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Duct cleaning - ventilation
If your kitchen extraction seems to struggle on windy days - pulling less, letting more heat and smoke linger - you are not imagining it. Wind acts on the point where the extraction discharges to outside, pushing back against the fan and making it harder to move the air out. It can act on the fresh-air side too. Here is why extraction works worse on windy days, and what can help.
The short answer
Kitchen extraction works worse on windy days because of what the wind does at the point where the extraction discharges to outside. The extraction fan pushes the air out through a duct to a discharge terminal - an outlet on the roof or an external wall - and normally the air leaves there freely. When it is windy, the wind blows against that discharge terminal, and if it is pushing towards the outlet, it raises the pressure there, creating backpressure that opposes the outflow of air from the fan. The fan now has to push its air out against that added pressure, which is harder - so a fixed-speed fan, which cannot simply work harder to compensate, moves less air than it does in calm conditions. The result is that the extraction pulls less: less heat, smoke, grease and moisture removed from the kitchen, so more of it lingers - exactly the struggling you notice on a windy day. The effect can be worse in gusts, because each gust raises the backpressure in a burst, so the extraction weakens and recovers as the wind rises and falls. Wind can also act on the make-up air side, disturbing the fresh-air intake and unbalancing the supply that the extraction depends on, which adds to the problem. What helps is partly in the design of the discharge - a wind-resistant terminal (one shaped to shed wind rather than catch it, or fitted with measures to resist backpressure) reduces how much the wind pushes back - and partly in the system's overall strength and condition: a well-designed system with margin, kept clean so it is not already weakened by grease-clogged ductwork, has more capacity to cope with the added resistance a windy day brings. So extraction works worse on windy days because wind at the discharge fights the fan, and it copes better when the discharge resists the wind and the system is strong and clean to begin with.
Key points
What the wind does
To understand why wind hurts extraction, start with where the extraction meets the outside: the discharge terminal, the outlet on the roof or an external wall where the extracted air is released. The fan pushes the hot, greasy air along the ductwork and out through this terminal, and in calm conditions the air leaves freely - the fan only has to overcome the resistance of the ductwork itself. Wind changes this. When it blows against the discharge terminal - particularly if it is pushing towards the outlet - it raises the air pressure at that point, so the air the fan is trying to push out now meets a higher pressure resisting it. This is backpressure: pressure at the discharge that opposes the outflow. The fan is no longer pushing into calm air but against a wind-raised pressure, which makes it harder to get the air out.
This is the core mechanism: wind at the discharge creates backpressure, and backpressure opposes the fan. The stronger the wind pushing against the terminal, the greater the backpressure, and the harder the fan has to work to discharge its air against it. Because the extraction depends on the fan being able to push the air out, anything that resists that outflow reduces the extraction - and wind, by raising the pressure at the discharge, does exactly that. So the reason extraction works worse on windy days is not mysterious: the wind is physically pushing back against the point where the air leaves, making it harder for the fan to expel the air, so less air is extracted. The kitchen end of the system (the canopy pulling air in) is unchanged; what has changed is the outside end, where the wind is now fighting the discharge. That fight is what weakens the extraction when the wind blows.
Why the fan cannot just push harder
The natural question is why the fan does not simply push harder to overcome the wind - and the answer is that a typical fixed-speed extraction fan cannot. A fixed-speed fan runs at one speed and delivers air according to the resistance it faces: the more resistance (or backpressure) it meets, the less air it moves, following its performance characteristic. It does not sense that the kitchen needs the same airflow and ramp up to maintain it; it simply moves less air when the resistance rises. So when wind adds backpressure at the discharge, the fixed-speed fan moves less air - it cannot compensate by working harder, because it has no mechanism to do so. This is why the extraction visibly weakens on a windy day: the fan is doing the same as always, but against more resistance, so its output falls.
