Phoenix Journal · Ductwork
Extraction that draws beautifully all week can wilt the moment a front blows in. The wind is quietly working against both ends of your system - here is why, and what actually restores the draw.
From the service diary
The chef rings on a Tuesday, and the first thing you hear is the wind behind him.
It is always the same story. The canopy has been fine all week - quiet, drawing well, kitchen comfortable through the lunch rush. Then a front rolls in off the coast, the gusts pick up, and suddenly the smoke off the chargrill is loafing about under the hood instead of vanishing up the duct. A back door that swung shut on its own yesterday now needs a shoulder. By service the pass is hazy and the extract, which has not changed one bolt since the last visit, feels like it has quietly given up.
Nothing has broken. The fan is turning at the same speed it always does, the ductwork is the same ductwork. What has changed is the sky - and the sky, it turns out, has a real say in how a kitchen breathes. Understanding why the weather can rob you of extraction on the worst possible day is the first step to making sure it does not leave you stranded mid-service.
Your extract fan does not move air in a vacuum. It pulls against whatever pressure sits at the discharge point on the roof, and it relies on an equal volume of fresh air finding its way back into the room. Wind interferes with both ends of that bargain at once, which is why a breezy day can hit performance harder than you might expect.
When wind strikes a building it does not slide past politely. It piles up against the windward face as positive pressure and peels away from the leeward face as suction, and your roof-mounted discharge sits somewhere in that shifting map. If a gust is pressing down onto the cowl or driving straight at the outlet, your fan is no longer just lifting grease-laden air up the riser - it is shoving it out against a headwind. That added back-pressure sits on the fan curve like extra load, and the volume it can actually shift drops away. The fan sounds the same because it is doing the same work; it is simply getting less air for it.
The quieter half of the problem is what is meant to replace the air you throw away. Every cubic metre pulled off the cooking line has to be made up from somewhere, and a well-designed kitchen brings it in deliberately through a make-up air supply. BESA's DW/172, the recognised UK specification for kitchen ventilation, is built around keeping extract and replacement air in balance for exactly this reason. When wind depressurises the building envelope, or slams shut the gaps and doors that were quietly feeding the room, that supply dries up. Run an extract with nothing to draw on and the whole system labours in negative pressure - and industry guidance notes that severe negative pressure can strip a meaningful slice of a hood's real-world capture away. That is when doors turn heavy, the burners can splutter, and the smoke you paid to remove drifts back into the room.
You cannot stop the wind, and you would not want to spend your energy trying. What you can do is make sure your system has enough margin in hand that a rough day is an inconvenience rather than a shutdown. That margin comes from two things working together - a duct that is not fighting itself, and a supply of make-up air that keeps turning up when the weather turns.
Wind adds resistance you have no control over. Grease adds resistance you do. As deposits build on the internal walls of the ductwork, the clear bore narrows and the surface roughens, and the fan has to work harder for every cubic metre - on a calm day and a windy one alike. A system already running close to its limit because the duct is furred up has nothing left to give when a gust piles on more back-pressure, so it is the greasy kitchens that notice the weather first. TR/19 Grease, the standard our sector cleans to, uses a mean grease depth of around 200 microns as the trigger point for cleaning, because past that the deposits are both a fire risk and a drag on airflow. Clearing them back to bare metal is the single most effective way to hand your fan back its headroom.
Beyond the deep clean, a handful of practical checks decide whether the weather bothers you. If the make-up air path is blocked, throttled or simply missing, the kitchen has no defence against a windy afternoon. If the discharge termination is badly sited or the cowl is damaged, every gust is amplified. And if the fan is already sounding strained before the wind gets involved, that is worth investigating in its own right - a system that has grown noticeably louder is often telling you it is working too hard, which you can read more about in our note on why your extraction has got louder. In a rented unit it also pays to know who picks up that work before you are mid-service in a gale; our guide to who is responsible for extraction cleaning in a leased kitchen sets out where the line usually falls.
Questions
It rarely stops the fan outright, but it can noticeably cut how much air the fan actually shifts. Gusts create positive pressure at the roof discharge that the fan has to push against, and they can depressurise the building so the make-up air the system relies on dries up. The fan keeps spinning at the same speed, but the real-world extraction volume at the canopy drops - which is why the smoke lingers even though nothing has broken.
Yes, more than most people expect. Wind adds resistance you cannot control, but grease inside the duct adds resistance you can remove. A furred-up system is already running near its limit, so it has no reserve left when a gust piles on more back-pressure. Cleaning the ductwork back to bare metal in line with TR/19 Grease restores the fan's headroom, so the same windy day that used to defeat it becomes a manageable dip rather than a stall.
Phoenix Duct Clean · by the numbers
Phoenix surveys and cleans kitchen and building ductwork to the TR19 standard - measured, cleaned and certificated, UK-wide.