Mechanical ventilation of rooms   Diagram 4 Provisions for background ventilation

The requirement will be satisfied for:

a. _ occupiable rooms, where mechanical ventilation is provided at the rate of not less than 8 litres/second of fresh air per occupant. This assumes that the provision is for no smoking, an appropriate provision for rooms designed for light smoking would be a fresh air supply of 16 litres/ second per occupant. For specific rooms designed ~or heavy smoking, such as rest rooms where smoking is permitted, see paragraph 2.7(e).

b. _ kitchens, bathrooms and sanitary accommodation without windows (eg internal rooms), where provision is made for mechanical extract ventilation rated as in Table 2 and the fan has a 15 minutes' overrun and is either controlled automatically or manually. For such rooms, it would be appropriate for the operation of the fans to be controlled by the light switch, or occupant detecting sensor. An air inlet to the room should be provided, eg a 10mm gap under the door.

2.4 _ For the design and commissioning of mechanical ventilation and air-conditioning plant, see paragraphs 2.11 to 2.17.

see para 2.2b

The following: are examples of provisions for background ventilation. In each case, the ventilation opening should be located (typically 1 .75m above floor level) so as to avoid discomfort due to cold draughts.
  1. Trickle ventilators
  _ a) In window frames

Ventilation of common spaces in non-domestic buildings

2.5 _ These provisions only apply to common spaces where large numbers of people are expected to gather, such as shopping malls and foyers. It does not apply to common spaces used solely or principally for circulation.

The requirement will be satisfied if there is provision to spaces where large numbers of people are expected to gather for either:

a. _ natural ventilation by appropriately located ventilation opening(s) with a total area of at least 1/50th of the floor area of the common space; or

b. _ mechanical ventilation designed to provide a supply of fresh air of one litre/second per m2 of floor area.

Alternative approaches

2.6 _ As an alternative to paragraphs 2.2 and 2.5 the requirement will be satisfied:

a. _ by following the relevant recommendations of BS 5925: 1991 Code of practice for ventilation principles and designing for natural ventilation. The relevant clauses are 5.1, 5.2, 6.1, 6.2, 7.3, 12 and 13; or

b. _ by following the relevant recommendations of CIBSE Guide A: Design data and CIBSE Guide B:
Installation and equipment data. The relevant sections are A4 Air infiltration and natural ventilation, B2 Ventilation and air-conditioning (requirements).

  _b) In glazed openings

_c) Above window frames

2. Airbrick with “hit and miss” ventilator


Notes to examples 1 and 2:
To minimise resistance to air flow, the main air passages (not insect screens or baffles, etc.) of an installed ventilation opening (when fully open) should have a smallest dimension of at least 5mm for slots or 8mm for square or circular holes.

3. _ Vertical sliding sash or top-hung windows

For background ventilation, the opening of the
window should be adjustable to provide the required area shown in Table 2.