Phoenix Journal · Extraction
Heat at the peak of service is a design outcome, not bad luck. Here is how balanced extraction, make-up air and thermal comfort keep a commercial kitchen workable when every burner is on.
Kitchen design
A commercial kitchen that feels comfortable at half past two in the afternoon can turn into an oven the moment every burner, every fryer and every rail of orders comes on at once - and that is exactly when your team can least afford it.
Heat under load is not bad luck. It is the sum of decisions made long before the first service - how much air you pull out, how much you put back, at what temperature, and how well the two are balanced against the heat your equipment throws off. Get those decisions right and the room settles itself as demand climbs. Get them wrong and no amount of open doors or desk fans will save you at the peak.
This is a design piece rather than a rescue plan. It walks through how a well-specified extraction and supply system keeps a kitchen workable when the pass is full, why the numbers matter, and where the standards draw their lines. If you are fitting out a new site, refurbishing an old one, or trying to work out why a room that looked fine on paper cooks its staff every Friday, the principles below are where to start.
The industry reference for all of this in the UK is DW/172, the Specification for Kitchen Ventilation Systems published by the Building Engineering Services Association. It is not law in its own right, but it is the document designers, installers and building control routinely work to, and it sets out how the extract rate over a canopy should be worked out in the first place.
DW/172 favours a capture-velocity approach - sizing extraction around the air speed needed across the face of the canopy to catch the plume of heat and grease rising off the appliances beneath it. There is an older air-changes rule of thumb, a minimum of forty changes an hour, but the specification is clear that this should not be used as the basis for designing the canopy itself. The reason matters for comfort: capture velocity ties the airflow to the actual heat load of the equipment, so a heavy bank of chargrills and fryers gets the extraction it genuinely needs rather than a figure derived from room volume alone.
Then comes the part that most often gets short-changed - replacing the air you have just thrown outside. If you extract hard and supply little, the kitchen falls into negative pressure. Doors become heavy, they slam, gas appliances can struggle to draw properly, and the shortfall gets pulled in through every gap it can find, often as cold draughts across ankles while the air at head height stays hot. DW/172 expects mechanical make-up air at roughly 85 to 90 per cent of the extract volume so the room stays close to balanced. The remaining slice is drawn from adjoining spaces on purpose, keeping cooking smells from drifting into the dining room.
It is tempting to reduce comfort to a single number on a thermostat, but the HSE is explicit that air temperature alone is not a reliable measure of whether a workplace is comfortable or whether staff are heading towards heat stress. It points to six factors that have to be read together: air temperature, radiant temperature, air velocity, humidity, the insulation of the clothing people wear, and the metabolic heat their own work generates.
A kitchen quietly maxes out several of these at the same time. Radiant heat pours off open ovens, chargrills and hot holding - you feel it on your skin before it ever shows on a thermostat on the far wall. Humidity climbs as pots boil and dishwashers vent steam. Chef whites, closed shoes and long sleeves add insulation you cannot simply peel off. And the work itself - moving fast, lifting, plating under pressure - pushes metabolic heat right up. This is why two kitchens can read the same air temperature and feel completely different to work in.
Good design pulls on the levers it can actually move. Extraction that captures radiant-heavy appliances at source stops that heat loading the room. Tempered, well-placed supply air lifts local air velocity where people stand, which helps the body shed heat even when the air is warm. Getting steam out quickly holds humidity down. None of it changes the weather outside, but together it keeps the six factors in a range the body can cope with through a long, busy service.
The legal backdrop is worth knowing. The Workplace (Health, Safety and Welfare) Regulations 1992 require a “reasonable” indoor temperature during working hours and, notably, set no legal maximum. The accompanying code of practice normally expects at least 16°C, or 13°C where the work is physically demanding - a floor, not a target for a hot kitchen. The Management of Health and Safety at Work Regulations 1999 then oblige you to assess the risk to staff, heat included, and act on it. Designing the room to stay workable under load is how you meet that duty before it becomes a problem, and it is closely tied to why some kitchens keep tripping their electrics under load when everything is running flat out and drawing hard.
A system that stays cool under load on its commissioning day will not stay that way on its own. The single biggest drift is supply air. DW/172 sets minimum discharge temperatures for make-up air precisely because untempered winter air blown straight onto the line is miserable to stand in, and untempered summer air simply adds to the load - a rough guide is around 10°C minimum where air is delivered through the canopy and 16°C where it comes down through a ceiling. If tempering fails, or if a plenum gets throttled to cut noise, the balance you designed for quietly collapses and the room heats up long before anyone links it back to the supply side.
The other slow killer is grease. As extract ductwork and canopy filters load up with deposit, airflow drops, capture velocity falls, and more of the heat and steam that should have gone up the duct spills back into the room. It is also, of course, the fire risk that TR/19 grease cleaning exists to manage. A kitchen that felt fine a year ago and now cooks its team by nine each evening has very often not been redesigned - it has simply been left to clog, so the airflow the design depended on is no longer there. Gas sites add another layer: a BS 6173 gas interlock is meant to shut the gas down if the fans are not proving airflow, so a system that is not moving air properly should not be firing at full tilt anyway.
The practical takeaway is that cool-under-load is a maintained state, not a fixed feature. Keep the supply air tempered and at its designed volume, keep the extract path clean so capture velocity holds, keep the interlocks honest, and verify airflow after any change to the equipment beneath the canopy. Do that and the room you designed on paper is the room your team actually works in - even at the busiest twenty minutes of the night.
Questions
Because heat builds faster than a system sized for average conditions can shift it. Each appliance adds radiant heat, steam and metabolic load from staff working harder, and if extraction cannot capture the plume at source the room absorbs it. A design based on capture velocity, sized to the actual bank of equipment rather than room volume alone, is what keeps a kitchen level as demand climbs.
It is necessary. If you pull air out without replacing most of it, the kitchen goes into negative pressure - doors slam, gas appliances struggle to draw, and cold draughts get sucked in through every gap while the air at head height stays hot. DW/172 expects mechanical make-up air at around 85 to 90 per cent of the extract rate, tempered so it is not miserable in winter or a burden in summer.
No. The Workplace (Health, Safety and Welfare) Regulations 1992 require a reasonable indoor temperature but set no legal maximum. The code of practice normally expects at least 16°C as a minimum, and separately the Management of Health and Safety at Work Regulations 1999 require you to assess and control the risk of heat stress - which for a hot kitchen means designing and maintaining the ventilation to keep the room workable under load.
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