Phoenix Journal · LEV Testing
The canopy is the one component that has to physically sit over the plume and draw it in - change it and you move the result more than any fan or duct. Here is how hood design decides whether your kitchen LEV actually controls what it should.
LEV design & performance
Change one thing about a commercial kitchen extraction system and the hood will move the result further than anything else - further than the fan, further than the ductwork, further than the filtration behind it.
Local exhaust ventilation works by pulling a moving cloud of contaminant into a capture zone before it can drift into the room and into someone's breathing space. In a kitchen that cloud is the thermal plume rising off the range: grease-laden vapour, steam, combustion products and heat. The hood - the canopy - is the mouth of the system. It is the one component that has to physically sit over that plume and draw it in. Get the hood right and a modest fan does the job cleanly. Get the hood wrong and no amount of extract volume downstream will rescue it.
The HSE makes this point plainly in HSG258, its guide to controlling airborne contaminants at work. The hood is described as the business end of the system - the place where the contaminant enters the LEV. Everything after it merely transports and cleans air that the hood has already succeeded, or failed, in capturing. That is why we treat canopy design as the first question on any kitchen, not the last.
For commercial kitchens specifically, the design benchmark is DW172, the specification published by the Building Engineering Services Association. It is not law in itself, but environmental health officers, insurers and consultants use it as the yardstick for what a competent kitchen ventilation system should look like - and its rules about hoods are the ones most often broken.
A canopy over a range is what HSG258 calls a receiving hood. It is not sucking the contaminant off a cold surface - it is receiving a hot plume that is already rising towards it under its own buoyancy. Your job is to design a mouth wide enough, low enough and hungry enough to swallow that plume before the room's air currents can pull it sideways.
Three numbers do most of the work:
Height matters just as much as width. A receiving canopy should sit as low over the appliances as is practical for the people working under it, because every extra 100 mm of gap gives the plume more room to widen and more chance to escape the capture zone. Mount a hood too high to look generous and you have quietly made it worse.
Here is the frustrating part: a hood can be perfectly sized and still fail on the day, because capture is a tug of war between the extract pulling up and the room pushing sideways. HSG258 is blunt that through-draughts are one of the biggest reasons receiving canopies leak. A capture velocity of half a metre per second sounds robust until a propped fire door, a pedestal fan aimed at the chef, or an open pass creates a cross-flow that simply peels the plume out from under the lip.
The most common self-inflicted version of this is make-up air. Every cubic metre you extract has to be replaced, and if it is not supplied deliberately the kitchen goes into negative pressure - doors become hard to open, appliances can be starved of combustion air, and replacement air comes howling in through whatever gap it can find, usually straight across the cooking line. DW172 recommends supplying replacement air at around 85 to 90 per cent of the extracted volume, introduced at low velocity and away from the capture zone, so the room stays close to balanced and the plume can rise undisturbed.
Grease filtration sits inside this balance too. The baffle filters in the canopy throat are a deliberate restriction - they knock grease out of the airstream so it does not coat the ductwork - but as they load up they raise system resistance and pull the delivered airflow down. A hood that met its capture velocity when it was commissioned can quietly drop below it after a few weeks of neglected filters, which is exactly why hood performance is a maintained property rather than a one-off design tick.
So the honest answer is that hood design is necessary but not sufficient. A well-designed canopy gives you the margin to survive real kitchen conditions; a poorly designed one has no margin to give, and every draught becomes a breach.
Because the hood is where control succeeds or fails, it is also where a proper LEV inspection spends its attention. Under Regulation 9 of the COSHH Regulations you have a legal duty to keep any engineering control in efficient working order, and for LEV that means a thorough examination and test at intervals not exceeding 14 months. That 14 months is a legal ceiling, not a recommendation - many kitchens sit well inside it.
A thorough examination is not a quick look at the fan. A competent assessor measures capture velocity at the plane of the canopy, checks face velocity through the filters, reads static pressure across the system and compares all of it against the commissioning benchmark and the DW172 design intent. That is how a hood that looks fine reveals itself as underperforming - the numbers at the capture zone tell the truth that a spinning fan hides. If you want the detail of what that visit covers, our explainer on what a thorough examination and test means walks through it, and the judgement behind those readings rests on the ILEVE competence framework that defines who is qualified to make the call.
The records matter as well: examination reports must be kept for at least five years, and they build the history that shows a hood is still controlling exposure rather than having drifted below the line since it was installed.
Treat the hood as the single biggest variable and the rest of the system falls into place around it. Size it, mount it low, protect it from draughts, balance the make-up air and keep the filters clean - then test it against real numbers so you know it is still doing what it was drawn to do.
Questions
DW172, the BESA specification used as the UK benchmark, expects a receiving canopy to extend roughly 250 to 300 mm beyond the footprint of the appliances on every open side. The thermal plume widens as it rises, so a canopy flush with the range edge lets the outer skirt of grease-laden vapour roll out past the lip and into the room. Mounting the hood as low over the appliances as is practical for the people working under it further reduces that escape.
Under Regulation 9 of the COSHH Regulations, LEV must have a thorough examination and test at intervals not exceeding 14 months. That figure is a legal maximum rather than a target, and higher-risk processes are examined more often. The report should measure capture velocity and system pressure against the original design intent, and records must be kept for at least five years.
Phoenix Duct Clean · by the numbers
Phoenix examines and tests local exhaust ventilation to HSG258 and COSHH - measured, reported and certificated, UK-wide.