In this article
An air balance is the simplest sum in your building: everything your kitchen blows out has to come back in somewhere. If your hood pulls 4,800 m³ an hour, then 4,800 m³ an hour arrives — either through a supply duct built for the job, or through your front door, your dining room and every gap the building has.
Four complaints that come up in almost every independent restaurant share one cause. There is a draught at table 2 and nobody knows why. In winter the front door is heavy to pull open and slams when you let go. On busy evenings the dining room smells of the kitchen. And your gas bill is higher than the place two doors down, though you cook much the same food. That is not a collection of irritations. That is one design error, showing up four times.
The hood over your line was sized: somebody looked at what stands underneath it and found an airflow to match. But the second number — how much air has to come back in on purpose — appears nowhere on the drawings of most independent venues. And without that number your dining room is the air supply. Literally: the hood pulls, and the only way in runs past your guests.
This guide follows one cubic metre of air from the hood to your bank account, in five stops. At the bottom you put your own kitchen into the calculator: which appliances sit under the hood, how big your kitchen and dining room are, how many hours the hood runs, and how much supply air you have today. You get your required airflow, how much air is dragged through the room, and what that air costs you a year. Everything is worked out in your own browser: nothing is sent anywhere and nothing is stored.
The half nobody calculated
A hood does not make air. It moves air, and moving is always a two-way story: every cubic metre that leaves through your roof is, at that same moment, pulled in somewhere else. That is not a rule of thumb, it is conservation of mass, and there is no third option.
If no supply system does that work, the building does it itself. The negative pressure that builds up pulls air through window gaps, under doors and — the moment a door opens — straight across your whole dining room in one movement. Hence the draught by the window, hence the heavy front door. And hence the smell: once the building sits under negative pressure, air stops running politely from your room into your kitchen and takes the shortest path instead, which is not always yours.
Then the money. That air arrives at outdoor temperature and has to reach indoor temperature, every running hour of every year. With a hood at 4,800 m³/h running ten hours a day, one heating season moves more air through your building than you will ever win back with better insulation or LED lighting. It is usually the largest unknown line on a restaurant's energy bill, and the only one that has no line of its own anywhere in the books.
Five stops on the path of one cubic metre of air
Each number below follows from the one before it. Stop 1 sets stop 2, stop 2 sets stop 3, and money only enters the story at stop 4. Do not skip one: it is precisely the jump from stop 1 straight to stop 4 — from the hood to the bill — that causes the misunderstanding this whole page is about.
1. What the hood has to pull — and why its own size is irrelevant
A hood's airflow does not follow from the hood. It follows from what stands underneath. Every appliance throws a plume of hot, greasy air upwards, and the hood has to catch that plume completely before it reaches the rim. A single-well fryer asks for roughly 900 m³ an hour. A four-burner range about 1,000. A ten-grid combi oven around 1,000 as well. A charcoal or lava-rock grill 1,500, because it is in a class of its own: its plume is hotter and dirtier than anything standing beside it.
Add up the line of an ordinary bistro — range, fryer, griddle, combi and a hood-type dishwasher — and you land around 4,800 m³ an hour. That is well over a cubic metre a second, every second you are open. If your hood hangs free in the room instead of against a wall, add roughly a quarter: a wall helps catch the plume, and without one the airflow has to do that work instead.
One floor always overrules that sum. A small kitchen with few appliances must not end up at 900 m³/h, because the room itself asks for roughly twenty air changes an hour. The calculator below therefore always takes the higher of the two. Size on appliances alone and you ventilate a four-by-eight-metre kitchen with the airflow of one fryer — and you find out in July.
2. What comes back on purpose — and why you do not want a hundred per cent
The second half of the design is called make-up air or supply air: a duct with its own fan that brings outdoor air to the kitchen to replace what the hood takes away. Zuluft in German, air compensé in French, korvausilma in Finnish, pótlevegő in Hungarian. Nearly every European language has its own trade term for it, which tells you how ordinary a component it is supposed to be.
