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Duct cleaning - ventilation design
A kitchen extraction system pulls hot, greasy air out of the kitchen - but for it to do that, an equal amount of air has to come in to replace what leaves, because air cannot simply be removed from a sealed space. That replacement air is make-up air, and without enough of it the extraction cannot work properly. Far from an accessory, make-up air is half the ventilation system. Here is the role make-up air plays in kitchen ventilation, and why it matters.
The short answer
Make-up air is the replacement air supplied to a kitchen to balance the air the extraction removes. The reason it is needed is basic physics: a kitchen extraction system removes air (the hot, greasy, humid air over the cooking), and that air has to be replaced by an equal amount coming in, because you cannot keep pulling air out of a space without letting air in - the space would simply go into negative pressure and the extraction would stall. So for the extraction to move air at its designed rate, make-up air has to be supplied at close to that rate. The industry specification for commercial kitchen ventilation (DW/172) recommends providing make-up air at roughly 85 to 90 per cent of the extract volume, with the kitchen held slightly negative relative to the dining area (so cooking smells flow towards the kitchen extract, not out into the restaurant). When there is enough make-up air, the extraction works as designed - capturing the grease, heat and moisture at the canopy and removing them. When there is too little, the extraction is starved: it cannot pull air out at its rated rate, so its capture falls, and the kitchen goes into negative pressure, which causes a string of problems - doors that are hard to open or slam, gas appliances that do not get the combustion air they need, cold draughts pulled in through every gap, and the extraction under-performing. So make-up air is not an optional extra bolted onto the extraction; it is half the ventilation system - the supply side that makes the extract side work. A kitchen ventilation system is a balanced pair, air out and air in, and make-up air is the in.
Why make-up air is needed
The role of make-up air rests on a simple physical fact: you cannot remove air from a space without replacing it. A kitchen extraction system works by drawing air out - the canopy over the cooking line captures the hot, greasy, humid air and the fan pulls it up the ductwork and out of the building. But every cubic metre of air the extraction removes has to be replaced by a cubic metre coming in, because the kitchen is a physical space that cannot lose air indefinitely. If air is pulled out and not replaced, the pressure in the kitchen drops - it goes into negative pressure relative to outside and the surrounding areas - and that negative pressure fights the extraction, so the fan can no longer move air at its designed rate. So an extraction system without a matching supply of replacement air cannot work properly: it is trying to remove air from a space that has no way to let air in.
This is why make-up air exists: it is the deliberate supply of replacement air to balance the extraction. Rather than relying on air leaking in through gaps (which is uncontrolled and insufficient for a large extract rate), a proper kitchen ventilation system supplies make-up air actively - bringing in outside air to replace what the extraction removes, so the extraction can move air freely at its rated rate. So make-up air is the supply side of the ventilation, paired with the extraction's removal side, and the two have to balance for the system to work. This is the fundamental role: make-up air is what makes it physically possible for the extraction to do its job, by replacing the air the extraction takes out. Without it, the extraction is trying to work against a growing vacuum; with it, the extraction works against a balanced supply. That balance is the heart of kitchen ventilation, and make-up air is half of it.
The balance
The role of make-up air is defined by the balance it strikes with the extraction, and the industry specification for commercial kitchen ventilation, DW/172, sets out how. It recommends supplying make-up air at roughly 85 to 90 per cent of the extract volume - so for most of the air the extraction removes, replacement air is supplied deliberately, with a small shortfall made up by air drawn in from the surrounding areas (the dining room and beyond). That deliberate slight shortfall is not an accident; it is what keeps the kitchen very slightly negative in pressure relative to the dining area. The reason for the slight negative is that it makes air flow from the dining area into the kitchen, and from the kitchen out through the extract - so cooking smells, heat and grease are pulled towards the kitchen's extraction and away from the restaurant, rather than drifting out into the dining space. So the balance is deliberately tuned: enough make-up air for the extraction to work (85 to 90 per cent supplied), but slightly less than the extract, so the kitchen stays marginally negative and the smells go the right way.
