G4/AS1 Ventilation
A household unit room by room: what each room needs to be ventilated, the extract its fixtures call for, and the trickle ventilators the unit needs, read from G4/AS1 Part 2.
- G4/AS1Fifth Edition28 Jul 2025
The unit
The unit's type sets how many people it is taken to hold, which is what sizes its trickle ventilators. Whether it has only one external wall changes most of what follows.
Household unit type
The unit
- Occupants
- 3
- Table 2.2.2.5A
Rooms
Add each room. What it needs depends on what it is, whether it has an external wall, and what is in it.
- Openable
- 1m²
- 2.2.1.2
- Extract
- –
- 2.2.1.5
Net openable area
At least 1 m², 5% of 20 m².
2.2.1.2
- Openable
- 0.6m²
- 2.2.1.2
- Extract
- 50L/s
- 2.2.1.5
Net openable area
At least 0.6 m², 5% of 12 m².
2.2.1.2
Extract flowrate
No mechanical extract is required of this Kitchen by G4/AS1: it has an external wall, so 2.1.1.3 does not send it to Section 2.3, and 2.2.1.5 names only cooktops, showers and baths.
2.1.1.3, 2.2.1.5
- Openable
- 0.25m²
- 2.2.1.2
- Extract
- 25L/s
- 2.2.1.5
Net openable area
At least 0.25 m², 5% of 5 m².
2.2.1.2
Extract flowrate
No mechanical extract is required of this Bathroom by G4/AS1: it has an external wall, so 2.1.1.3 does not send it to Section 2.3, and 2.2.1.5 names only cooktops, showers and baths.
2.1.1.3, 2.2.1.5
- Openable
- 0.6m²
- 2.2.1.2
- Extract
- –
- 2.2.1.5
Net openable area
At least 0.6 m², 5% of 12 m².
2.2.1.2
- Openable
- 0.6m²
- 2.2.1.2
- Extract
- –
- 2.2.1.5
Net openable area
At least 0.6 m², 5% of 12 m².
2.2.1.2
Gas-fuel appliances
Part 3 is a space with gas appliances in it: the permanent vents a non room-sealed appliance needs on top of whatever Part 2 already asks of the space, or the space's mechanical ventilation.
Gas appliance flues
Where on the dwelling a gas appliance's flue may end, whether the appliance is room-sealed or not: a water heater on an outside wall as much as a fire.
G4 requirements
The requirements the unit and its rooms carry, numbered so one can be pasted onto a sheet as it stands. A requirement several rooms share is written once and names them. The four questions about the unit are answered here or on the Tool face, and the answers print. The values are on the Tool face; the reading and the guidance do not print.
The unit
Not applicable: where the household unit has only one external wall with opening windows2.2.2.1
Not applicable: where the building contains a Type 5 fire alarm system2.1.1.4
Rooms
- Net openable area Windows or other openings to the outside with a net openable area of no less than 5% of the floor area. Applies to Living or dining 1, Kitchen 1, Bathroom 1, Bedroom 1, Bedroom 2.
2.2.1.2
- Openable elements Openable building elements must be able to remain fixed in the open position as a means of ventilation during normal occupancy. Applies to Living or dining 1, Kitchen 1, Bathroom 1, Bedroom 1, Bedroom 2.
2.2.1.3
- Cooktop extract At least 50 L/s, including the ducting. Applies to Kitchen 1.
2.2.1.5 b), 2.2.2.3 b)
- Intermittent extract at a cooktop Where the extract serving a cooktop is intermittent rather than continuous, it exhausts to the outside at no less than 50 L/s. Applies to Kitchen 1.
2.3.1.1 c) iii)
- Shower and bath extract At least 25 L/s, including the ducting. Applies to Bathroom 1.
2.2.1.5 a), 2.2.2.3 a)
Gas-fuel appliances
Not applicable: where a space contains a gas-fuel appliance working under natural draught3.1.1.1
Gas appliance flues
Not applicable: where a gas appliance's flue terminates on the dwelling3.3.1.1
G4 requirements to show on the drawings
What has to be drawn or named for each requirement and value to be checkable. It follows the unit's rooms and the answers on Requirements. Tick as you go; the ticks print.
Not applicable: 2.2.2.1; 2.1.1.4; 3.1.1.1; 3.3.1.1
Show on the drawings
- 2.2.1.2, 2.2.2.2 a), 2.4.2.1 b)
- 2.2.1.3
- 2.1.1.3, 2.2.1.5, 2.3.1.1 c) ii), 2.3.1.1 c) iii)
Terminology
The words the notes and the values lean on, defined where the document gives them their meaning.
- OccupantsTable 2.2.2.5A, 2.2.2.5 g)
- How many people a household unit is taken to hold, read off Table 2.2.2.5A by the unit's type: a studio, one bedroom, two, or three people plus one for every bedroom after the second. What sizes trickle ventilators, in every space fitted with them.
- A unit with one external wall2.2.2.1, 2.2.2.2
- A household unit with only one external wall that has opening windows. Subsection 2.2.2 is written for it and reaches no other unit: the depth limit, the trickle ventilators and the route through another room are all its.
- Small space2.2.1.1
- A hallway or lobby in a household unit. 2.2.1.1 asks no specific ventilation of it, provided natural ventilation is available to it through the spaces beside it.
- Ventilated through another room2.2.2.4, 2.2.2.4 COMMENT
- 2.2.2.4's route for a habitable room in a one-wall unit that has no opening of its own: it takes its air through an adjoining habitable room, with a permanent opening between the two, both sized on their combined floor area. Never through a kitchen, bathroom, toilet or laundry.
- Combined ventilation2.4.2.1
- 2.4.2.1's arrangement for a habitable room in a one-wall unit with a permanent opening to a wet room served by the unit's mechanical extract: the 5% openable area and high-level trickle ventilators both, with the depth limit the extract system sets.
- Non room-sealed appliance3.1.1.1 b)
- A gas-fuel appliance that draws its combustion air from the space it is in. Part 3 sizes permanent vents for it once the space holds more than 1 kW of gas input for each m³; a room-sealed appliance draws its air from outside and is not in Part 3.
Reading G4
Why the document says what it says, and the readings this tool has had to make of it. Screen only; none of it is a note.
The unit
- Total equivalent aerodynamic area2.2.2.1
- Trickle ventilators are asked for by Subsection 2.2.2, which is about units with one external wall. This unit has more, so 2.2.2 does not reach it and no room takes a trickle area.
Where the rest is
What this page reads, and what it points at rather than reads.