Cold water storage cistern: what is it & how does it work?

The cold water storage cistern, often referred to as the cold water tank, is an important part of a gravity-fed domestic water system. It supplies cold water to the hot water cylinder, and to potentially every cold water outlet in the home, except the kitchen and outside taps. It is usually made out of plastic, although more rigid materials such as galvanised steel and asbestos cement have been used in the past. A typical cold water storage cistern may hold anywhere from 15 to 100 gallons of water.

The cold water storage cistern is often referred to as the cold water tank. This is technically incorrect, however, as a tank is a sealed vessel, unlike cisterns, which are open to atmospheric pressure. Fed from the rising main, it is usually situated in the loft because of its size and in order to minimise the annoyance of the sound of running water. Nevertheless, in some homes it can be found above the hot water cylinder in the airing cupboard.

Modern cold water storage cisterns are generally made from black plastics. This makes them light, flexible, and resistant to algae growth. They can be round, square, or rectangular in shape, and should be insulated to prevent the water from freezing in the winter, and to keep it cool during the summer. Certain fittings are also required in order to minimise the risk of contamination, such as a lid.

Contents – cold water storage cistern

General overview

The flow of water into the cold water storage cistern is controlled by a ballcock. The float on the arm of the ballcock is usually plastic, although copper floats are sometimes used. Should the ballcock fail, an overflow pipe carries the water away, discharging outdoors. The overflow is extremely important and serves two purposes. It alerts the household to a fault, and prevents the cistern from being overwhelmed with water, leading to property damage. The overflow pipe should be fitted at a constant fall, and its exit outside should be clearly visible. It should be at least 19mm in diameter, and should be capable of evacuating all of the excess water under maximum fault conditions, such as in complete failure of the ball valve.

Tank connectors

Tank connectors connect the cold water storage cistern to 15, 22, or even 28mm distribution pipes via a compression fitting, supplying most cold water outlets and devices in the home apart from the kitchen tap. These are fitted 50 – 75mm from the bottom of the cistern in order to prevent any sediment or settled limescale from getting into the system. The outlet supplying the hot water cylinder should be fitted higher than the cold outlet, so that if the water in the cistern is exhausted, the hot water will stop flowing before the cold does. This means that an individual taking a shower will not be scalded if the water in the cistern runs out: the water will run cold, not hot. However, water regulations recommend that, where possible, outlets should be fitted to the bottom of the cistern in order to reduce the collection of sediment.

Isolation valve

An isolation valve must be fitted to the pipe supplying the cold water storage cistern. This allows for the cistern to be isolated without disrupting the mains water supply to the property. It is also far more convenient for an individual working on the cistern, saving them from having to go up and down between the loft and mains valve access points, such near the kitchen sink or out onto the street or driveway. Gate valves or quarter-turn lever valves on the distribution pipes prevent the need for the cistern to be drained down when work is carried out further along the system.

Expansion pipe

Water expands when heated. In order to accommodate the expansion of water heated by the hot water cylinder, a pipe rises from the top of the cylinder and curves down into the cistern, without coming into contact with the water. This pipe is known as the expansion pipe. If the hot water expands to the extent that it cannot be accommodated by the hot water cylinder or the expansion pipe, it vents into the cold water cistern. It is very important that the expansion pipe does not dip into the water, as this may create a gravity circuit that fills the cistern with hot water from the hot water cylinder.

Cold water storage cistern construction

Modern cold water storage cisterns are generally made from plastic. The most common plastics used are polyethylene, polypropylene, or polyvinyl chloride (PVC). Glass-reinforced polyester (GRP) may also be encountered. Older cisterns may be made from galvanised steel, which are highly susceptible to corrosion. They also may be impossible to remove from the roof space via the loft hatch because they were installed before construction of the roof was completed. Once drained down, an older galvanised cistern can be cut up. This should be done manually so as not to create a stream of sparks in the loft – a serious fire hazard. However, this can be a laborious task. If there is no particular reason to remove it, it may be more convenient to simply it to a corner of the loft and leave it there.

Other materials

Some older cisterns may be made from asbestos cement. You should never attempt to cut up or even dispose of such a cistern yourself. Even in tiny amounts, asbestos particles are extremely dangerous to human health; skin contact alone can cause nasty dermatological problems. Contact a professional asbestos remover – it’s just not worth the risk.

