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Duct cleaning - airflow and balance
When a kitchen's extraction seems to be under-performing - struggling to clear the air, doors hard to open, the system straining - it is natural to blame the extraction. But often the real problem is the other side of the balance: the make-up air. If not enough air comes in to replace what is extracted, the extraction cannot work properly, however good it is. Here is when make-up air is the real problem.
The short answer
A kitchen ventilation system is a balance: the air the extraction removes has to be replaced by make-up air coming in. If there is not enough make-up air to replace what is extracted, the kitchen goes into negative pressure - and this starves the extraction, so it cannot move the air it should, however capable the extraction itself is. The symptoms look like an extraction fault: the extraction seems weak, the air is not cleared, doors are hard to open (pulled shut by the negative pressure), and the system strains. But the real problem is the make-up air, not the extraction. Without enough air coming in, the extraction has nothing to draw and works against a vacuum. So when extraction under-performs, it is worth checking whether the make-up air is adequate before concluding the extraction is at fault - because often the extraction is fine, and the real problem is too little make-up air to let it work. The fix is to restore the balance: enough make-up air to replace what is extracted.
Ventilation is a balance
A kitchen ventilation system is fundamentally a balance between air going out and air coming in. The extraction removes air - the grease-laden, hot, humid air from the cooking - and pushes it out of the building. But that air has to be replaced: for every volume of air the extraction removes, an equal volume has to come into the kitchen from somewhere, or the kitchen would run out of air. The air that comes in to replace what is extracted is the make-up air, and it is meant to be supplied deliberately - through a supply air system, or through adequate openings - to balance what the extraction takes out.
This balance is essential, because the two sides are linked: the extraction can only remove as much air as is replaced. If plenty of make-up air comes in, the extraction can freely remove air and the system moves the air it should. But if too little make-up air comes in, the extraction is starved - it cannot remove air faster than the air is replaced, so it cannot achieve its intended airflow, no matter how powerful it is. So ventilation is not just about the extraction; it is about the balance of extraction and make-up air. Understanding this is the key to diagnosing ventilation problems, because a shortfall on either side shows as a problem - and a shortfall of make-up air shows as an extraction problem, even though the extraction is not at fault.
What too little make-up air does
When there is not enough make-up air to replace what the extraction is trying to remove, the kitchen goes into negative pressure - a partial vacuum, where the air pressure inside is lower than outside because more air is being taken out than let in. This negative pressure produces a set of characteristic signs that are often mistaken for an extraction fault. The extraction seems weak, because it cannot move the air it should against the vacuum. Doors become hard to open, or slam shut, because the negative pressure pulls them closed - a classic sign of a kitchen starved of make-up air. Air may be pulled in through any gap it can find, sometimes with whistling or draughts.
These signs point to negative pressure, which points to a make-up air shortfall - but they are easily read as the extraction failing, because the visible symptom is that the extraction is not clearing the air. That misreading is the crux of the problem: a kitchen with too little make-up air looks like it has weak extraction, so the extraction gets blamed and perhaps investigated, when the actual cause is the air not coming in. The negative pressure and its signs - the strained extraction, the hard-to-open doors, the draughts - are the tell-tale of a make-up air problem. Recognising them for what they are is what lets you look in the right place: not at the extraction, which may be fine, but at the make-up air, which is the real shortfall.
Why it starves the extraction
The reason too little make-up air makes the extraction under-perform is straightforward once the balance is understood: the extraction can only remove air as fast as it is replaced. If the make-up air coming in is less than the extraction is designed to remove, the extraction is limited by the make-up air - it cannot pull out more air than is coming in to replace it, so its actual airflow drops to match the limited make-up air, well below its capacity. The extraction fan may be running at full power, capable of much more, but it is throttled by the lack of air to draw. So the extraction under-performs not because it is faulty but because it is starved.
This is why the extraction can be in perfect order and still seem weak: its performance is capped by the make-up air, not by its own capability. Investigating the extraction in this situation finds nothing wrong with it, because there is nothing wrong with it - the problem is upstream, in the air supply. This is the important insight: a make-up air shortfall does not just accompany extraction problems, it causes what looks exactly like an extraction problem, by starving a healthy extraction. So when extraction under-performs, the make-up air has to be considered as a possible - often the actual - cause, because the extraction being starved of make-up air produces the same symptoms as the extraction being faulty. Ruling the make-up air in or out is essential to diagnosing the real problem rather than chasing a fault in an extraction that is working fine.
