How to Spec a Commercial Extraction System From Scratch

Eight steps to a system that works from day one

1. Start with the cooking line, not the canopy. Before you think about airflow you need a full appliance schedule - what is being cooked, on what, and how hard. Open-flame chargrills, solid-fuel ovens and woks throw off far more heat and grease-laden vapour than an electric griddle or a bank of combi ovens, and DW/172 grades the extraction demand by exactly this kind of cooking intensity. Nail the duty of the line first and every number after it falls into place.
2. Size the extract to the thermal load. BS EN 16282-1 sets out a recognised method for calculating the air volume a canopy needs to draw, driven by the convective heat rising off the appliances rather than a rule of thumb. As a working guide, medium-duty cooking sits around 0.4 to 0.6 m³/s of extract per metre of canopy length, rising for heavy open-flame work. Undersize here and the canopy simply cannot capture the plume; oversize it and you waste energy and over-draw your make-up air.
3. Design the canopy to capture, not chase. A canopy works by containing the rising plume long enough to pull it into the filters, so it should overhang the cooking equipment on all exposed sides - typically by around 250 to 300 mm - and sit at a sensible height above the line. Specify baffle-type grease filters rather than mesh, because they separate grease more effectively and are far easier to remove and wash. The canopy plenum behind the filters is where much of your future cleaning access is won or lost, so plan it now.
4. Plan the replacement air deliberately. Every cubic metre you extract has to be replaced, and if you do not design that make-up air in, the kitchen will drag it through doors, gaps and neighbouring spaces - slamming fire doors, chilling staff and starving gas appliances of combustion air. Aim to supply slightly less than you extract so the kitchen stays at a gentle negative pressure relative to the dining area, and temper the incoming air so winter supply does not turn the line into a wind tunnel. This is one of the most common things a cheap spec skips.
5. Route the ductwork for grease and access. Keep runs as short and direct as you can, avoid unnecessary bends, and maintain enough air velocity through the duct to carry grease along rather than let it settle on horizontal surfaces. Following BESA’s TR/19 Grease guidance, build in access panels of adequate size and number - at every change of direction and at regular intervals - so the whole internal surface can be reached, cleaned and inspected. Ductwork you cannot get inside is ductwork that will eventually fail an inspection.
6. Build in the fire strategy from the outset. Grease-laden extract is a genuine fire risk, so the specification must address it head-on: cleanable filters, fire-rated ductwork where it passes through compartment walls or floors, and, where the risk warrants, a fixed fire suppression system designed to BS EN 16282-7. All of this is far cheaper to engineer in now than to retrofit, and your insurer and fire risk assessor will expect to see it. The Regulatory Reform (Fire Safety) Order 2005 puts the legal duty squarely on you as the responsible person.
7. Interlock the gas supply. If any appliance on the line burns gas, BS 6173:2020 and Gas Safe practice require a gas interlock that shuts off the supply automatically whenever the ventilation is not proving adequate airflow. This protects against a build-up of combustion products - including carbon monoxide - if a fan fails or is switched off, and it is checked at commissioning and thereafter. Treat it as non-negotiable, not an optional extra.
8. Commission, balance and document. A system is not finished when it is installed; it is finished when it is commissioned, the airflows are measured and balanced against the design figures, and the results are recorded. Insist on a full operation and maintenance manual, as-fitted drawings and a logbook, because these are the documents an environmental health officer, insurer or future cleaning contractor will ask for. Without them you cannot prove the system ever met its own specification.