This is also why the effect comes and goes with the wind, especially in gusty conditions. Wind is rarely steady; it rises and falls in gusts, and each gust raises the backpressure at the discharge in a burst, so the fan's output dips with the gust and recovers as it passes. The result is extraction that seems to fluctuate - weaker in the gusts, stronger in the lulls - tracking the wind rather than staying constant. To someone in the kitchen, this feels like the extraction struggling and then briefly recovering, in time with the wind outside. So the combination of a fixed-speed fan (which moves less air against more resistance and cannot compensate) and gusty wind (which raises the backpressure in bursts) explains both why the extraction weakens on windy days and why it does so unevenly, coming and going with the gusts. The fan is not faulty; it is simply responding to the changing resistance the wind imposes, in the only way a fixed-speed fan can - by moving less air when the wind pushes back.
The make-up air side too
Wind can also act on the other side of the ventilation - the make-up air intake - which adds to the problem. The extraction depends on make-up air, the fresh air supplied to replace what is extracted, drawn in through an intake somewhere outside. Just as wind affects the discharge, it can affect this intake: wind blowing across or against the intake can disturb the pressure there, sometimes reducing the fresh air drawn in, sometimes over-pressurising it, either way unbalancing the supply the extraction relies on. Since the extraction can only move air as fast as make-up air comes in, a wind-disturbed intake can weaken the extraction from the supply side, on top of the backpressure fighting it at the discharge. So on a windy day, both ends of the ventilation - the discharge and the intake - can be affected by the wind, compounding the effect.
The extent of this depends on how the intake and discharge are positioned and designed relative to the prevailing wind, which is why some kitchens suffer more than others on windy days. A discharge terminal exposed to the wind, or an intake badly placed relative to it, will feel the wind more; a well-positioned, well-designed system feels it less. There can also be interactions - wind that raises the discharge backpressure and disturbs the intake at the same time hits the system from both directions. The practical point is that the wind effect is not only about the discharge; the whole balanced ventilation system is exposed to the weather outside, and wind can act on both the air going out and the air coming in. This is worth knowing because it means a windy-day extraction problem is not always solved by looking at the discharge alone - the make-up air side may be part of it too, and a proper assessment considers both. Still, the discharge backpressure is usually the main mechanism, and the make-up air disturbance an additional factor on top.
What helps
Two things help extraction cope with wind: reducing how much the wind pushes back at the discharge, and having a system strong enough to handle the added resistance. On the discharge, the design of the terminal matters - a wind-resistant discharge terminal, shaped to shed the wind rather than catch it, or fitted with measures that resist backpressure, reduces how much the wind raises the pressure at the outlet, so the fan has less to fight. Where a kitchen consistently struggles on windy days, the discharge terminal is worth examining, because an exposed or poorly shaped one lets the wind push back more than a well-designed one would. Improving the discharge is the most direct way to reduce the wind's effect, because it addresses the point where the wind acts.
The other thing that helps is the system's overall strength and condition - because a windy day adds resistance, and a system with margin and in good condition has more capacity to absorb that. A well-designed extraction system with some margin over the minimum will lose some airflow to wind backpressure but still have enough to keep the kitchen reasonably clear; a system running at its limit even in calm conditions has nothing to spare, so the wind's added resistance tips it into visibly struggling. Condition matters here too: a system already weakened by grease-clogged ductwork is starting from a reduced airflow, so the wind's added resistance bites harder - the same wind that a clean, strong system shrugs off can leave a clogged, marginal one struggling badly. So keeping the ductwork clean, so the system delivers its full airflow, gives it the best chance of coping with a windy day, because it is not already handicapped before the wind adds its resistance. In short: resist the wind at the discharge, and keep the system strong and clean, so it has the capacity to cope when the wind pushes back.
The takeaway
Extraction works worse on windy days because of what the wind does at the discharge - the outlet where the extracted air leaves to outside. Wind blowing against the discharge terminal raises the pressure there, creating backpressure that opposes the fan's outflow, so the fan has to push against more resistance. A typical fixed-speed fan cannot compensate by working harder; it simply moves less air against the added resistance, so the extraction pulls less and more heat, smoke and moisture linger. Gusts make it uneven, weakening the extraction in bursts as the wind rises and falls. Wind can also disturb the make-up air intake, unbalancing the supply the extraction depends on, which adds to the effect.