Diagnosing and fixing it
So when a kitchen's extraction is under-performing, the sensible diagnosis considers the whole balance, not just the extraction - and specifically checks whether the make-up air is adequate before concluding the extraction is at fault. The signs of negative pressure - hard-to-open doors, draughts pulled through gaps, the extraction straining - point toward a make-up air shortfall. Where these are present, the make-up air is the thing to investigate: is there enough air coming in to replace what the extraction removes, through the supply system or adequate openings? If not, the make-up air is the real problem, and no amount of attention to the extraction will fix it.
Fixing it means restoring the balance: providing enough make-up air to replace what is extracted, so the extraction can work freely and the negative pressure is relieved. This might mean a make-up air supply system, or ensuring adequate provision for air to come in, matched to the extraction rate. Once the balance is restored, the extraction - which was fine all along - performs as it should, because it now has the air to draw. So the practical lesson is to diagnose ventilation problems as balance problems: when the extraction under-performs, check the make-up air, because it is often the real cause. This does not mean the extraction is never at fault - it can be, and a genuinely blocked or failing extraction is a real problem too - but it means the make-up air must be considered, because a make-up air shortfall masquerades as an extraction fault. Getting the diagnosis right - extraction fault, or make-up air shortfall - is what leads to the right fix.
The takeaway
A kitchen ventilation system is a balance: the air the extraction removes has to be replaced by make-up air coming in. If there is not enough make-up air, the kitchen goes into negative pressure, which starves the extraction - it can only remove air as fast as it is replaced - so the extraction under-performs however capable it is. The signs look like an extraction fault: the extraction seems weak, doors are hard to open (pulled shut by the negative pressure), and the system strains. But the real problem is the make-up air, not the extraction, which may be in perfect order and simply starved.
So when extraction under-performs, it is worth checking the make-up air before concluding the extraction is at fault - because often the extraction is fine and the real shortfall is the air coming in. The signs of negative pressure point the way: hard-to-open doors, draughts, a straining extraction. Fixing it means restoring the balance - enough make-up air to replace what is extracted - after which the extraction performs as it should. This does not mean the extraction is never the problem, but it means the make-up air must be considered, because a make-up air shortfall masquerades as an extraction fault. Diagnose the balance, not just the extraction, and you look in the right place - which is often at the air coming in, not the air going out.
Questions
The air that comes into a kitchen to replace the air the extraction removes. A ventilation system is a balance - for every volume the extraction takes out, an equal volume has to come in, supplied deliberately through a supply air system or adequate openings. If too little make-up air comes in, the extraction cannot work properly, because it can only remove air as fast as it is replaced.
Because too little make-up air starves the extraction, so it under-performs - and the visible symptom is the extraction not clearing the air, which looks like the extraction failing. The kitchen goes into negative pressure, making doors hard to open and pulling draughts through gaps, while the extraction strains. These signs are easily blamed on the extraction, when the real cause is the air not coming in.
A partial vacuum where the air pressure inside is lower than outside, because more air is being extracted than is being replaced by make-up air. It produces characteristic signs: doors hard to open or slamming shut (pulled closed by the pressure difference), draughts whistling through gaps, and a straining extraction that cannot move the air it should. These signs point to a make-up air shortfall.
Because the extraction can only remove air as fast as it is replaced. If the make-up air is less than the extraction is designed to remove, the extraction is throttled by the lack of air to draw - its actual airflow drops to match the limited make-up air, well below its capacity. The fan may be at full power, capable of much more, but it is starved of air to move.
Look for the signs of negative pressure: doors hard to open, draughts pulled through gaps, a straining extraction. Where these are present, check whether there is enough air coming in to replace what the extraction removes. If not, the make-up air is the real problem - and investigating the extraction will find nothing wrong, because there is nothing wrong with it; it is simply starved.
By restoring the balance - providing enough make-up air to replace what is extracted, so the extraction can work freely and the negative pressure is relieved. This might mean a make-up air supply system or ensuring adequate provision for air to come in, matched to the extraction rate. Once the balance is restored, the extraction performs as it should, because it now has the air to draw.
When extraction under-performs, it can be a make-up air shortfall - or genuinely blocked, grease-laden ductwork. Our cleaning rules out and removes the duct build-up, so you can diagnose the real cause.