This is why make-up air is a designed quantity, not just any air let in: it has to be enough for the extraction to achieve its rate, but arranged so the pressure balance is right. Getting it wrong in either direction causes problems. Too little make-up air, and the kitchen goes strongly negative and the extraction is starved (the problems below). Too much make-up air, and the kitchen goes positive relative to the dining area, so the cooking smells and heat are pushed out into the restaurant instead of being pulled into the extract - the opposite of what you want. So the make-up air is balanced to the extract at the DW/172 proportion, achieving both the airflow the extraction needs and the slight negative pressure that keeps the smells contained. Every cubic metre extracted is replaced by close to a cubic metre supplied, deliberately, and that balance is the role make-up air plays: not just replacing air, but replacing it in the right amount to make the whole ventilation system work as it should.
What goes wrong without it
When there is not enough make-up air, the problems are numerous and connected, all stemming from the kitchen being pulled into negative pressure by an extraction that has nothing to replace the air it removes. The first is that the extraction itself under-performs: starved of replacement air, the fan cannot move air at its rated rate, so the extraction captures less grease, heat and moisture at the canopy than it should - the very job it exists to do is compromised. So a kitchen short of make-up air has a weakened extraction even if the extract fan is correctly sized, because the fan cannot achieve its output against the negative pressure. This is a common and under-recognised reason for an extraction that seems to under-perform: not the extract fan, but the missing make-up air.
The negative pressure causes a string of further problems. Doors become hard to open, or slam shut, because the kitchen is sucking air in through every opening - a nuisance and sometimes a safety issue. Cold draughts are pulled in through every gap and grille, making the kitchen uncomfortable and hard to heat, because uncontrolled air is being drawn in wherever it can. And, most seriously, gas appliances can be starved of the combustion air they need: a strongly negative kitchen can interfere with the proper burning of gas and the clearing of combustion products, which is a genuine safety concern with flued and open-flued appliances. So a kitchen short of make-up air is not just an inefficient one; it is one with slamming doors, cold draughts, an under-performing extraction and a potential gas-safety problem - all because the air the extraction removes is not being properly replaced. These problems are the practical demonstration of make-up air's role: their appearance is usually the sign that the make-up air is inadequate, and putting the make-up air right resolves them.
Make-up air in practice
In practice, make-up air is provided by supplying outside air into the kitchen to balance the extraction - often through the ventilation canopy itself (some canopies supply make-up air as well as extracting), or through separate supply air units and grilles, bringing in and distributing the replacement air. In a well-designed kitchen ventilation system, the make-up air is designed in alongside the extraction, sized to the DW/172 proportion of the extract, and often tempered (heated in cold weather) so that bringing in large volumes of outside air does not make the kitchen cold. So make-up air is a designed, supplied part of the system, not an afterthought - which is exactly the point: it has to be planned and provided deliberately, because relying on incidental air leaking in cannot balance a proper extract rate.
Like the extraction, the make-up air has to be maintained to keep working: its supply routes kept clear, its units serviced, its filters kept clean, so it delivers the replacement air the extraction needs. And when a kitchen's ventilation is assessed - for instance if the extraction seems to be under-performing, or the kitchen shows the negative-pressure symptoms - the make-up air should be checked alongside the extract, because the two are a balanced pair and a problem in one shows up in the other. A kitchen ventilation system is not just the extraction; it is the extraction and the make-up air together, balanced, and both have to be right and maintained for the system to work. So the role of make-up air, in practice, is as an equal half of the ventilation - designed, supplied and maintained alongside the extraction - and treating it as such is what keeps a kitchen properly ventilated. Ignore it, and the best extraction in the world cannot work; provide and maintain it, and the extraction can do its job.
The takeaway
Make-up air is the replacement air supplied to a kitchen to balance the air the extraction removes, and its role rests on a simple fact: you cannot keep pulling air out of a space without letting air in, or the space goes into negative pressure and the extraction stalls. So for the extraction to move air at its designed rate, make-up air has to be supplied at close to that rate - the industry specification DW/172 recommends about 85 to 90 per cent of the extract volume, leaving the kitchen slightly negative to the dining area so cooking smells are pulled towards the kitchen's extract rather than out into the restaurant.
