Domestic cold water storage cisterns are normally located in the roof space, where the noise of them re-filling is less apparent and their high point creates a static head of pressure needed to ensure a strong flow at outlets. This cistern supplies all the cold taps in the house except for the one used for drinking water in the kitchen (which is supplied by the rising main) and water to the hot water cylinder. Cisterns provide a reservoir of water which equalizes the 'pull' on the mains, and provides against any reduction or shut down of the main supply. Old cisterns were made of galvanized steel. Electrolytic action - a chemical reaction in hard water conditions - and the build up of limescale means that the outlet pipes can become constricted and the cistern itself eventually corrodes.

New circular cisterns of lightweight polythene - much easier to lift into the roof space because they can be folded to pass through narrow access hatches - are standardized to a normal capacity of 227 litres (50 gallons). Make sure the cistern you buy has a lid to keep the water clean. You will need to drill holes in the new cistern to fit the inlet and outlet pipes. If you plan to take out an old steel cistern, it's a good idea to saw it into pieces using a metal-sheet saw.

Inlet Supply

The inlet supply to the cistern is controlled by a float valve which shuts off the water by means of an arm operated by a plastic or metal ball called a 'float'. This lies on the surface of the water and rises as the water flows in, raising a lever arm that closes the inlet valve at the determined level. The hole to fit the float valve assembly should be about 75mm (3in) below the top of the cistern.

One or two feed or distribution pipes can be taken from the cistern: one may feed the cold water supply to the hot water cylinder, while the other feeds bathroom washbasins and baths. The holes for these pipes should be drilled about 50mm (2in) above the bottom of the cistern. It's a good idea to fit gate valves on these pipes so you can cut off the water supply without having to drain the cistern.

Overflow

The overflow pipe is a safety valve. If the float arm fails to shut off incoming water, the level in the cistern rises. To stop water flowing out of the cistern and into your home, the overflow pipe leads excess water out of the cistern and discharges it outside the building. The hole for the overflow pipe should be drilled 25mm (1 in) below the level of the float valve inlet. The pipe must be a minimum of 22mm (/in) diameter and should extend into the cistern with a bend taking its inlet about 50mm (2in) below level. This makes a water seal that stops any icy draughts blowing up the overflow pipe into the roof space.

Vent Pipe

The storage cistern also operates as a form of safety valve for the hot water cylinder. Heated water expands and it would be dangerous to contain it, so an expansion pipe must be fitted to the cylinder. The expansion or vent pipe is taken from the crown of the cylinder via an off-set fitting and curved just above the cold cistern. This vent pipe should not dip in the water or a siphonage circulation would start pumping hot water into nearly every tap in your home.

Troubleshooting

Cistern overflows: Check the float isn't leaking. Unscrew it from the arm and shake it. If you hear water inside it needs to be replaced.The water in it makes it too heavy to rise to the correct level to shut off the supply. If it's not leaking, the float may need to be adjusted.

If an overflow persists after adjustment, then the valve needs a new washer.Turn off the water supply, flush out the cistern and take out the spit pin under the valve holding the arm. Insert the tip of a screwdriver into the slot beneath the valve body and slide the piston out. Unscrew the end cap of the piston with pliers and pick out the old washer. Clean the piston with wire wool. Clean the inside of the valve body with fine abrasive paper wrapped around a pencil - take care not to damage the valve seat at the far end. Smear the surface of the valve with petroleum jelly. Fit the new washer, reassemble the valve and connect the float. Restore the water supply then adjust the arm to regulate the cistern's water level.