Phoenix Journal · Extraction
The right extract fan for a commercial kitchen is a calculation, not a hunch. It starts with the heat rising off your appliances and ends with the resistance the air meets on its way out of the building.
Kitchen extraction
A commercial kitchen fan is sized from three things - what you cook on, how the heat rises off it, and how much resistance the air meets on the way out. Get the sum right and the canopy captures cleanly. Get it wrong and you either flood the kitchen with fumes or spend the next ten years fighting your own building.
The recognised basis for the sum in the UK is DW/172, the Building Engineering Services Association's specification for kitchen ventilation systems. It sets out how the extract rate is worked out, how the canopy is dimensioned and how the make-up air is balanced - so that everyone from designer to installer to the person cleaning it later is reading from the same page.
The sizing starts at the appliances, not at the fan. Every cooking process turns most of its heat into a rising column of hot air - the thermal plume. As that plume climbs it pulls in surrounding room air, so it widens and cools the higher it goes. The job of the canopy is to sit in the path of that plume and catch it before it spreads across the kitchen, and the job of the fan is to pull air through the canopy fast enough to hold it there.
DW/172 works this out through the thermal convection method, which follows CIBSE Guide B2. Rather than guessing at air changes for the room, it looks at the surface area of each appliance and applies a coefficient for its type and fuel, giving a plume flow rate in cubic metres per second. Those figures are added across the whole line. Then a canopy factor is layered on top - a margin that lets the canopy overcome the cross-draughts from opening doors, moving staff and nearby supply grilles that would otherwise peel the plume away from the hood. The overhang matters here too: keeping the canopy edge at least 250mm proud of the cooking equipment, and further over combi and baking ovens, gives the plume somewhere to be caught rather than escape at the sides.
Only once the extract volume is settled does the fan itself come into it, and this is where sizing quietly goes wrong. A fan is not chosen on airflow alone - it has to deliver that airflow against the total resistance of the system. Grease filters, the friction of every metre of duct, each bend, any fire damper and acoustic attenuator, and the discharge cowl all add static pressure, and they are added up along the longest, most awkward run. The fan is then selected from its performance curve to hit the required volume at that pressure. Duct sizing feeds straight into this: main runs are usually designed around 6 to 8 m/s with branches a little lower at 5 to 7 m/s, fast enough to keep grease droplets moving to the filters, slow enough to avoid turbulence and noise. Skinny ducts push the resistance up and force a bigger, louder fan to compensate.
It is tempting to think the safe move is simply to fit the biggest fan available, but a fan sized past what the plume calls for is its own problem. It over-extracts, pulls conditioned air straight out of the kitchen, runs loud and burns money doing it - which is exactly the hidden cost of an oversized extraction fan. The aim is a fan matched to the calculation, not one chosen to feel reassuring.
None of it holds together without the other half of the balance - the air coming in. A canopy pulling air out of a sealed room quickly drops the space into negative pressure, and the moment that happens the fan can no longer shift its rated volume, doors stick, and gas burners struggle for the oxygen they need. That is why DW/172 expects replacement air to be supplied at roughly 85 to 90% of the extract rate, tempered and delivered so it doesn't blow across the cooking line and disturb the very plume you are trying to catch. Understanding the role of make-up air in kitchen ventilation is really the difference between a system that captures on paper and one that captures in the room.
A well-sized system is a chain: appliances set the plume, the plume sets the extract volume, the canopy and duct set the resistance, the fan is matched to both, and the make-up air keeps the whole thing in balance. Break one link and the rest can't recover. It's also worth remembering that a system is only sized correctly while it is clean - filters and ducts loaded with grease change the resistance the fan was chosen for, which is why keeping on top of the extract system is part of keeping the sizing honest.
Questions
DW/172, the Building Engineering Services Association's Specification for Kitchen Ventilation Systems, is the recognised basis. It sets out the thermal convection (plume) method for working out the extract rate, along with guidance on canopy dimensions, ductwork, grease filtration and make-up air, so the whole system is designed to one consistent standard.
Because a fan sized beyond what the calculation calls for over-extracts. It drags heated or cooled air straight out of the kitchen, runs noisily, uses more energy and can pull the room into negative pressure that upsets doors and gas appliances. The goal is a fan matched to the plume and the system resistance, not simply the biggest one available.
DW/172 expects replacement air of roughly 85 to 90% of the volume being extracted. Without it, the room falls into negative pressure, the fan can't move its rated airflow, and capture at the canopy suffers - so make-up air is sized alongside the extract, not bolted on afterwards.
Phoenix Duct Clean · by the numbers
The right canopy and filters only help if the system stays clean. Phoenix degreases canopies, filters and ductwork to TR19 Grease - UK-wide, overnight.