BS 5440-2:2009 pdf free download – Flueing and ventilation for gas appliances of rated input not exceeding 70 kW net (1st, 2nd and 3rd family gases) – Part 2: Specification for the installation and maintenance of ventilation provision for gas appliances.
5.1 Design
5.1.1 General
The apertures of air vents shall allow the entry of a 5 mm diameter ball. No gauze or fly screen (i.e. mesh with apertures of less than 5 mm) shall be incorporated or subsequently fitted to an air vent.
NOTE Domestic debris can block vents of little less than 5 mm and restrict the free area.
5.1.2 Free area
A proprietary air vent which is marked with the free area shall be used wherever possible.
COMMENTARY ON 5.1.2
The development of materials, technology and manufacturing techniques has led to a wide diversity of ventilation products, including complex multiple assemblies dedicated for specific purposes such as minimizing draughts orsound transmission, or warming air. The free area of such products, by virtue of their internal aerodynamic characteristics, might not be calculated reliably from physical measurement. There is currently no formally agreed standard for the design or production of air vents for use with gas appliances and thus manufacturers are not obliged to mark their products with an area calculated in a uniform manner. The equivalent area stated in the vent manufacturer’s literature and/or marked on the air vent, should be used wherever it is available, as this can differ considerably from the free area measured at one end of the air vent.
In the absence of any such marking on a grille, louvre or duct, the sum of the free cross-sectional areas of the apertures should be used (see Figure 2). For a louvre, the areas should be measured at right angles to the blades or vanes. For an air vent assembly, the free cross-sectional area of the most restrictive component in an assembly should be used to calculate the free area.
Where two or more components are employed to construct a non-proprietary assembly, changes in section and cross-sectional area should be avoided wherever possible. Where such changes do occur, adapters should be used to facilitate a smooth and efficient transition which minimizes restriction at any of the components. The free area should be determined from the free area of the component with the least free area available.
In Figure la), for example, if the external grille or louvre has 60 holes of 9 mm x 9 mm (i.e. area = 48 cm2), the duct is 150 mm in diameter (i.e. area = 176 cm2), and the internal louvre has 6 slots each of which is 156 mm long and 9 mm wide (i.e. area = 84 cm2), then the free area for the whole air vent should be taken as that of the component with the minimum area (i.e. 48 cm2). If the areas are similar then caution should be used since components with similar aerodynamic resistances placed in series are more restrictive than the individual components by themselves, If the free area cannot be established, an alternative air vent should be used.
Although a non-preferred practice, where grilles/louvres and ducts of dissimilar shape or proportion are used together [see Figure 2b)], e.g. a 225 mm x 150 mm rectangular louvre and a 125 mm diameter duct, sufficient space should be allowed between the rear face of the louvre and the end of the duct to prevent an obstruction to the free area (20 mm is typically a satisfactory clearance).
Grilles, louvres or louvered doors used internally should be marked or labelled with, “IMPORTANT- DO NOT BLOCK THIS VENT”.