Phoenix Journal · Ductwork
A well-behaved commercial kitchen depends on a quiet discipline most people never see: matching the air you pull out with the air you put back. Here is how balance is set, why it matters for safety, and how it is held over time.
On the floor
There is a moment, usually mid-service, when a kitchen tells you whether its ventilation is balanced - and it rarely does so politely.
You feel it before you can measure it. The fire door to the corridor drags on its closer and then snaps shut with a bang. A cold draught knifes across the pass every time someone opens the back door. The chefs are working under a canopy that sounds busy but somehow the room is still hazy, and a faint smell of last night's service lingers in the dry store. None of these are separate faults. They are all the same fault wearing different clothes - a building pulling out far more air than it is letting back in.
Balancing supply and extract is the quiet discipline that sits underneath every well-behaved commercial kitchen. When it is right, nobody notices it. When it is wrong, it shows up as slamming doors, sputtering pilot lights, high energy bills and a grease-laden haze that no amount of extra fan speed seems to shift. Getting it right is not guesswork - it is a set of numbers that have to add up, and a handful of British standards that tell you what those numbers should be.
Every cubic metre of air a canopy drags out of a kitchen has to be replaced from somewhere. That is not a design preference, it is physics. If you do not deliberately design where that replacement air comes from, the building will find its own routes - under doors, down flues, through the letterbox and around window frames - and every one of those uninvited paths causes a problem.
In a commercial kitchen the extract rate is usually calculated first, because it is driven by what you are cooking. The recognised UK specification for this is DW/172, published by the Building Engineering Services Association, which sets out several methods for sizing a canopy - among them the capture-velocity approach, where a face velocity is chosen across the canopy opening according to how hard the appliances beneath it are working. Light-duty equipment such as steaming ovens, boiling pans and bains-marie is typically served at around 0.25 m/s, with heavier, hotter appliances demanding more. CIBSE Guide B, meanwhile, points to a minimum in the order of 30 air changes an hour for a kitchen, with many busy operations sitting comfortably between 30 and 40.
Once you know how much you are pulling out, you know how much you have to put back. The principle is simple even when the ductwork is not: the total leaving the room and the total entering it should be within a small, controlled margin of each other.
DW/172 and the wider catering guidance are consistent on the figure that matters most here. When a kitchen is mechanically extracted, the bulk of that extracted volume should be mechanically replaced with tempered fresh air rather than left to leak in - commonly expressed as supplying somewhere between 75% and 95% of the extract rate, with many designers aiming for around 85% to 90%. That deliberate shortfall is the clever part. You do not supply 100%. You leave the kitchen very slightly negative relative to the spaces around it, so that cooking smells, heat and grease-laden air are drawn towards the canopy and away from the dining room, rather than drifting out to greet the customers.
Keep a few things in view when you look at how that balance is set up:
Understanding the distinction between air that is drawn out, air that is pushed in and air that is simply recirculated is where a lot of well-meaning designs go wrong, and it is worth reading up on the difference between supply, extract and recirculated air before you start moving dampers.
An unbalanced kitchen is not just uncomfortable. It quietly undermines gas safety, fire safety and the performance of the very extract system you paid for. The symptoms are worth learning to read, because they nearly always point back to the same root cause.
When a kitchen runs too negative, the building starts hunting for air down any available path - and flues are an available path. A gas appliance or a combustion flue can back-draught, pulling products of combustion, including carbon monoxide, back into the space instead of venting them safely outside. This is precisely why BS 6173, the standard for the installation of gas catering appliances, requires that appliances are interlocked with the mechanical ventilation serving them - the gas supply should not be available unless the ventilation is proving airflow. Balance is what makes that interlock meaningful rather than a box-ticking gesture.
The same negative pressure that threatens a flue also makes the room hard to work in. Doors become heavy and slam. Draughts chill the section. And, counter-intuitively, the extract itself gets weaker - a fan that cannot draw enough replacement air simply cannot move its rated volume, so capture over the canopy falls off and the haze you were trying to remove starts to win. Environmental health officers and insurers both treat DW/172 as the benchmark here, so a system that cannot demonstrate a sensible supply-to-extract relationship is a system that invites awkward questions.
Putting balance right is part design, part commissioning and part maintenance. The design fixes the intent, commissioning proves it, and ongoing cleaning and testing stop it drifting. A duct that has silted up with grease behaves like a smaller duct - resistance climbs, the fan falls behind, and a system that was commissioned in balance slowly slides negative without anyone touching a control.
Practical steps that keep a kitchen sitting where it should:
That last point matters most in premises where the kitchen shares a structure with bars, dining rooms, hotel floors or offices, because the pressures interact in ways a single-room calculation will miss. If that is your situation, it is worth thinking about balancing ventilation across a mixed-use building as a single system rather than treating the kitchen as an island. Do that, keep the ductwork clean, and the building stops arguing with you - the doors close softly, the flames burn steady, and the only thing anyone notices about the ventilation is that they no longer notice it at all.
Questions
UK catering ventilation guidance, including DW/172, points to mechanically supplying the large majority of the extracted volume - commonly cited as around 75% to 95%, with many designers aiming for roughly 85% to 90%. The deliberate shortfall keeps the kitchen very slightly negative relative to the dining areas, so cooking smells and grease-laden air are drawn towards the canopy rather than out to the customers. Supplying a full 100% would remove that helpful pressure difference, and supplying too little starves the extract and pulls the room negative.
When a kitchen runs too negative it draws replacement air down any available path, and that can include combustion flues. A gas appliance can back-draught, pulling products of combustion such as carbon monoxide back into the room instead of venting outside. This is why BS 6173 requires gas catering appliances to be interlocked with their mechanical ventilation, so gas is only available when the system is proving airflow. Proper supply-and-extract balance is what makes that protection reliable.
Phoenix Duct Clean · by the numbers
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