air has to come from somewhere. If it is not supplied deliberately, it comes in through the door, the loading bay and every gap in the envelope, at whatever temperature the weather provides.
The balance is simple:
make-up air = exhaust air − air that can be drawn through the space by design
In most kitchens the design intent is that the hood’s exhaust is largely replaced by tempered make-up air, with a deliberate and small negative pressure in the kitchen relative to the dining area so that cooking odours do not migrate into the guest space. Both halves of that sentence matter: too little make-up air and the doors will not close, the hood will not capture, and the kitchen will pull air through the dishwash area; too much and odours travel into the restaurant.
Distribution matters as much as volume. Make-up air delivered directly into the hood’s capture zone dilutes the plume and defeats the hood. It should be introduced away from the hood, low or at the perimeter, and at a temperature the staff can work in. In cold climates that means tempering, which is a capital cost and an energy cost that belongs in the project budget at the start.
| Make-up air approach | Advantage | Constraint |
|---|---|---|
| Tempered, ducted supply at the perimeter | Comfort and reliable capture | Capital cost, duct space, plant capacity |
| Tempered supply through a displacement plenum | Good comfort at low velocity | Needs floor area and a clear perimeter |
| Untempered supply near the hood | Low cost | Cold draughts, condensation, staff complaints |
| Transfer air from the dining area | No plant cost | Depends on the building; can pull odours the wrong way |
| Short-cycle (supply and exhaust at the hood) | Simple balancing | Poor comfort; limited benefit in a busy kitchen |
Settle make-up air at the concept stage, with the mechanical engineer and the equipment layout in the same drawing. Retrofitting a tempered supply into a finished kitchen is one of the most expensive changes a project can make.
What else does the hood system have to do?
The hood is part of a fire safety system, not only a ventilation device. NFPA 96, the Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations, is the reference most jurisdictions adopt for this, and the current edition is the 2027 edition. Its scope is preventive and operative fire safety for commercial cooking operations, which brings duct construction, grease accumulation, cleaning access and the interaction with the suppression system into the same conversation.
In the European Union the hygiene framework adds a separate requirement: Regulation (EC) No 852/2004 requires suitable and sufficient natural or mechanical ventilation, prohibits mechanical airflow from a contaminated area to a clean area, and requires ventilation systems to be constructed so that filters and other parts needing cleaning or replacement are readily accessible. That last clause is a design instruction: if the filter bank or the duct access panel cannot be reached without a ladder and a dismantled ceiling, the system will not be maintained.
| System element | Design question to settle early |
|---|---|
| Grease filters | Type, access height, and where spare filters are stored during cleaning |
| Duct route | Length, number of bends, clean-out access, and clearance from combustibles |
| Fire suppression | Nozzle layout coordinated with the appliance positions, not just the hood footprint |
| Shut-off and fuel isolation | Location relative to the exit route and the cooking line |
| Lighting | Mounted so that it does not obstruct filter removal |
| Condensate drainage | Slope, trap and discharge point for steam-producing appliances |
Coordination is the recurring theme. The suppression contractor needs the appliance schedule, the hood manufacturer needs the appliance positions, and the electrical contractor needs to know where the isolators sit. When those three sets of information are issued at different times, the result is a hood that fits the ceiling but not the kitchen.
What should the specification include?
| Specification item | Why it matters |
|---|---|
| Canopy length, depth and mounting height | Determines capture; must be fixed against the appliance lineup, not a nominal size |
| Filter type and face area | Affects pressure drop, cleaning frequency and fire behaviour |
| Exhaust volume at design static pressure | Fan selection needs both figures; volume alone is not a specification |
| Material and gauge | Stainless steel where cleaning and corrosion matter; gauge affects rigidity over years |
| Condensate channel and drain | Essential for steam-producing appliances |
| Access panels and clean-outs | Required by the standards, and used only if they are reachable |
| Integrated lighting | Must not obstruct filter removal |
| Suppression coordination | Nozzle positions and appliance schedule issued together |
| Make-up air interface | Supply location, volume and temperature stated on the same drawing |
The two items buyers most often leave out are the exhaust volume at design static pressure, and the make-up air interface. A fan quoted without its operating point cannot be checked, and a hood specified without a make-up air plan will be balanced on site by opening a door.
Where does Zhongji Metal fit?
Zhongji Metal has manufactured commercial kitchen and refrigeration equipment in Shandong, China since 2010, and exports to more than 130 countries. We supply the cooking line that sits under the hood, and we work with the ventilation and suppression contractors who specify the hood itself, because the two decisions constrain each other.
Three points from project work:
- The appliance schedule drives the hood. Our western kitchen cooking equipment range is issued with dimensions, plume behaviour and clearance requirements, so the hood length can be set from real numbers rather than a nominal allowance.
- High-intensity appliances need their own capture. A electric fryer bank and a smart steam oven do not belong under the same capture assumption; one produces grease, the other produces steam that has to drain.
- Layout comes before the ceiling. We issue equipment layouts with the hood line, the make-up air route and the service clearances marked, so that the mechanical drawings and the kitchen drawings agree before fabrication.
Send the menu, the appliance schedule and the room section, and we will return a layout showing the cooking line, the hood line and the service clearances together.
Frequently asked questions
How do I calculate the length of a commercial kitchen hood?
Add the widths of all appliances that need capture, add the clearance between units, then extend the canopy beyond the lineup at each open end so the plume is captured as it spreads. Check the result against the room: a hood that overlaps a doorway or a supply diffuser will not capture reliably however long it is.
How is hood exhaust volume determined?
By the capture requirement in the mechanical code the project is permitted under, applied to the hood’s face area or the appliance length. It is a project input from the mechanical engineer, not a figure to be taken from a supplier’s brochure. The fan is then selected against that volume at the system’s design static pressure.
How much make-up air does a commercial kitchen need?
Enough to replace what the hood exhausts, tempered and delivered away from the hood, while keeping the kitchen slightly negative relative to the dining area. The practical test is that doors close normally and the hood captures at working temperature.
Why does my hood fail to capture smoke?
Usually cross-drafts, insufficient overhang at the open ends, appliances pushed forward of the hood edge, or loaded filters. Two hoods sharing one fan can also starve each other if the system was never balanced on site. Check those four before increasing the fan size.
Can make-up air be supplied straight into the hood?
It can be, and it is common in short-cycle systems, but it dilutes the plume and does little for staff comfort. Tempered supply delivered at the perimeter is more comfortable and captures more reliably, at a higher capital cost.
What standard covers commercial kitchen hoods?
In the United States, NFPA 96 covers ventilation control and fire protection for commercial cooking operations, and the current edition is the 2027 edition. Equipment standards such as NSF/ANSI 2 cover hood construction among food equipment. Your adopted mechanical code sets the airflow figures, so confirm them with the authority having jurisdiction.
Sources
- National Fire Protection Association, NFPA 96, Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations — scope and current edition: nfpa.org
- Regulation (EC) No 852/2004 on the hygiene of foodstuffs, Annex II — ventilation requirements and accessibility of filters: eur-lex.europa.eu
- NSF, Food Equipment Standards — NSF/ANSI 2, Food Equipment, including hoods: nsf.org
- US Food and Drug Administration, FDA Food Code — the model code adopted, with amendments, by state and local jurisdictions: fda.gov
Local codes vary; verify requirements with your authority having jurisdiction.
Author: China Zhongji Metal
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Website: https://zhongjimetal.com
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