A fan that looks powerful on paper can still leave a commercial kitchen hot, smoky and difficult to work in. The question is not simply what size extraction fan to buy, but how much air must be moved through the complete system once the canopy, ductwork, filters, bends and discharge point are taken into account.
For restaurants, production kitchens, factories and workshops, correct fan sizing protects air quality, supports safer working conditions and prevents costly alterations after installation. A properly specified system should clear contaminants at source without creating excessive noise, draughts or energy use.
Start with the air you need to remove
Fan duty is usually expressed as airflow in cubic metres per hour (m³/h), or litres per second (l/s). This figure is the starting point, not the final answer. It needs to reflect the heat, steam, grease, fumes, dust or vapours produced by the process.
In a commercial kitchen, the cooking equipment beneath the extraction canopy is the main driver. A light-duty prep area with ovens or a dishwasher needs far less extraction than a line of gas chargrills, fryers and solid-fuel equipment. The canopy size, cooking duty and appliance layout all affect the required capture performance.
For workshops and industrial areas, look at the process rather than the room alone. Welding bays, grinding stations, chemical processes and dust-generating machinery usually need local extraction close to the source. Trying to ventilate the whole building with one oversized wall fan is often inefficient and may fail to control exposure where it matters.
A basic room-air calculation can provide an early indication. Multiply the room volume by the desired number of air changes per hour:
Room length × width × height × air changes per hour = required airflow in m³/h
For example, a 10 m by 6 m kitchen with a 3 m ceiling has a volume of 180 m³. At 25 air changes per hour, the calculation gives 4,500 m³/h. That may be a useful sense check, but it is not a substitute for designing around the cooking line and canopy. Source capture is what delivers effective extraction.
What size extraction fan is right for a canopy?
The correct fan size for a canopy depends on the canopy's capture zone, its height above equipment, the type of cooking and the condition of the surrounding space. A larger canopy does not automatically need a dramatically larger fan if it is well positioned and the cooking duty is light. Equally, a compact canopy over high-heat appliances can demand substantial airflow.
The fan must draw air evenly across the canopy length. Poorly designed plenum sections, undersized spigots or badly arranged duct connections can leave weak spots where smoke escapes into the room. That is why a system should be assessed as a complete assembly rather than a fan bolted to the end of a duct.
For grease-producing commercial kitchens, specify suitable grease filtration before the fan and allow proper access for cleaning. Filters protect the duct and fan, but they also add resistance as they load with grease. A fan selected with no allowance for that resistance may perform well on day one and poorly after a period of service.
Airflow is only half the specification
A fan can deliver a high airflow at zero pressure, yet lose much of that capacity once connected to real ductwork. Static pressure is the resistance the fan must overcome to move air through the system. It is created by duct length, changes in direction, branches, grilles, filters, attenuators and terminal outlets.
Long duct runs need particular attention. Every sharp bend, reducer and poorly sized section adds pressure loss. Flexible ducting, where it is used inappropriately, can be especially restrictive. A short, straight route with correctly sized rigid ductwork is generally easier for a fan to serve, quieter in operation and more energy efficient.
The required fan duty should therefore be stated as both airflow and pressure. For example, 4,500 m³/h at a specified static pressure is meaningful. Simply asking for a 4,500 m³/h fan is not, because the quoted figure may relate to free-air performance rather than installed duty.
A professionally selected centrifugal or mixed-flow fan is often better suited to systems with significant duct resistance than a basic axial fan. Axial fans can be effective for short, low-resistance routes, but they are not a universal solution. Fan type should follow the duty, not the other way round.
Check the ductwork before increasing fan power
When a system is underperforming, the instinct is often to fit a larger extraction fan. This can create new problems if the ductwork, canopy or replacement-air strategy has not been addressed. Higher fan speed can increase noise, pull conditioned air out of the building and make doors difficult to open. It may also expose an existing duct installation as the real restriction.
Duct velocity must be balanced. If ducts are too small for the intended volume, air moves too fast, pressure loss rises and noise becomes intrusive. If ducts are too large, air can slow down enough for grease, dust or moisture to settle where it should be carried away. The right duct dimensions depend on the air type and application.
Pay close attention to transitions and bends. Smooth changes in duct size and swept bends are usually preferable to abrupt fittings. These are not cosmetic details. They directly affect the fan duty required and the running cost of the system.
Replacement air cannot be ignored
Extraction removes air from the building. That air must be replaced, either naturally through controlled openings or by a dedicated supply-air arrangement. Without sufficient make-up air, an extraction fan can become starved and canopy capture can suffer.
A kitchen or workshop under excessive negative pressure may feel stuffy despite having a large fan. It can draw odours from neighbouring areas, create uncomfortable draughts and interfere with combustion appliances. In some cases, air may simply rush in through the nearest open door rather than flowing where it supports the extraction canopy.
A balanced approach is usually more controllable. Tempered supply air can improve comfort during colder months, while carefully positioned replacement air helps direct contaminants towards the extraction point. The goal is not the highest possible extraction rate. It is stable, effective airflow in the occupied working area.
Consider noise, controls and energy use
An extraction system may operate for long shifts, so electrical consumption matters. Selecting a fan far larger than required and throttling it back is rarely the best long-term solution. A correctly sized fan with speed control can offer better adjustment as operating conditions change.
Variable-speed controls are particularly useful where a kitchen has quieter prep periods and busier service periods, or where industrial processes are not continuously active. However, reducing fan speed must not compromise capture at peak demand. Commissioning should establish the safe operating range rather than leaving staff to guess.
Noise control should also be designed in from the start. Fan position, duct velocity, vibration mounts and attenuation can all make a major difference. A system that clears air effectively but makes conversation impossible is not a finished solution.
Plan for access, cleaning and compliance
Extraction systems work hard in demanding environments. Grease, dust and process residue will accumulate unless the system is designed for practical maintenance. Include access doors at sensible intervals, allow space around the fan for servicing and use components suited to the air being extracted.
For commercial kitchens, discharge location needs careful planning to reduce odour nuisance and prevent contaminated air being drawn back into the building. Industrial applications may need additional treatment equipment, such as filtration or electrostatic precipitator units, depending on the process and site requirements.
Building layout, local authority expectations, fire safety considerations and equipment manufacturers' requirements can all affect the final design. Treat these as part of the sizing process, not an afterthought once the fan has been ordered.
Get a complete duty, not a guess
Before choosing a fan, gather the key site information: the process or appliances involved, canopy dimensions, duct route, approximate duct lengths, number of bends, filtration, discharge position and available replacement air. Photographs and a measured layout are often enough to identify major constraints before a survey.
CanopyMan designs and manufactures extraction systems around the actual site duty, from premium-quality stainless steel canopies to ductwork, fans and air-treatment equipment. That joined-up approach avoids the common mismatch between a capable fan and an unsuitable system around it.
The best extraction fan is not the biggest unit on the catalogue page. It is the one that delivers the required airflow at the required pressure, works with the duct route, remains serviceable and gives your team a cleaner, more comfortable place to work every day.