Phoenix Journal · Extraction
A kitchen canopy can only extract as much air as the room can replace, so make-up air is not an extra - it is half the ventilation system. Here is how replacement air should be planned from the start.
Kitchen ventilation
An extract canopy can only pull out as much air as the room is able to replace, which is why make-up air is not an add-on to a kitchen design - it is half of the system.
Get the balance right and the canopy captures grease-laden vapour cleanly, gas appliances burn safely and the space stays workable through a long service. Get it wrong and you inherit slamming fire doors, cold draughts across the pass, pilot lights that will not stay lit and a canopy that simply cannot reach its rated extract volume. This piece sets out how replacement air should be planned from the start, and how it ties into the standards a UK commercial kitchen is judged against.
Before any ductwork is drawn, a competent design should be able to answer for each of the points below. If it cannot, the numbers on the extract fan are optimistic at best.
It is tempting to treat kitchen ventilation as a single number - the extract rate - and to size a canopy and fan to hit it. CIBSE Guide B points designers towards high air change rates for commercial kitchens, in the order of 30 air changes an hour, and it is easy to focus on the extract side alone to get there. But a fan is only a pump. It can only move the air the room lets in. If replacement air is not designed in, the fan tries to draw a partial vacuum and the room fights back through every gap it can find: under doors, down flues, through letterboxes and around windows.
You feel the consequences long before you measure them. Fire doors that will not close against the pressure difference, which is both a nuisance and a serious life-safety failure. A whistling draught along the floor that makes the pot-wash a miserable place to stand. Worst of all, appliances that are meant to draw their own combustion air can be pulled into a starved condition, so flames lift, pilots blow out and, in the wrong circumstances, products of combustion spill back into the room. This is exactly why the make-up air question is bound up with gas safety, not just comfort.
The widely applied working rule is that where make-up air is supplied mechanically it should be capped at around 85% of the volume being extracted. The remaining share is allowed to transfer in from surrounding spaces. That is not a fudge - it is deliberate. Keeping the kitchen slightly negative means that when a door opens, air moves into the kitchen and up into the canopy, carrying grease vapour and smells with it, rather than rolling out over the diners. A kitchen pushed positive by over-generous supply does the opposite, and the first complaint is usually about smells in the restaurant.
Introducing large volumes of outside air has an obvious side effect: in winter that air is cold, and in a busy kitchen it is a lot of air. Dumping it in raw creates a cold, fast draught precisely where people are working hardest. Good design tempers the supply with a heater battery so incoming air arrives at a reasonable temperature, and it delivers that air gently and broadly - low-velocity ceiling supply, four-way diffusers or a plenum behind the canopy - rather than as a jet. The aim is a calm blanket of replacement air feeding the canopy, not a wind across the burners that pulls the capture zone apart and drops your extraction efficiency.
Delivery position is part of this too. Supply thrown at the open face of a canopy will disturb the very plume the canopy is trying to catch, so grease and steam escape into the room instead of being drawn away. Placed thoughtfully, make-up air does the opposite - it gently pressurises the space around the canopy and feeds a clean, stable capture pattern.
None of this is guesswork in the UK. BESA DW/172 is the reference specification for commercial kitchen ventilation systems and covers design, installation, commissioning and the balance between supply and extract. The BS EN 16282 series deals with the ventilation components themselves, including duct design and dimensioning. IGEM/UP/19 governs the interlock that ties gas appliances to proven ventilation, and it is increasingly paired with CO₂ monitoring so that gas is isolated if air quality in the working area deteriorates. Building Regulations Part F sits over the top on ventilation provision generally. Designing to these from the outset is far cheaper than discovering, after opening, that the extract cannot reach its rated figure because the room was never given a way to breathe.
There is also a maintenance dividend. A system that is balanced, tempered and delivered well runs closer to its design point, so the canopy captures more grease at source and less is left to accumulate along the ductwork. That makes routine extract cleaning more effective and the whole installation easier to keep compliant. Supply plant needs attention too - filters and heater batteries on the make-up side are part of the system that keeps the kitchen safe and pleasant to work in, and they should be designed for access from day one. If you want the wider picture of how replacement air shapes the whole ventilation strategy, our companion piece on the role of make-up air in kitchen ventilation goes further into how the pieces fit together.
Questions
The extract and supply volumes have to be reconciled so the canopy can actually reach its design rate. The common working rule is that mechanically supplied make-up air is kept to around 85% of the total extract volume, with the remaining share transferring in from adjacent spaces. That keeps the kitchen at a slight negative pressure relative to front-of-house, so odours and heat are drawn up into the canopy rather than out into the dining area.
Those are classic signs of too little make-up air. When the extract fan cannot draw enough replacement air through the supply system, it pulls the room into a partial vacuum and air rushes in through every gap - under doors, around windows and down flues. The fix is to design proper replacement air into the space rather than fighting the fan, and to temper and diffuse that air so it does not arrive as a cold jet across the line.
Yes, directly. If a kitchen is starved of replacement air, gas appliances can be pulled into a poorly ventilated condition where flames lift, pilots blow out or combustion products spill back into the room. This is why IGEM/UP/19 requires a gas interlock that only allows gas appliances to run when both extract and supply ventilation are proven to be working, often alongside CO₂ monitoring that isolates the gas if air quality drops.
BESA DW/172 is the reference specification for commercial kitchen ventilation systems and covers the balance between supply and extract. The BS EN 16282 series deals with the ventilation components and duct design, IGEM/UP/19 covers the gas interlock, and Building Regulations Part F sits over ventilation provision generally. CIBSE Guide B also informs the high air change rates typical of commercial kitchens.
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