Specific requirements for cold water storage cisterns incorporating a Type AB air gap

A cold water storage cistern can sometime incorporate a Type AB air gap. Where this is the case it is important to minimise any light penetration through the weir overflow. This is because light is known to promote the growth of algae which could lead to not only taste and odour concerns but also provide nutrients which could support the growth of bacteria. The most common way of addressing this issue is to fit a cowl or shroud that covers the weir slot.

To demonstrate the cowl or shroud arrangement does not impede any discharge it should mirror the overflow weir in size and shape to at least below the lowest point of the weir and any surfaces below or to the side of the cistern as shown below. Information can also be found here

Please note: cold water storage cisterns incorporating a Type AB air gap are sometimes being used as backflow protection arrangements for high risk installation downstream and should not be removed or altered without the agreement of the local water undertaker.