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Kitchen extraction cleaning - fan sizing
An extractor fan that is too small can't clear the kitchen; one too big wastes energy and unbalances the airflow. Sizing it right means matching the fan's airflow to what the kitchen needs - based on the canopy, the cooking, and the resistance of the ductwork. Here is how extractor fans are sized for a kitchen, in outline - and why a clean system is what lets a well-sized fan perform. This is general information; sizing should be done by a competent designer.
The short answer
Getting the extractor fan the right size is fundamental to a kitchen's ventilation working - and it is a design calculation, not a guess. Here, in outline, is how extractor fans are sized, and where cleaning fits in. This is general information; the actual sizing is for a competent ventilation designer. What 'sized' means: the airflow. Sizing a fan means selecting one that will move the right volume of air - the airflow (often measured in cubic metres per second or per hour) - to do the kitchen's extraction job: capturing and clearing the heat, smoke and grease-laden vapour the cooking produces. Too little airflow and the kitchen isn't cleared (smoke and heat linger); too much and energy is wasted and the airflow balance upset. So the core of sizing is determining and delivering the right airflow. Step one: the airflow the canopy and cooking need. The required airflow is driven by the canopy and what is cooked under it. The canopy's size and type (wall-mounted, island, its dimensions and design) determine the airflow needed to capture the rising plume of vapour effectively across its face - a bigger or more open canopy needing more air. And the cooking beneath it matters: heavier, hotter, greasier cooking (ranges, chargrills, fryers, woks) produces a bigger, hotter plume needing more extraction than light cooking. So the airflow requirement is calculated from the canopy and the cooking load - the volume of air needed to capture and clear that kitchen's output. (Design guidance and standards inform these calculations.) Step two: the resistance the fan must overcome. A fan doesn't just move air in free space - it has to pull the air through the whole system, overcoming its resistance (the pressure drop). This resistance comes from the ductwork (its length, diameter and bends - longer, narrower, more convoluted ducts resist more), the filters (which resist airflow), any fire dampers, and the discharge. So the fan must be selected to deliver the required airflow against this system resistance - a fan's capacity is airflow at a given resistance (pressure), so both matter. A fan that could move the air in free space might fall short against a resistive system. Getting it wrong: too small or too big. Too small: the fan can't move enough air against the resistance, so the extraction under-performs (smoke, heat and grease not cleared - an ineffective, and less safe, kitchen). Too big: the fan over-extracts, wasting energy and potentially unbalancing the airflow (pulling more air than the make-up air can replace, causing negative pressure and its problems - as covered in the oversized-fan page). So both errors cause problems - sizing aims for the right match. Where cleaning fits: keeping the resistance low. Here is the tie to cleaning: the system resistance the fan works against includes the filters and ductwork - and grease build-up increases that resistance (clogged filters and grease-narrowed ducts resist more airflow). So a grease-choked system adds resistance beyond what the fan was sized for, starving the airflow (the well-sized fan can no longer deliver its designed airflow against the raised resistance). Keeping the system clean keeps the resistance low, so a well-sized fan performs as intended. So a fan is sized to deliver the airflow the canopy and cooking need against the system's resistance - and cleaning keeps the resistance low so the fan performs as sized. So sizing matches the fan to the kitchen, and cleaning lets it perform. This is general information; sizing is for a competent designer.
Key points
Sizing means the right airflow
At its heart, sizing an extractor fan means choosing one that moves the right volume of air - the airflow - to do the kitchen's extraction job. The fan's purpose is to pull air through the system to capture and clear the heat, smoke and grease-laden vapour the cooking produces. How much air it needs to move (its airflow, measured in cubic metres per second or per hour) is the key sizing quantity: enough to capture and clear the kitchen's output effectively, but not wastefully more.
So sizing is a balance: too little airflow and the extraction under-performs (the cooking's heat, smoke and vapour not cleared - lingering in the kitchen); too much airflow and energy is wasted and the airflow balance can be upset (over-extraction). The right airflow is the amount that matches the kitchen's actual need. Determining that need, and then selecting a fan to deliver it against the system's resistance, is what sizing involves - a design calculation based on the canopy, the cooking and the ductwork, covered in the next sections. So sizing means the right airflow - enough air to clear the cooking's output, no more. This is the goal the calculation works toward. So the right airflow clears the cooking's output. This is general information; the calculation is for a competent designer.
What sets the airflow needed
The airflow a fan needs to deliver is set primarily by the canopy and the cooking beneath it - these determine how much air must be moved to capture and clear the kitchen's output. The canopy: its size and type (a wall-mounted or island canopy, its dimensions, how open it is) determine the airflow needed to capture the rising plume of cooking vapour effectively across the canopy face. A larger or more open canopy needs more airflow to draw in the vapour over its whole area; the canopy design guidance and standards inform how much.
