The cold water tank (cold water storage cistern) can be found in the attic. This is how it works:
Water arrives at the cistern via the incoming mains supply; which is then available to provide water to various outlets around the home.
The size of a cold water storage cistern is dictated by the demands it will need to meet such as the size of the home, number of bathrooms and whether it needs to provide a supply of water to the hot water cylinder.
Cold water storage cisterns are designed and plumbed so that the stored water cannot become contaminated because this could cause health issues.
In the past, cold water storage cisterns were constructed of galvanised metal and were open. Today, cisterns are made of black plastic and have a close fitting cover. This makes them lightweight and durable in addition to being semi-flexible; which makes it easier to get them into the attic.
The black plastic resists algae growth and the plumbing of the cistern is designed to prevent water from stagnating. Cisterns are also insulated and protected from overflowing by an overflow pipe.
When installing a modern cistern, it is important that the base is fully supported. A solid platform between joists must be constructed because the weight of the water within a cistern is considerable.
The position of the cistern should be such that its outlets are higher than any of the draw off points for fittings and appliances.
The inlet to the cold water storage cistern on modern installations is usually a diaphragm valve. These provide more protection against contamination or backflow than the older style ballcock or ball valve (float valve).
Diaphragm valves are designed to prevent backflow and to enable the cistern to be filled and shut off automatically dependent on the water level. The valve can be adjusted to enable a specific maximum water level can be maintained.
In addition to the inlet, a cistern has an outlet and this should be located close to the base of the cistern or even underneath it. This helps to prevent debris from accumulating in the bottom of the water.
The inlet and outlet should always have a shut off valve for obvious reasons. This should be located as close to the cistern as practical so that you can turn off the supply of water to the cistern.
Cisterns have an overflow pipe; which allows water to be discharged should the inlet valve fail. This pipe runs downhill and is routed through an outside wall. This enables the homeowner to see that there is a problem.
The overflow pipe should be protected with mesh to prevent dirt and insects from entering the system and it should be insulated.