When cold water needs to be stored to supply an indirect system of cold water, or to feed a system of dhw, the water is stored in a cistern, usually located in the roof space.

The storage cistern should have a minimum capacity of 100 litres. If the cistern is also to act as a feed cistern for the hot water supply (being a combined storage and feed cistern), it should have a minimum capacity of 230 litres.

Cold distribution pipes from storage cisterns should be connected so that the lowest point of the water outlet is a minimum of 30 mm above the base of the cistern. This is to prevent sediment passing into the pipe work. Connections of feed pipes to hot water apparatus from cisterns should be at least 25 mm above cold distribution pipes, if applicable. This should minimise the risk of scalding should the cistern run dry.

The supply ball valve is fitted as high as possible and must comply with BS 1212, parts 2 or 3, thus maintaining an air gap and preventing back-siphonage. Overflow pipes should have a minimum internal diameter of 19 mm and in all cases be greater in size than the inlet pipe.

To prevent the ingress of insects, a tight-fitting lid must be provided, with a screened air inlet. Where a vent pipe passes through the lid, the pipe must be sleeved. Overflow warning pipes must also incorporate a filter or screen. Finally, the whole installation (cistern and pipes) must be insulated to prevent freezing.

 

 

Sometimes it is desirable to have two or more cisterns coupled together instead of one large cistern. This is beneficial because one of them can be isolated and drained down, if required. Equally, on a smaller scale, lack of space in a house sometimes limits the size of a storage cistern. In such a case, two smaller cisterns can be joined together to give the required capacity. Different methods are used for the above examples. If the need arises to isolate one cistern, an isolating valve is fitted at each point in or out of the cistern, which can be shut off. For the purposes of a domestic house, it would be uncommon for one cistern to be isolated, so the mains supply is usually taken into one cistern and the cold distribution or feed pipe is taken out of the other. By designing it in this way the water in the second cistern would not become stagnant.

The washout pipe shown in the figure is only used on large cisterns (those holding over 2300 litres), for the purpose of draining down and cleaning out any sludge deposits, etc.

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