The instinctive reaction is to set that supply to a hundred per cent and be done with it. That is exactly the mistake not to make. A kitchen should sit at slightly negative pressure relative to your dining room, so air always runs from the room into the kitchen and never the other way. At a hundred per cent supply — or worse, above it — you push steam, grease and frying smells into your dining room, and you have bought a new problem with the money meant to fix the old one.
The target is therefore around 85 to 90 per cent mechanical supply, with the remaining ten to fifteen per cent coming from the room on purpose. That remainder is not sloppiness, it is the design: exactly enough negative pressure to keep the kitchen smell in, and far too little for anyone to feel a draught.
3. What gets dragged through your dining room
The difference between what the hood takes and what the supply brings back has no name on any drawing, and it is the only number in this article that says something real about your own building. Call it transfer air: the air your building has to find for itself.
Set it against the volume of your dining room and you get the figure a guest actually notices. A room of seventy square metres at three metres high holds 210 cubic metres of air. Drag 4,800 m³ an hour through it and the whole air volume of your dining room is replaced almost twenty-three times an hour — once every two and a half minutes, in January, with outdoor air at four degrees. Above roughly four times an hour a guest starts to feel a draught; below two it turns stuffy. Twenty-three is not ventilation any more, it is a dining-room temperature problem no radiator can win against.
The three bars below show the same kitchen at three levels of supply air. Watch what does not change: the bar is the same length every time. That is the whole point.
One bistro kitchen extracting 4,800 m³ an hour, at three levels of mechanical supply air. The bar is the same length every time — it is the same airflow. Only the split moves.
Nothing disappears and nothing is added: three bars of the same length, because it is three times the same airflow. What moves is where that air comes in. In the top bar your dining room is the supply duct; in the bottom one it is a dining room again, with just enough transfer air to keep the kitchen smell where it belongs.
4. What that air costs to condition — and why supply air changes none of it
Now the money. Heating air costs roughly 0.34 watt-hours per cubic metre per degree. That sounds like nothing until you multiply it: 4,800 m³ an hour, ten hours a day, a hundred and fifty heating days, twelve degrees between outside and inside. That is nearly 29,000 kilowatt-hours of heat a year, purely to replace the air your hood removes — and in summer cooling is added on top, which in southern Europe is the larger of the two.
And here is the number almost everybody gets wrong. That cost does not change when you install make-up air. What the building has to condition is all the air that enters, and that is by definition exactly as much as the hood removes — whether it arrives neatly through a duct or past your front door. A make-up air unit moves the problem to where it belongs. It does not solve it.
That makes supply air a comfort measure, not an energy measure, and this is not a small message: it is precisely the investment sold with a payback period that does not exist. Only two things genuinely lower the bill, and they are in the next stop. One nuance belongs here: a tempered supply that stops at eighteen degrees rather than twenty-one does shave something off. Something — not half.
5. What the fan itself costs — and the two levers that do work
Beside the heat there is a motor running. An extract fan at 4,800 m³ an hour draws roughly 2.4 kilowatts; at ten hours a day over 320 running days that is nearly 8,000 kilowatt-hours of electricity. Put a supply fan next to it and you add well over half of that again. Which is exactly why the middle column below is higher than the left one, not lower.
The two levers that do work are demand control and heat recovery. Demand control measures temperature or smoke under the hood and drops the airflow the moment nobody is cooking — which is by far the larger part of your opening hours. Thirty to forty per cent less air moved over a year is normal, and because a fan's power follows roughly the cube of its airflow, its electricity use falls harder still.
Heat recovery takes heat out of the extracted air and puts it into the supply. On kitchen extract that cannot be done with an ordinary plate exchanger — it blocks with grease inside a month — but with a coil behind the grease filters, linked by a water circuit to a second coil in the supply. Forty to sixty per cent is realistic, provided the cleaning schedule is followed; let that schedule slip and efficiency falls while the fire risk of grease in your ductwork rises. And notice how the two combine: thirty per cent less air plus fifty per cent recovery is not an eighty per cent saving but sixty-five. They multiply, they do not add.
The same kitchen, three times, per year. The lower block is heating and cooling the replacement air, the upper one the electricity for the fans.
