3.14.6 Trickle ventilators
A trickle ventilator, sometimes called ‘background ventilation’, is a small ventilation opening, normally provided with a controllable shutter. Although routinely provided in the head of a window frame this is often not the best location as the free movement of air can be restricted, for example by curtains or blinds. They should be provided in naturally ventilated areas to allow fine control of air movement. The location of trickle ventilators should be carefully considered so that they are capable of providing the intended ventilation, taking into account factors such as the size and shape of the room and availability of external walls. A permanent ventilator is not recommended since occupants like control over their environment and uncontrollable ventilators are usually permanently sealed up to prevent draughts.

Trickle vent efficiency – it is recognised that the air flow performance through trickle ventilators can vary, dependent on the design and arrangement of air routes through the ventilator. For the purpose of performance, the recommended areas in the table to clause 3.14.3 should be achieved by the use of ventilators that are sized by the equivalent area, as determined using BS EN 13141-1:2004. When determining the equivalent area, the whole ventilator installation, including the external grille or canopy, should be considered as a single unit.

Where the trickle ventilator has to be ducted, e.g. to an internal room, the equivalent area of the trickle ventilator should be increased to double that shown in the table to clause 3.14.3, to compensate for the reduced air flow caused by friction. This may over-provide ventilation in some cases but can be regulated by the fine control.

Alternatives to proprietary trickle ventilators – fitting proprietary trickle ventilators is the preferred method of fine tuning room ventilation. However in some cases it may be acceptable for background ventilation to be provided through small windows, such as top hoppers, but other issues need to be considered if this method is to be adopted:

A partially open window on a night latch is a possible point of forced entry to a dwelling even when the window is locked in position and because of this it is less likely to be left open at night or when the dwelling is empty, even for short periods. Small, upper floor windows in a well lit, open location that are difficult to access may be appropriate it tends to be windier where flats and maisonettes are at high level and windows on night latches do not have fine adjustment to reduce draughts. They are therefore more likely to be kept closed manufacturers will need to show that the opening area when on the night latch is to the recommended sizes in the table to clause 3.14.3.

Some windows might be too small to incorporate the recommended size of trickle ventilator in the frame and careful thought will need to be given to the design and location of trickle ventilators in the window itself or additional trickle ventilators through the external wall trickle ventilators supply replacement air for mechanical extract and passive stack ventilation systems and routes for extracting air provided by mechanical input air systems. It is recommended that proprietary trickle ventilators are used in rooms where such systems are installed since it is more likely that they, rather than windows, will be left open.

Location of trickle ventilators – should be positioned to encourage movement of air within the dwelling and reduce stratification. To assist air movement consideration should be given to providing two or more trickle ventilators within rooms, installed at different heights.

Although ventilation should normally be to the external air, a trickle or permanent ventilator serving a bathroom or shower room may open into an area that does not generate moisture, such as a bedroom or hallway, provided the area is fitted with a trickle ventilator in accordance with the guidance in clause 3.14.3. In these cases, noise transmission may need to be limited, see Section 5.

A trickle ventilator should be provided in an area fitted with mechanical extraction to provide replacement air and ensure efficient operation when doors are closed. This will prevent moist air being pulled from other ‘wet areas’. The trickle ventilator should be independent of the mechanical extract so that replacement air can be provided when the extract fan is operating. Consideration should be given to the location of the ventilator and the fan so as to prevent short-circuiting of the air.

To assist air movement within dwellings with an air infiltration rate of less than 10m3/hr/m2@ 50 Pa, trickle ventilation to rooms with dMEVs could be formed by “undercutting” the room door to achieve an air space of at least 8,000mm2. This air space should be clear of any actual or notional floor coverings.