The cold water tank in the attic which is used to store cold water is actually referred to as a cold water storage cistern. Water is fed from the incoming mains supply to the storage cistern to provide readily available water to various outlets in the house. The size will be worked out according to the demands it will need to meet which may also include supply of water to the hot water cylinder. They are designed and plumbed so that the stored water cannot become contaminated which could give rise to health issues.
Cisterns nowadays are made of black plastic with a close fitting cover rather than the open galvanised metal ones which were used in the past. The use of plastic makes them durable as well as being light. If you’ve ever removed an old metal storage cistern, you will know how heavy they can be. Being plastic also makes them semi-flexible which can make it easier to get them into the loft – particularly if the hatch is a bit tight. The reason they are made from black plastic is that this resists algae growth. The plumbing of a cistern is also designed to prevent the water from stagnating. Cisterns are insulated – obviously enough this prevents the water from freezing, but it also minimises warming of the stored water. An overflow pipe is used to guard against the cistern overflowing should the inlet shut off fail.
Locating a Storage Cistern
As mentioned earlier, modern plastic cisterns are flexible so when they are installed, the base must be fully supported. It is unacceptable to stand them spanning across joists without providing a solid platform between the two. The weight of water inside a cistern is considerable - without this solid base, it would be possible for the cistern to distort with potentially disastrous consequences. The position of the cistern should be such that it’s outlets are higher than any of the draw off points for fittings and appliances.
The inlet to the cistern is controlled by a float valve or what we commonly used to call a ballcock or ball valve. With newer plumbing installations this will most likely be a diaphragm valve as the older style valves have insufficient protection against backflow and contamination. Diaphragm valves discharge water from the top and this allows a sufficient air gap to prevent backflow. The purpose of this valve is to enable the cistern to be filled and shut off automatically according to the water level. Adjustment of the valve once it is fitted enables a specific maximum water level to be maintained.
The connection for the outlet from a cistern should close to the base or even underneath so that debris does not accumulate in the bottom of the water
Both the Inlet and the outlet should have a shut off valve and this should be as close to the cistern as reasonably practical. The point of these is that you will be able to turn off the supply to the cistern for maintenance and stop water flowing from the outlet if work needs to be carried out on the pipe work fittings or appliances beyond. The inlet one is usually a plug valve type. The outlet one will normally be a regular gate valve.
An overflow pipe is fitted to cisterns above the maximum water level but sufficiently below the inlet so as to maintain the required air gap. However, the end of this pipe inside the cistern is continued down below the maximum water level. This prevents draughts coming back along the pipe from outside. The purpose of this pipe is fairly obvious – it allows water to be discharged should the inlet valve fail. The pipe runs downhill so that water flows freely down it and it will be routed out through an outside wall so that it becomes obvious if there is a problem. The pipe should be protected with a mesh screen to stop insects and dirt entering the system. One thing that is often neglected is to insulate this pipe. Whilst it is empty most of the time, a small flow of water might otherwise freeze and block the pipe. The overflow or warning pipe as it is sometimes called is commonly 19mm but must be capable of coping with the potential amount of water.
Modern plastic cisterns have a top, further preventing risk of water contamination. There is an access panel in this with a screwed down lid which can be removed allowing for maintenance and inspection. Where pipes pass through the lid – for example an open vent pipe, there will be a seal. A small air vent is also fitted in the top of the cistern which will also include an insect and dust screen.
A new cistern will need holes cut in the sides for the inlet and outlet or outlets. These are cut with a hole saw – various sizes are available allowing accurate holes to be cut through the plastic. Where the pipe passes through the wall of the cistern, a rubber washer will be used on either side to seal the hole.