Section 2 Natural ventilation and intermittent extract systems

  1. Section 2

    Section 2

    2.1 System 1 – Background ventilators and intermittent extract fans

    The information provided in Tables 1 and 2 gives guidance on good installation practice and on procedures for the inspection and air flow measurement testing of systems comprising background ventilators and intermittent extract fans (including cooker hoods) for dwellings. The relevant design guidance is given in ADF, in particular Table 5.2a.

    2.2 System 2 – Passive stack ventilation

    The information provided in Tables 3 and 4 gives guidance on good installation practice and the procedures for the inspection of passive stack systems for dwellings. The relevant design guidance is given in ADF, in particular Table 5.2b.

    Note: If intermittent extract fans are installed in dwellings where the ventilation system follows System 2, air flow measurement testing should be carried out and recorded as for System 1.

    ****Table 1: System 1 installation guidelines****

    ****Diagram 1: Duct installations****

    The inner radius of any bend should be greater or equal to the diameter of the ducting being used. If the radius is reduced, the resistance of the bend will increase and the volume of air being extracted will decrease (see “Do” diagram above).

    Ensure flexible ducting is installed without peaks or troughs (see “Don’t” diagram above).

    ****Table 2: System 1 inspection and testing****

    ****Table 3: System 2 installation guidelines****

    ****Table 4: System 2 inspection****

    ****Diagram 2: Suitable layouts for PSV systems****

    The layouts shown in Diagram 2 are considered to be suitable for the majority of dwellings of up to four storeys. Placing the outlet terminal at the ridge of the roof (Diagram 2(a)) is the preferred option for reducing the adverse effects of wind gusts and certain wind directions. A tile ventilator may be used to terminate a PSV system on the roof slope but the terminal should be positioned no more than 0.5 m from the roof ridge. If the duct penetrates the roof more than 0.5 m from the ridge, it should extend above the roof slope to at least ridge height to ensure that the duct terminal is in the negative pressure region above the roof (Diagram 2(b)).

    ****Diagram 3: Suitable and unsuitable bends for passive stack ducts****

    Ducts should use no more than one offset (i.e. no more than two bends) and these should be of the “swept” rather than “sharp” type to minimise flow resistance. Offsets at an angle should be no more than 45° to the vertical (Diagram 3).