Kitchen extraction & ventilation
For every cubic metre the canopy hauls out, one has to come back in. How that replacement air is supplied quietly decides whether the whole system works or fights itself.
Ask most people what a kitchen ventilation system does and they will point at the canopy pulling smoke and steam away from the cookline. That is only half of it. For every cubic metre of air the canopy hauls out, a cubic metre has to come back in, and how that replacement air is supplied quietly decides whether the whole system works or fights itself. That supply is make-up air, and it is the half of the system nobody looks at.
A commercial kitchen extract system is really two flows in balance. The extract side captures heat, grease-laden vapour and combustion products at the cookline and drives them out of the building. The supply side, the make-up air, brings in fresh outdoor air to replace exactly what was removed. Air is not compressible in any useful way at these pressures, so what leaves must be replaced from somewhere; the only question is whether that replacement is designed and controlled, or whether it sneaks in uncontrolled through doors, gaps and flues. Make-up air is the decision to control it.
Why it is not optional
If a canopy extracts a large volume of air and nothing deliberately replaces it, the kitchen simply becomes negatively pressured: the pressure inside drops below that of the spaces around it, and air is dragged in from wherever it can find a path. UK guidance in BESA's DW172, the specification for kitchen ventilation systems, addresses this directly, calling for make-up air to be introduced to prevent that negative pressure from building. A common design brings back most of the extract volume, often up to around 85 per cent, as deliberate make-up air, leaving a small deficit so the kitchen stays gently negative and its odours do not migrate into dining areas. That small, controlled negativity is healthy; a large, uncontrolled one is not.
An under-supplied kitchen announces itself. Doors become heavy to open because the room is effectively sucking against them; cold draughts are pulled in through every crack, chilling staff at the pass; and, most seriously, the extract fan cannot develop its designed capture velocity because there is not enough air to pull against, so grease and smoke that should be captured spill back into the room. Where gas appliances are present, strong negative pressure can disturb their flues and combustion, which is why make-up air ties directly into gas safety and the BS6173 interlock arrangements that shut the gas off if ventilation fails.
Because make-up air is real outdoor air, it needs conditioning before it reaches the cookline. Straight from outside it is cold in winter and unfiltered, so well-designed systems temper it with a heater battery and filter out dust before it enters the space. A neat and common solution is to build a replacement-air plenum into the extraction canopy itself, feeding the fresh air in through diffusers in the canopy face, some with directional vanes so staff can steer it, so make-up air arrives gently and close to where it is needed rather than as a draught across the room.
Getting the balance right
The lesson that experienced designers keep relearning is that extract and make-up air have to be conceived together, as a single balanced system, not bolted on one after the other. Size the canopy for the cooking load, then size the make-up air to match the extract, temper and filter it, and route it so it supports capture rather than disrupting it. Get that balance right and the kitchen sits at a comfortable, slightly negative pressure with clean capture and no draughts; get it wrong and no amount of fan power fixes the symptoms. It is the same balancing discipline that any complex building applies when it shares air between spaces, the ventilation equivalent of the problem faced when balancing ventilation across a mixed-use building.
Keeping both flows clear
It is easy to focus all the cleaning attention on the greasy extract side and forget that make-up air has its own ductwork, filters and plenum that must stay clear to do their job. A clogged make-up air filter chokes the supply and pushes the kitchen negative just as surely as an oversized extract fan does, undoing the balance the system was commissioned with. Both flows deserve inspection and cleaning on a proper schedule, which is exactly why an enclosed kitchen relying wholly on mechanics, like a basement or windowless kitchen, has to maintain its supply side as diligently as its extract. Treat make-up air as an equal partner to extraction, and the system stays in the balance it was built to hold.
Questions
Make-up air, or replacement air, is fresh outdoor air deliberately supplied to replace what the extraction canopy removes. A kitchen extract system is two flows in balance: the extract captures heat, grease and fumes at the cookline, and the make-up air brings back what was taken out.
Because what leaves must be replaced from somewhere. Without deliberate make-up air, a kitchen goes negatively pressured and drags air in through doors, gaps and flues. BESA's DW172 calls for make-up air to prevent that, typically returning up to around 85 per cent of the extract volume.
Doors that are heavy to open, cold draughts pulled in through gaps, and an extract fan that cannot reach its capture velocity, so grease and smoke spill back into the room. With gas appliances, strong negative pressure can also disturb flues and combustion, which is a safety risk.
Yes. It is real outdoor air, so it is cold in winter and unfiltered. Well-designed systems temper it with a heater battery and filter it before it reaches the cookline, often introducing it through a plenum built into the canopy so it arrives gently rather than as a draught.
Yes. The supply side has its own ductwork, filters and plenum that must stay clear. A clogged make-up air filter chokes the supply and pushes the kitchen negative just as an oversized extract fan would, so both flows should be inspected and cleaned on a proper schedule.
Phoenix Duct Clean · by the numbers
The supply side needs clean ductwork and filters just as much as the extract. Talk to Phoenix about maintaining both halves of your kitchen ventilation system.