The cooking beneath it: what is cooked, and how heavily, affects the plume that must be captured. Heavier, hotter, greasier cooking - ranges, chargrills, fryers, woks, solid-fuel cooking - produces a bigger, hotter, more vigorous plume of heat and vapour, needing more extraction airflow than light cooking (a few light appliances). So the cooking load is part of the calculation. Together, the canopy and the cooking determine the required airflow: the volume of air needed to capture and clear that particular kitchen's output. This is the demand side of the sizing - what the fan must achieve. The supply side (selecting a fan to deliver it against the resistance) comes next. So what sets the airflow needed: the canopy and the cooking beneath it. So the canopy and cooking set the airflow needed. This is general information; the calculation is for a competent designer.
The resistance to overcome
Determining the airflow needed is only half of sizing - the fan must also overcome the system's resistance to deliver that airflow, so the resistance is the other key factor. A fan does not move air in free space; it has to pull the air through the whole extraction system, and the system resists that airflow (a pressure drop the fan must overcome). This resistance comes from several things: the ductwork (its length, diameter and bends - a longer, narrower or more convoluted duct resists more), the filters (which the air must pass through, resisting it), any fire dampers in the duct, and the discharge arrangement.
So a fan is not selected on airflow alone but on airflow at a given resistance (pressure): its capacity is its ability to move a certain volume of air against a certain pressure. A fan that could move plenty of air in free space might fall short when it has to pull that air through a long, resistive duct system. So the sizing must account for the system's resistance - selecting a fan powerful enough to deliver the required airflow against the actual resistance of that kitchen's ductwork, filters and dampers. This is why fan sizing needs the whole system considered, not just the airflow target. So the resistance to overcome - ductwork, filters and dampers add pressure - is the second half of sizing. So ductwork, filters and dampers add resistance. This is general information; the calculation is for a competent designer.
Too small, too big
Getting the size wrong in either direction causes problems - which is why matching the fan to the need matters. Too small: if the fan can't move enough air against the system's resistance, the extraction under-performs - it doesn't clear the kitchen's heat, smoke and grease-laden vapour effectively. The result is a hot, smoky kitchen with poor smoke and odour control and grease not properly captured - an ineffective and less safe extraction. So an under-sized fan simply can't do the job.
Too big: an over-sized fan over-extracts - moving more air than needed. This wastes energy (a bigger fan running harder than necessary) and, importantly, can unbalance the airflow: if the fan pulls more air than the make-up air system can replace, the kitchen goes into negative pressure (a partial vacuum), causing draughts, doors hard to open, and air pulled back down flues - the problems of an oversized extraction fan (covered in the dedicated page). So an over-sized fan is not 'safe margin' but a source of its own problems. So both too small and too big cause problems - the aim is the right size, matched to the kitchen's need and system, delivering the required airflow without over-extracting. So too small, too big - both wrong sizes cause problems. So both wrong sizes cause problems. This is general information.
Where cleaning comes in
There is a direct tie between fan sizing and extraction cleaning: the system resistance the fan is sized against includes the filters and ductwork, and grease build-up increases that resistance - so a clean system is what lets a well-sized fan actually perform as intended. A fan is sized to deliver its airflow against the system's designed resistance (with clean filters and clear ductwork). But as grease builds up, it raises the resistance beyond that design point: clogged filters resist more, and grease-narrowed ductwork resists more. So a grease-choked system presents the fan with more resistance than it was sized for - and against that raised resistance, even a well-sized fan can no longer deliver its designed airflow (the airflow is starved).
So grease build-up effectively undoes the sizing: the fan that was correctly sized for the clean system under-performs in the grease-choked one (moving less air, the extraction weakened) - and works harder and strains doing so. Keeping the system clean keeps the resistance at the design point, so the well-sized fan delivers its intended airflow and performs as designed. So cleaning is what lets a correctly-sized fan perform - and neglect can make even a well-sized system under-perform as if it were under-sized. This is a practical reason (beyond fire safety) to keep the extraction clean: to preserve the airflow the system was designed and sized to deliver. So where cleaning comes in: a clean system lets a well-sized fan perform. So a clean system lets a well-sized fan perform. This is general information; sizing is for a competent designer.
Questions
It is sized to deliver the airflow needed to clear the cooking's output - set by the canopy and the cooking - against the resistance of the system it must pull through (the ductwork, filters and dampers). Sizing means selecting a fan that moves the right volume of air (the airflow) to capture and clear the heat, smoke and grease-laden vapour the kitchen produces. The required airflow is worked out from the canopy (its size and type determine the air needed to capture the rising vapour) and the cooking beneath it (heavier, hotter cooking needing more). The fan is then selected to deliver that airflow against the system's resistance - the pressure it must overcome from the ductwork (length, diameter, bends), the filters, and any fire dampers. So sizing matches the fan's airflow-at-a-pressure to the kitchen's need and system. It is a design calculation for a competent ventilation designer. So it is sized to deliver the needed airflow against the system's resistance. This is general information; sizing is for a competent designer.