When there is enough make-up air, the extraction works as designed, capturing the grease, heat and moisture at the canopy. When there is too little, the kitchen is pulled into negative pressure and a string of problems follow: the extraction is starved and under-performs, doors are hard to open or slam, cold draughts are pulled in, and - most seriously - gas appliances can be starved of combustion air. So make-up air is not an accessory bolted onto the extraction; it is half the ventilation system, the supply side that makes the extract side work. A kitchen ventilation system is a balanced pair - air out and air in - and make-up air is the in. Design it, supply it and maintain it alongside the extraction, and the system works; neglect it, and the extraction cannot, however well the extract fan is sized.
Questions
Make-up air is the replacement air deliberately supplied into a kitchen to balance the air the extraction removes. When the extraction pulls hot, greasy air out of the kitchen, an equal amount of air has to come in to replace it - otherwise the kitchen goes into negative pressure and the extraction cannot work. Make-up air is that replacement air, brought in and distributed on purpose (through the canopy or separate supply units) rather than left to leak in through gaps. So it is the supply side of the kitchen ventilation, paired with the extraction's removal side. The two together - air out and air in, balanced - make up the complete ventilation system, and make-up air is the in half of that pair.
The industry specification for commercial kitchen ventilation, DW/172, recommends supplying make-up air at roughly 85 to 90 per cent of the extract volume. That deliberately leaves a small shortfall, made up by air drawn in from the dining area, which keeps the kitchen slightly negative in pressure relative to the restaurant - so cooking smells and heat are pulled towards the kitchen's extract rather than out into the dining space. So the make-up air is matched to the extraction at that proportion: enough for the extraction to work at its rated rate, but slightly less than the extract so the pressure balance keeps the smells contained. It is a designed quantity, tuned to both the airflow the extraction needs and the pressure balance the kitchen wants.
The kitchen is pulled into negative pressure, and several connected problems follow. The extraction is starved - it cannot move air at its rated rate against the negative pressure, so it captures less grease, heat and moisture than it should, under-performing even if the fan is correctly sized. Doors become hard to open or slam shut as the kitchen sucks air in through every opening. Cold draughts are pulled in through gaps, making the kitchen uncomfortable and hard to heat. And most seriously, gas appliances can be starved of the combustion air they need, which is a genuine safety concern. So too little make-up air is not a minor inefficiency - it weakens the extraction and creates a string of comfort and safety problems, all from the air not being properly replaced.
Yes. If too much make-up air is supplied, the kitchen goes positive in pressure relative to the dining area, which reverses the intended airflow: instead of smells being pulled from the dining area into the kitchen and out through the extract, the cooking smells and heat are pushed out of the kitchen into the restaurant. That is the opposite of what you want - the dining area fills with kitchen smells. So make-up air is not simply more-is-better; it is balanced to the extract at the DW/172 proportion (about 85 to 90 per cent), deliberately leaving the kitchen slightly negative so the smells go the right way. Both too little and too much cause problems, which is why the make-up air is a designed, balanced quantity rather than just any air let in.
Because it may be starved of make-up air. An extract fan can only move air as fast as replacement air comes in; if the make-up air is inadequate, the kitchen goes negative and the fan cannot achieve its rated output against that negative pressure, so the extraction under-performs regardless of the fan being correctly sized. This is a common and under-recognised cause of poor extraction: people look at the extract fan when the real problem is the missing supply air. So when an extraction seems weak, the make-up air should be checked alongside the fan, because the two are a balanced pair - a correctly sized fan starved of make-up air will still under-perform, and putting the make-up air right often restores the extraction without touching the fan.
Yes - it is an equal half of the ventilation system and needs keeping up just as the extraction does. Its supply routes have to be kept clear, its supply units serviced, and its filters kept clean, so it delivers the replacement air the extraction needs. If the make-up air supply degrades - filters clogging, units failing - the balance is lost and the extraction is starved, with all the negative-pressure problems that brings, even though nothing has changed on the extract side. So when a kitchen ventilation system is maintained or assessed, the make-up air should be included, not just the extraction. Treating the make-up air as an afterthought, in maintenance as in design, is how kitchens end up short of it and their extraction under-performing.
The extraction can only work if its ductwork is clear and its make-up air is balanced - we clean and assess kitchen ventilation as the balanced system it is. Ask us to look at your extraction and its air supply.
