Gas ventilation calculator
Work out the permanent ventilation a domestic gas appliance needs, in cm² of free area, and how many airbricks or core vents that comes to. Five situations, from an open-flued boiler in a cupboard to a flueless cooker.
- Works out
- Free area in cm², high and low where needed
- Covers
- Open-flued, room-sealed and flueless
- Then
- How many airbricks or core vents to fit
- Room size
- Measured, or the volume if you know it
Gas ventilation
Work out the permanent ventilation a domestic gas appliance needs, and what to fit.
Room volume 33.6 m³
Splits the requirement between them.
Required free area
25cm²
What to fit
9 × 3 airbrick is 229 × 76 mm. Free area is not the size of the opening: a 9 × 3 airbrick is 174 cm² of brickwork and roughly a quarter of that in free area. These are typical figures; use the free area printed on the product you actually fit.
Required free area
25cm²
What to fit
9 × 3 airbrick is 229 × 76 mm. Free area is not the size of the opening: a 9 × 3 airbrick is 174 cm² of brickwork and roughly a quarter of that in free area. These are typical figures; use the free area printed on the product you actually fit.
- Guidance only. Follow Gas Safe procedures and the manufacturer's instructions.
- Your figures stay on this device. They're kept as you type, and Save keeps a check by name.
- The same calculator engineers use in OnPoint's Tools.
01What it works out
Enough air, in the right places.
A gas appliance needs air to burn safely, and one in a cupboard needs air to stay cool. The calculator works out the permanent ventilation the appliance and its room or compartment call for, as free area in cm².
Then pick the vent you're fitting, from a 9 × 3 airbrick to a 125 mm core vent, and it tells you how many it takes. For a compartment, the high and low vents are worked out separately.
When you'd use it
- An open-flued applianceA back boiler, open-flued fire or heater takes its air from the room, so the room needs air from outside.
- A boiler in a cupboardThe compartment needs vents at high and low level, even for a room-sealed boiler, to keep it cool.
- Flueless appliancesFor a flueless cooker or water heater, the size of the room decides whether a vent is needed.
- Gas safety checksThe gas safety record asks whether the ventilation is satisfactory for each appliance.
02The rules
Five situations, five different sums.
Pick the wrong situation and the answer is wrong, however carefully you measure. These are the exact rules the calculator applies, using the appliance's net heat input from the data plate.
01Open-flued appliance in a room
free area = 5 cm² × (kW − 7) with adventitious air
free area = 5 cm² × kW without it
at least 100 cm² under 5 m³, 50 cm² under 10 m³
Normal gaps around doors and floors let in some air, so the first 7 kW needs no vent. Don't count on it in a sealed, draught-proofed room. A very small room needs a vent whatever the appliance.
02Open-flued appliance in a compartment
to outside: high 5 cm² × kW, low 10 cm² × kW
from the room: high 10 cm² × kW, low 20 cm² × kW
The low vent brings in air to burn and the high one lets warm air out, so they're different sizes. Taking the air from inside the building doubles both.
03Room-sealed appliance in a compartment
to outside: high 5 cm² × kW, low 5 cm² × kW
from the room: high 10 cm² × kW, low 10 cm² × kW
The boiler takes its combustion air through the flue, so the vents are only there to keep the compartment cool. High and low are the same size.
04Flueless cooker
under 5 m³: 100 cm²
5 to 10 m³: 50 cm², or none with a door straight to outside
over 10 m³: none
The cooker's size doesn't come into it, only the room's. An openable window, or something that does the same job, is needed whatever the vent.
05Flueless water heater, up to 11 kW net
under 5 m³: not covered
5 to 10 m³: 100 cm²
10 to 20 m³: 50 cm²
over 20 m³: none
Above 11 kW, or in a room under 5 m³, the calculator says the check doesn't cover it. An openable window is needed here too.
From cm² to vents
vents = free area needed ÷ free area of one vent, rounded up
Free area is not the size of the hole. A 9 × 3 airbrick is 174 cm² of brickwork and roughly a quarter of that in free area. These are typical figures: use the free area printed on the product you actually fit.
- 9 × 3 airbrick · 229 × 76 mm44 cm²
- 9 × 6 airbrick · 229 × 152 mm88 cm²
- 9 × 9 airbrick · 229 × 229 mm132 cm²
- 100 mm core vent · 100 mm bore40 cm²
- 125 mm core vent · 125 mm bore65 cm²
03A worked example
An open-flued boiler in a cupboard.
A 20 kW open-flued boiler in an airing cupboard, with vents going straight through the outside wall. It needs 100 cm² at high level and 200 cm² at low level, not one figure split in two.