Look at the lower block of the first two columns: it is exactly the same height. Supply air changes where the air comes in and not one cent of what it costs — and because a second fan joins the payroll, the bill actually goes up slightly. What you get for it is a dining room you can sit in by the window in January. The saving is in the third column, and those two measures only work with a supply: there is nothing to recover from air that arrives past your front door.
Work out your own air balance
Put your own line in below: how many of each appliance sit under the hood, how big your kitchen and dining room are, and how many hours a day the hood runs. The three sliders are your levers — the supply air you have today, and the two measures from stop 5.
While you drag them, watch one thing. Take the supply air from zero to a hundred and look at the amount in the right-hand tile: it does not move. The dining-room figure in the middle does move, and it goes from red to green. That difference is what this whole article is about.
Air balance calculator
Your airflow, what gets dragged through your dining room, and what that air costs a year. Everything is worked out in your own browser.
What stands under the hood?
Your kitchen, your room and your hours
Your three levers
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The per-appliance airflows are the rules of thumb a kitchen designer starts from; the final figure follows from the thermal load of your exact layout and from the standard your installer has to work to. The energy prices and the heating season are Irish averages. Use this to know whether there is a problem and roughly how big it is — not to order an installation with.
Two figures are worth writing down. The airflow in the first tile is the number every conversation with an installer starts from. And the middle figure — how often the air in your dining room is replaced each hour — is the number that explains why table 2 has been empty for three winters.
If you are sitting on zero supply air, the order is not 'save first, comfort later'. Heat recovery needs a supply duct to put the recovered heat into, and demand control only works properly when supply and extract come down together. So the comfort measure is also the measure that makes the two savings possible — that is the only correct order.
Three things that are nearly always wrong
- The hood was sized on itself, not on the line underneathA hood is ordered to the width of the suite, and the airflow follows from a table against that width. Put a charcoal grill or a second fryer underneath later and it no longer holds — and nothing in the building will tell you except a kitchen that keeps steaming and a maintenance contract that keeps getting dearer. Add your appliances up again every time the line changes.
- The supply air is switched off because it blows coldThis is the most common of the three and the most expensive. There is a supply unit, but it blows untempered outdoor air onto somebody who stands under it for a whole service, so one winter morning the switch went off and never came back. The result: you have paid for the investment and you have the imbalance back. The fix is not to leave it off but to move its discharge — low, away from the sections, or through a tempering coil.
- The hood runs flat out for the whole serviceBetween twelve and two there is cooking; at four o'clock one pot simmers and the same hood moves exactly the same volume. A manual two-speed switch already helps, but everybody forgets it — which is precisely why demand control exists: measure it under the hood and let the system do it itself.
Five questions for your installer
These are the five questions that make two quotations comparable. All five can be answered with one number or one sentence, and a supplier who starts hesitating at question two has already told you what you wanted to know.
- What extract airflow is this hood sized for, and against which list of appliances? Ask for the list, not just the number.
- How much mechanical supply air is in this proposal, expressed as a percentage of the extract? If the answer is zero, or 'it comes from the dining room', you know where you stand.
- Where and at what temperature does that supply discharge, and what happens on a morning at minus five?
- Can the system drop its airflow when nobody is cooking, and what does it control on — time, temperature or smoke?
- Is heat recovery possible on this extract, and how is the exchanger protected from grease and cleaned?
The number worth writing down
Of everything above there is one figure you can remember tonight: how often the air in your dining room is replaced each hour. Above four, something is wrong, and it is not your heating.
The reason this goes unnoticed for so long is that every consequence gets its own explanation. The draught is 'an old building'. The heavy door is 'the closer'. The smell is 'the kitchen door standing open'. The bill is 'energy prices'. All four have the same cause, and it is turning on your roof while you read this.
None of this has to become a refit tomorrow. Work out your balance, write the two numbers down, and put them next to the next quotation that lands on the table — whether it is for a hood, a boiler or air conditioning. A serious share of what you pay on quotations like that is about air nobody has ever counted.