Phoenix Journal · Extraction
Get the extract volume wrong and a kitchen either fills with heat and grease or haemorrhages energy. Here is how the two UK sizing methods compare, and which one DW/172 now asks you to use.
Getting the numbers right
Size a kitchen extraction system too small and the space fills with heat, steam and grease-laden air; size it too big and you burn energy, pull conditioned air straight out of the building and unbalance every neighbouring room.
Correct sizing is the difference between a canopy that quietly does its job for years and one that fights you every service. In the UK the reference point is BESA's DW/172, the recognised specification for commercial kitchen ventilation systems. It is not a statute, but Environmental Health Officers, insurers and building services consultants all treat it as the benchmark, so a system sized against it is far easier to sign off, insure and defend.
Where designers disagree is on the method used to reach the extract volume. There are two in common use: the thermal convection method that the 2018 edition of DW/172 asks you to adopt, and the older canopy velocity, or loading-factor, method that many people still reach for because it is quick. Both can land you close to the right figure. They fail in different ways, so it is worth understanding what each one buys you before you commit a design.
This is the method the second edition of DW/172 states should be the only one used. Rather than working from the size of the canopy, you work from the appliances underneath it. Each piece of equipment is given a thermal convection coefficient based on its heat output and whether it is gas or electric - a char grill running at a high surface temperature throws off far more of a rising thermal plume than a bain-marie. You take the plan area of each appliance, multiply by its coefficient to get the air it needs carried away, add the appliances together, then apply a canopy factor that accounts for the hood shape and how well it is positioned over the cooking line.
The result is an extract volume expressed in cubic metres per second that reflects what is actually being cooked, not just the footprint of the steelwork. For a mixed cooking suite this is the most defensible figure you can put in front of an EHO or an insurer.
The older approach sizes from the canopy itself. You take the length and depth of the hood, apply a loading factor for the cooking duty - broadly 0.25 for light loads such as steaming, 0.35 for medium duty, and 0.5 for heavy work like solid-fuel and char grilling - and arrive at an extract rate. It is often quoted as flow per linear metre of canopy, with medium-duty runs landing around 0.4 to 0.6 cubic metres per second per metre and duties across the range spanning roughly 1,500 to 5,000 cubic metres per hour per metre. The aim is to hold a capture velocity of about 0.3 to 0.5 metres per second across the open face so the plume is drawn in rather than spilling into the room.
It is fast, it needs very little information, and for a simple single-duty canopy it will usually get you within sensible bounds. That is exactly why it persists - but its simplicity is also where it goes wrong.
Whichever route you take to the extract volume, a handful of DW/172 and CIBSE reference points keep the rest of the design honest. Getting the headline flow right is wasted if the canopy overhang, make-up air and grease-filter velocities are not sized to match.
A few practical points sit around those numbers. Grease-filter face velocity matters as much as the total flow - DW/172 gives recommended figures, and running too fast or too slow across the filters hurts grease capture and pushes more residue into the duct. Make-up air needs a genuine supply path, not just a gap under a door, or the fan will pull air back down the flue and unbalance adjacent rooms. And the extract volume is only ever as good as the appliance list it was based on, so if the cooking line changes, the sizing should be revisited rather than assumed. EN 16282 covers similar ground and is sometimes cited alongside DW/172 on larger or multi-national projects.
Questions
The 2018 second edition of DW/172 states that the thermal convection method should be the only method used. It sizes the extract volume from each appliance's heat output, fuel type and plan area rather than from the canopy dimensions alone. The older velocity or loading-factor method is still used for quick estimates, but it is no longer the recommended basis for a design you need to sign off.
An oversized system runs its fans harder than needed and pulls large volumes of heated or cooled make-up air straight out of the building, which adds to energy bills for the life of the system. It can also put the kitchen under excessive negative pressure, causing doors to slam, flues to draw backwards and neighbouring rooms to be starved of air. Right-sizing to the actual cooking load avoids paying to move air you never needed to move.
DW/172 recommends providing replacement, or make-up, air equal to roughly 85 to 90 per cent of the volume being extracted. That air needs a proper supply route into the space rather than relying on gaps around doors. Without it, the kitchen sits under heavy negative pressure, the extract fan cannot achieve its designed flow, and comfort and appliance performance both suffer.
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