Phoenix Journal · Extraction
On the hottest day of the year, one kitchen closed at half eleven - not because a part broke, but because a struggling extract finally ran out of slack. Here is why the heat finds the weakness first, and how to make sure it is not yours.
From the pass
The forecast said thirty-four degrees by noon, and by half eleven the head chef was standing in the car park with his whole brigade, watching the extraction fan tick over above the flue and doing nothing at all.
It had not failed dramatically. There was no smoke, no alarm, no dramatic bang. The canopy simply stopped pulling. On an ordinary day nobody would have noticed for an hour, but this was not an ordinary day. The outside air was already warmer than the kitchen was supposed to be at full tilt, the fryers were on, the chargrill was on, and with the extract barely moving the heat had nowhere to go. Within twenty minutes the space behind the line had climbed past anything a person could reasonably stand in. The kitchen did not close because of a broken part. It closed because a system that had been quietly struggling for months finally met a day that gave it no slack.
That is the thing about a heatwave. It does not create new problems in a commercial kitchen so much as it collects the ones you were already carrying and presents the bill all at once. A fan that was working twice as hard as it should because the ductwork was furred with grease. A motor running hot because nobody had cleaned the impeller in a year. A room that was borderline uncomfortable in April and genuinely dangerous in July. The weather was the trigger. The condition was already there.
A commercial extraction system is a heat engine as much as it is a grease trap. Its whole job is to lift hot, laden air off the cooking line and move it out of the building, and it does that against the resistance of the ductwork it has to travel through. When that ductwork is clean, the air moves freely and the fan works within its designed limits. When it is coated in grease, the effective bore of the duct shrinks, the resistance climbs, and the fan has to strain to shift the same volume of air. On a mild day it copes. On the hottest day of the year, with the motor already sitting in a hotter plant space and the incoming air offering no cooling, that strain is the difference between running and stopping.
Most kitchens run with far less headroom than the people working in them imagine. The system was specified for a certain airflow against a certain resistance, and every millimetre of grease deposit eats into the margin between what the fan can do and what it is being asked to do. You do not feel that margin disappearing because the fan keeps going right up until it does not. Grease inside the extract also carries a fire risk that rises with heat: fats, oils and grease build up on canopy, filters, ductwork and fan, and around seventy per cent of commercial kitchen fires are linked to build-up in the extract system. A heatwave stacks a thermal load on top of a fire load that was already accumulating.
The industry benchmark for this is TR19® Grease, published by BESA - the Building Engineering Services Association - and it is refreshingly specific about what clean means. The system should be maintained so that the mean grease deposit measures less than 200 microns across it. If a single reading exceeds 500 microns, that area needs immediate localised cleaning. After a clean, verification should confirm deposits below 50 microns, measured with a deposit thickness test or a wet film thickness test rather than a glance and a wipe. Those numbers matter beyond fire safety, because a system held to that standard is also a system that moves air the way it was designed to - which is exactly the capacity you are relying on when the outside temperature is doing its best to overwhelm you.
Once the extract falters, the problem stops being mechanical and becomes human very quickly. A kitchen is one of the hardest indoor environments the law recognises, and the heat inside it is never just the number on a thermometer on the wall. There is radiant heat pouring off the chargrill and the fryers, humidity from everything on the boil, and the metabolic heat of people working hard on their feet. All of that is supposed to be carried away by the extract. When the extract slows, that load stays in the room with your staff.
There is no legal maximum working temperature in the UK. Under the Workplace (Health, Safety and Welfare) Regulations 1992 the temperature indoors during working hours must be "reasonable", which sounds vague until you remember that reasonable is judged against the actual conditions of the job. The guidance sets a minimum - usually at least 16°C, or 13°C for strenuous work - but leaves the upper end to a thermal comfort assessment that weighs radiant heat, humidity, air movement and workload together. The TUC has long argued for a legal maximum of 30°C, dropping to 27°C for strenuous work, and while that is a campaign rather than a statute, it tells you where informed opinion sits. A kitchen at the wrong end of a heatwave with a struggling extract can pass straight through all of those figures.
The kitchen that closed did not have to. A working extract, cleaned to standard and moving air at its designed rate, is your first line of defence against a heatwave because it removes both the heat and the humidity that make the room unbearable. That protects the food safety chain as much as the people, since a kitchen that cannot shed heat is also a kitchen putting extra strain on its cold storage - which is why it pays to specify refrigeration that will not fail in a heatwave alongside an extract you can trust. And the two together protect the thing that is hardest to replace, which is the brigade themselves, because good conditions are a large part of how you keep kitchen staff through a heatwave rather than losing them to a rival with a cooler pass. The lesson from that car park is not that heatwaves are unpredictable. It is that they are the one time of year when every corner you have quietly cut comes due, and the extract is usually the first bill to land.
Questions
Yes, and it is more common than most operators expect. Grease inside the ductwork narrows the effective bore and forces the fan to work harder and run hotter for the same airflow, and a heatwave removes the cooling margin that fan relies on. When the extract slows or stops, the heat and humidity it should be removing stay in the room, and the space can quickly pass the point where it is safe or bearable to work. The weather is the trigger, but the neglected system is the real cause.
There is no legal maximum working temperature in the UK. The Workplace (Health, Safety and Welfare) Regulations 1992 require indoor temperatures to be "reasonable", judged through a thermal comfort assessment that weighs radiant heat, humidity, air movement and how strenuous the work is - not the air temperature alone. The TUC campaigns for a legal maximum of 30°C, or 27°C for strenuous work, which gives a useful benchmark even though it is not law. In a kitchen, a well-maintained extract is one of your main controls for keeping conditions on the right side of reasonable.
Phoenix Duct Clean · by the numbers
The right kit only helps if the system stays clean. Phoenix degreases canopies, filters and ductwork to TR19 Grease - UK-wide, overnight.