The same 20 kW boiler, set up differently
- CompartmentHighLow
- Open-flued, vents to outside100200
- Open-flued, air from the room200400
- Room-sealed, vents to outside100100
- Room-sealed, air from the room200200
cm² of free area.
Ventilation check
Boiler in the airing cupboard
Open-flued · 20 kW net · vents straight through the outside wall
- 1High level100 cm²5 cm² × 20 kW
- 2Low level200 cm²10 cm² × 20 kW
- 3In 9 × 9 airbricks, 132 cm² each3 vents1 high (132 cm²), 2 low (264 cm²)
Compartment requires permanent high and low level ventilation.
04Reading the result
What the result is telling you.
Under the figure, the calculator says what it means in a sentence. A figure bigger than you expected is usually the air coming from inside rather than outside, or adventitious air switched off.
Guidance only. The calculator covers the common domestic cases, not every appliance-specific exception. Confirm the manufacturer's instructions and the appliance category, check whether an existing vent is blocked before you size a new one, and follow Gas Safe procedures. In the UK, gas work must be done by a Gas Safe registered engineer.
- No ventNo purpose-provided vent required on this simplified check.
The appliance is small enough, and the room big enough, that the air getting in is enough.
- Vent neededPurpose-provided ventilation required to outside air.
Fit at least the free area shown, to outside air. Split it between vents if you set more than one.
- Vent neededCompartment requires permanent high and low level ventilation.
Fit both, each to its own figure. The picture shows which goes where.
- Window tooOpenable window still required alongside any permanent vent.
For flueless appliances, a permanent vent doesn't replace a window you can open.
- CheckOpenable window or equivalent purge ventilation is required.
There's no openable window, and a flueless appliance needs one.
- Not coveredThis simplified flueless water-heater check only covers appliances up to 11 kW net.
Outside what this check covers. Go to the manufacturer's instructions and the standard.
More free calculators.
All free, with no account, and the same ones engineers use inside OnPoint.
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- Every calculator in OnPointThese four, plus the tightness test, IGEM/UP/1 testing and unsafe situations walk-throughs in the app.
Questions people ask.
Spotted something wrong in a sum? Ring +44 7884 697405 or email us.
How do you work out ventilation for a gas appliance?
Start with the situation: an open-flued appliance in a room or a compartment, a room-sealed appliance in a compartment, a flueless cooker or a flueless water heater. That decides the sum. Then use the appliance's net heat input from the data plate, the size of the room, and whether the air comes from outside or from inside the building.
Does a boiler in a cupboard need ventilation?
Yes. An open-flued boiler in a compartment needs 5 cm² per kW at high level and 10 cm² per kW at low level when the vents go straight outside. A room-sealed boiler in a compartment still needs vents to keep it cool: 5 cm² per kW at high level and the same at low level. Vents that take air from inside the building need twice as much.
Does a room-sealed boiler need an air vent?
Not for combustion: it takes its air from outside through the flue. In a room, that usually means no vent. In a cupboard or compartment it still needs vents at high and low level to keep it cool, and the calculator works those out.
What is free area?
The part of a vent that's actually open to the air, once the grille or louvres are taken off. It's much less than the size of the hole: a 9 × 3 airbrick is 174 cm² of brickwork and roughly a quarter of that in free area. Use the free area printed on the product you fit.
What is adventitious air?
The air that gets in through normal gaps around doors, windows and floors. For an open-flued appliance in a room, the calculator lets it cover the first 7 kW when you say it's there. Don't count on it in a sealed, draught-proofed room.
Does a gas cooker need an air vent?
It depends on the room. In a room under 5 m³ the calculator asks for 100 cm². From 5 to 10 m³ it's 50 cm², unless there's a door straight to outside. Over 10 m³ no permanent vent is needed, but the room still needs a window you can open.
How many airbricks do I need?
Divide the free area you need by the free area of one airbrick and round up. A 9 × 3 airbrick gives about 44 cm², so 100 cm² takes 3. The calculator does this for you once you pick the vent you're fitting.
Is the gas ventilation calculator free?
Yes. There's no sign-up and no limit. Your figures are kept on your device as you type, and you can save each check with a name. Nothing you enter is sent to OnPoint.
Then put it on the gas safety record.
OnPoint has 18 gas certificate types, from the landlord gas safety record to commissioning and service records. They're filled in on the phone, checked as you go, and signed and sent from the job.
- 14 days free, no card needed
- £19 a month for you, £9 for each person you add
- Every feature, no contract
Or pay yearly: £228 a year for you and £108 for each extra person.
More about certificates in OnPoint, or see pricing.