It can't move enough air against the system's resistance, so the extraction under-performs - the kitchen's heat, smoke and grease-laden vapour aren't cleared effectively, giving a hot, smoky, less safe kitchen. The fan has to pull the air through the whole system against its resistance. If the fan is under-sized (too little capacity for the airflow needed against that resistance), it simply can't move enough air - so the extraction fails to capture and clear the cooking's output effectively. The result is a kitchen that stays hot and smoky, with poor smoke and odour control and grease not properly captured - an ineffective extraction, and a less safe and less pleasant kitchen. So an under-sized fan is a real problem, not solved by anything but correct sizing (or, if the shortfall is from grease raising the resistance, by cleaning). So if too small, the extraction under-performs and can't clear the kitchen. So too small means an under-performing extraction. This is general information.
No - an over-sized fan wastes energy and can unbalance the airflow (over-extracting, causing negative pressure), so bigger is not better; the aim is the right size, matched to the need. It might seem safer to fit a bigger fan for margin, but an over-sized fan causes its own problems: it over-extracts (moving more air than needed), which wastes energy (a bigger fan running harder than necessary) and can unbalance the airflow - if it pulls more air than the make-up air can replace, the kitchen goes into negative pressure (a partial vacuum), causing draughts, doors hard to open, and air pulled back down flues. These are the problems of an oversized extraction fan. So bigger is not better - it trades under-extraction for over-extraction and imbalance. The aim is the right size: enough airflow for the need, matched to the make-up air, without excess. So no - a bigger fan is not always better; correct sizing is. So no - over-sizing causes its own problems. This is general information.
Because the fan has to pull the air through the whole system against its resistance, so it must be sized to deliver the airflow at that resistance - a resistive duct system needs a more capable fan for the same airflow. A fan doesn't move air in free space; it pulls it through the ductwork, filters and dampers, all of which resist the airflow (a pressure drop). A fan's real capacity is airflow at a given pressure (resistance) - so the same fan delivers less airflow against more resistance. This means the sizing must account for the system's resistance: a long, narrow or convoluted duct with resistive filters needs a more powerful fan to deliver the required airflow than a short, direct duct would. So duct (and system) resistance is a key sizing factor - a fan sized only on free-air airflow could fall short against a resistive system. And grease build-up raises this resistance further, which is why cleaning matters for airflow. So resistance matters because the fan must deliver its airflow against it. So resistance sets how capable the fan must be. This is general information; sizing is for a competent designer.
It raises the system resistance beyond the design point, so even a correctly-sized fan can no longer deliver its designed airflow - the airflow is starved and the extraction under-performs, as if the fan were under-sized. A fan is sized to deliver its airflow against the system's designed resistance (clean filters, clear ductwork). Grease build-up increases that resistance: clogged filters resist more, and grease-narrowed ductwork resists more. So a grease-choked system presents more resistance than the fan was sized for - and against that raised resistance, the well-sized fan delivers less than its designed airflow (the airflow is choked). So grease effectively undoes the sizing, making a correctly-sized system under-perform as if under-sized (and straining the fan). Cleaning removes the grease, restoring the resistance to the design point so the fan delivers its intended airflow again. So grease build-up starves a well-sized fan's airflow - which cleaning restores. So grease raises resistance and starves the airflow. This is general information.
A competent ventilation designer - fan and system sizing is a design calculation (canopy, cooking load, duct resistance, make-up air), not a guess or an off-the-shelf choice. Sizing an extractor fan properly involves calculating the required airflow (from the canopy and cooking) and selecting a fan to deliver it against the system's resistance (the ductwork, filters, dampers), while ensuring the airflow is balanced with the make-up air. This is a ventilation design task, best done by a competent designer or ventilation engineer who can do the calculations and account for the whole system - not something to guess or pick off the shelf. Getting it wrong (too small or too big) causes real problems. So the sizing (for a new or replacement system) should be done by a competent ventilation designer. The extraction cleaner's role is different - keeping the system clean so a correctly-sized fan performs. So a competent ventilation designer should size the fan. This is general information; sizing is for a competent designer.
A fan is sized to deliver its airflow against the system's resistance - but grease raises that resistance and starves the airflow. We clean the whole system so the resistance stays at the design point and your correctly-sized fan performs as intended. Ask us to keep your extraction clean and free-flowing. This is general information; fan sizing is for a competent designer.