Some cisterns may be made entirely from copper. Metal cisterns are required where the domestic hot water is heated by a device without any form of thermostatic control. This is because they may vent extremely hot water into the cistern, even as part of their normal operation.

Modern plastic cisterns are generally black in order to resist algae growth. They can be round, square, or rectangular in shape. Some cisterns are long and narrow, and relatively shallow in comparison to conventional designs. These “coffin tanks”, as they are known, are ideal for confined spaces.

Some cisterns are designed in such a way that they are no longer than a given dimension on any plane. This means that they can fit through even the smallest of loft hatches.

Joining cold water cisterns

In circumstances where extra capacity is needed but a bespoke cistern is not a viable option, it is possible to connect two cold water storage cisterns together, e.g., to connect a 25 gallon cistern to a preexisting 25 gallon cistern. However, it is extremely important that the outlets are on the cistern without the ball valve. This ensures a flow of water through the cisterns, preventing stagnation. The overflow and expansion pipe must be connected to the cistern with the ball valve.

Cold water storage cistern bases

Cold water storage cisterns are very heavy when full. A 50 gallon cistern will weigh around a quarter of a ton. Plastic cisterns of up to 500 litres must be situated on a base of at least 19mm marine plywood and ideally 25mm, providing flat, stable, continuous support under the entirety of the cistern. This base should be situated on timber bearers laid across the ceiling joists. Chipboard should never be used as a base, as it is liable to disintegrate if it gets wet.

It is absolutely fundamental that the entire underside of the cistern is supported, without any part of it overhanging. Older pattern immersion heater thermostats which fail in the “on” position will heat the water in the hot water cylinder indefinitely. This water will be discharged into cistern via the expansion pipe. Since the hot water cylinder is fed from the cistern, it is only a matter of time before the entire cistern is full of scalding hot water. In such circumstances, there have been incidents where an improperly supported plastic cistern splits, dumping its contents through the ceiling and injuring or killing the resident in a bedroom below. If a metal cistern has been replaced with a plastic one, it is advisable to check the immersion heater thermostat and replace it with one that has a cut-out.

Extra fittings for the cold water storage cistern

The storage cistern and all its adjoining pipes must be lagged. This will prevent the water from freezing during the winter and keep it cool during the summer. Loft insulation should not be placed underneath the cistern. This is because heat rising from the household below during the winter will help prevent the water from freezing.

A small backing plate, either metal or plastic, and located on the outside cistern wall, is secured via the ballcock’s rear nut. This reduces stress to the cistern wall from the upward force of the float.


You shower, bathe, and wash the dishes in water from the cold water storage cistern. You may even brush your teeth in water from it. A lid is therefore a must, and is usually supplied with the cistern. This will prevent dust, insects, and rodents from getting inside, as well as bits of loft insulation and other debris which could cause a blockage or damage, such as in a shower pump. The lid must be close-fitting and made from a substance which will not react with the water. It must also retain its structural integrity if it gets wet.

Byelaw 30 and byelaw 60 kits

Water byelaws require the fitting of certain components in order to minimise the risk of contamination. These are:

  • A rubber grommet, to create a sealed admission point for the expansion pipe.
  • A screened breather, to keep the cistern at atmospheric pressure.
  • A screened warning pipe unit, to prevent anything outside, such as insects, from entering the cistern.
  • A dip tube, allowing the warning pipe to dip into the water so that cold drafts cannot enter the cistern.

These parts can be bought as a kit. Byelaw 30 and Byelaw 60 kits as they are known, are often supplied with a new cistern, but you can also buy them separately if you are looking to upgrade an older cold water storage cistern.

Where to buy a new cold water storage cistern

Brand new cold water storage cisterns for new installations or to replace an existing cistern are available from most of the major UK hardware stores, including B&Q and Wickes.

All of the cisterns below from B&Q are from Polytank. They come with lids, an insulation jacket, a Part 2 ball valve and float, a tank connector and ball valve backing plate, and a Byelaw 30 kit. As most of the cisterns and included parts are over £75, you can also enjoy free next-day delivery.