Part F ventilation rates: extractor fans and whole dwelling rate

The whole dwelling ventilation rate is the minimum fresh air supply for a home, in litres per second. Approved Document F sets it at the higher of a figure by bedrooms (31 l/s for three) and 0.3 l/s per m² of floor area, alongside extract rates of 15 l/s for a bathroom fan.

What Approved Document F asks for

Approved Document F, Volume 1: Dwellings (2021 edition) gives the ventilation guidance for homes in England and took effect on 15 June 2022. It asks for three things: extract ventilation in the kitchen, utility room, bathrooms and WCs; a whole dwelling ventilation rate that supplies fresh air to the habitable rooms; and purge ventilation, usually opening windows, in each habitable room.

Rates are in litres per second. Multiply by 3.6 to compare with a fan sold in m³/h: 31 l/s is 111.6 m³/h.

Whole dwelling ventilation rate by bedrooms and floor area

Minimum whole dwelling rate from Approved Document F: the higher of the Table 1.3 figure for the number of bedrooms and 0.3 l/s for each m² of internal floor area, all storeys added together. Example: a three-bedroom house of 90 m² needs 31 l/s, but at 125 m² the floor area rule gives 37.5 l/s.

The whole dwelling rate: bedrooms or floor area

The minimum whole dwelling rate must satisfy two conditions, so you take the higher. Table 1.3 gives 19 l/s for one bedroom, 25 for two, 31 for three, 37 for four and 43 for five, plus 6 l/s for each extra bedroom. Paragraph 1.24 adds a floor area rule: 0.3 l/s for each m² of internal floor area, counting every storey.

Example: a three-bedroom house of 90 m² needs the higher of 31 l/s and 0.3 × 90 = 27 l/s, so 31 l/s. A four-bedroom house of 140 m² needs the higher of 37 and 42 l/s: 42 l/s, because the floor area now decides. A home with a single habitable room uses 13 l/s.

Extract rates room by room

Table 1.1 sets the minimum rates for intermittent extract fans, the usual choice when you replace a fan:

• Kitchen: 30 l/s with a cooker hood extracting to the outside, 60 l/s without one, or with a recirculating hood, which on its own does not count as ventilation.
• Utility room: 30 l/s.
• Bathroom: 15 l/s.
• WC (sanitary accommodation): 6 l/s, or the purge ventilation guidance instead.

For a continuous mechanical extract system, Table 1.2 gives the high rates: kitchen 13 l/s, utility room 8, bathroom 8 and WC 6. The continuous rates of all the extract points together should be at least the whole dwelling rate. In the three-bedroom example with one bathroom and one WC, the high rates add up to 13 + 8 + 6 = 27 l/s, but the continuous total still has to reach 31 l/s.

Special cases

• Large homes: above five bedrooms, add 6 l/s per bedroom, so six bedrooms give 49 l/s before the floor area check.
• Two-storey houses: add the ground floor and first floor areas for the 0.3 l/s per m² rule.
• Fan position: extract terminals and fans, other than cooker hoods, should be as high as practicable and no more than 400 mm below the ceiling.
• Air paths: internal doors should allow air through, with the free area of a 10 mm undercut in a 760 mm wide door.
• Wales and Scotland have their own guidance, not covered here.

Common mistakes

• Fitting a 60 l/s kitchen rate when a cooker hood already extracts outside: 30 l/s is enough in that case, and the reverse mistake is more common.
• Counting a recirculating cooker hood as extraction.
• Taking the bedroom figure without checking the floor area rule in a large home.
• Reading a fan box in m³/h as if it were l/s: 15 l/s is 54 m³/h.

A new fan needs a suitable supply, and better insulation, worked out with the insulation thickness calculator, makes ventilation even more important. Measure each room with the room area calculator, and estimate what the fans cost to run with the electricity usage calculator. If you redecorate the bathroom once the new fan is in, the paint calculator gives the quantity.