What helps is partly the discharge design - a wind-resistant terminal, shaped to shed the wind or fitted to resist backpressure, reduces how much the wind pushes back - and partly the system's strength and condition. A well-designed system with margin, kept clean so it is not already weakened by grease-clogged ductwork, has the capacity to absorb the extra resistance a windy day brings, whereas a marginal or clogged system tips into struggling when the wind adds its load. So extraction works worse on windy days because wind at the discharge fights the fan, and it copes better when the discharge resists the wind and the system is strong and clean to begin with. If your kitchen struggles noticeably on windy days, the discharge terminal and the system's condition are the things to look at.
Questions
No - it is a real effect with a clear mechanism. Wind blowing against the discharge terminal, where the extracted air leaves to outside, raises the pressure there and creates backpressure that opposes the fan's outflow. A typical fixed-speed fan cannot work harder to compensate, so it moves less air against the added resistance, and the extraction pulls less - so heat, smoke and moisture linger more than on a calm day. The effect is often uneven, weakening in gusts and recovering in lulls, so it tracks the wind. So if your extraction seems worse on windy days, you are noticing a genuine physical effect, not imagining it - the wind really is making the fan's job harder at the discharge.
Because a typical extraction fan runs at a fixed speed and cannot ramp up to compensate. A fixed-speed fan delivers air according to the resistance it faces - the more resistance or backpressure, the less air it moves - following its performance characteristic, without any mechanism to sense that the kitchen needs the same airflow and work harder to maintain it. So when wind adds backpressure at the discharge, the fan simply moves less air; it cannot push harder to overcome the wind. This is why the extraction visibly weakens on a windy day even though the fan is doing exactly what it always does. Some systems use variable-speed or pressure-controlled fans that can adjust, but a standard fixed-speed fan responds to added resistance only by moving less air.
Because wind is gusty rather than constant. Wind rises and falls in gusts, and each gust raises the backpressure at the discharge in a burst, so the fan's output dips with the gust and recovers as it passes. The result is extraction that fluctuates - weaker in the gusts, stronger in the lulls - tracking the wind rather than staying constant. In the kitchen this feels like the extraction struggling and then briefly recovering, in time with the wind outside. So the coming and going is simply the fan responding to the changing backpressure as the wind gusts and lulls: more resistance in a gust means less air moved, less resistance in a lull means more, and the extraction rises and falls with the wind.
It can. The extraction depends on make-up air drawn in through an intake outside, and wind can disturb that intake just as it disturbs the discharge - blowing across or against it to reduce the fresh air drawn in, or over-pressurise it, either way unbalancing the supply the extraction relies on. Since the extraction can only move air as fast as make-up air comes in, a wind-disturbed intake can weaken the extraction from the supply side, on top of the backpressure at the discharge. How much depends on how the intake is positioned relative to the wind. So while the discharge backpressure is usually the main mechanism, the make-up air side can be an additional factor, which is why a proper assessment of a windy-day problem considers both ends of the system.
Two things help. First, the discharge terminal - a wind-resistant terminal, shaped to shed the wind rather than catch it, or fitted with measures to resist backpressure, reduces how much the wind raises the pressure at the outlet, so the fan has less to fight. If a kitchen consistently struggles on windy days, the discharge terminal is worth examining, because an exposed or poorly shaped one lets the wind push back more. Second, the system's strength and condition - a well-designed system with some margin, kept clean so it delivers its full airflow, has the capacity to absorb the added resistance a windy day brings, whereas a marginal or grease-clogged system tips into struggling. So resist the wind at the discharge, and keep the system strong and clean.
Yes. A system already weakened by grease-clogged ductwork is starting from a reduced airflow, with no margin to spare, so when the wind adds backpressure at the discharge, the extra resistance bites harder and the extraction struggles more visibly. The same wind that a clean, full-strength system would largely shrug off can leave a clogged, marginal system struggling badly, because the clogged system had nothing in reserve to absorb the added load. So keeping the ductwork clean, so the system delivers its full airflow, gives it the best chance of coping with a windy day - it faces the wind's added resistance from a position of full strength rather than already handicapped. Cleaning the ductwork will not stop the wind, but it ensures the wind is the only thing the extraction is fighting, not the wind plus its own grease.
A grease-clogged system struggles most when wind adds resistance at the discharge - we clean the ductwork so the extraction delivers its full airflow and has the margin to cope with a windy day. Ask us.
