Determining equivalent area of ventilators

The equivalent area stated in the ventilator manufacturer's literature or marked on the air vent should be used whenever it is available, as this can differ considerably from the free area measured at one end of the air vent. When this is not available the equivalent area of a simple ventilator with no internal baffles can be taken as the total unobstructed cross-sectional area, measured in the plane where this area is at a minimum and at right angles to the direction of air flow. For an airbrick, grille or louvre with apertures no smaller than 5 mm, it should be the aggregate free area of the individual apertures as shown in Diagram 2.1.