The Cold Water Storage Cistern

The cold water storage cistern can be regarded as the ‘heart’ of the domestic hot and cold water services. Usually situated out of sight it should never be out of mind. It deserves regular inspection and maintenance.

Failure to fill properly can result, at the worst, in failure of all the domestic plumbing appliances. At the best it can result in intermittent supplies of water and recurring airlocks. A leak or overflow at a time when the family is away from home can result in hundreds of pounds worth of damage to carpets and furnishings.

Householders not infrequently query the necessity for the very existence of this potentially dangerous piece of equipment. They point out that it is technically possible to supply all cold water draw-off points direct from the rising main and that there are nowadays water heaters capable of providing a whole-house hot water supply that are designed for mains connection. Nevertheless the provision of a substantial cold water storage cistern offers real advantages both to the Water Authority and to the householder.

Demand for water is not constant throughout the day. There is a peak period of demand between 7.00 a.m. and 9.00 a.m. that few Water Authorities would be capable of meeting if all connections were made to the main. The cold water storage cistern provides a ‘shock absorber’ against peak period demand. At 7.00 a.m. all domestic storage cisterns are full. As the demand for water increases water pressure falls in the mains. Stored water is drawn off from storage cisterns which refill slowly during the peak period and more quickly as it passes.

From the householder’s point of view a major advantage is that it makes possible the installation of that most versatile means of domestic hot water supply, the cylinder storage system. The fact that water in the distribution pipes is at low pressure from a storage cistern reduces the possibility of leaks and, if they occur, makes them less devastating. A low pressure supply to w.c. flushing cisterns helps to reduce the noise of refilling. Above all though, the provision of a substantial water storage cistern means that temporary failure of the mains water supply does not immediately paralyse the domestic plumbing system. All householders receive, from time to time, a brief official notification from the Water Authority that, in order to carry out repairs or alterations to the main, water supply will be cut off for a few hours.

For a household without a storage cistern such a notice means the immediate filling of every available bucket and water container for lavatory flushing, food preparation and cooking. The householder with a storage cistern of reasonable capacity can view a notification of this kind with equanimity. He knows that, provided that water supply is resumed within a few hours, only the cold tap over the kitchen sink will fail.

Where should the cold water storage cistern be situated? The traditional site is in the roof space though in recent years it has been suggested that, as a frost precaution, it is better situated at a lower level, perhaps in an upper part of a hall or bathroom airing cupboard. On balance the storage cistern is probably better retained in its traditional position and other measures taken to protect it from frost.

In an airing cupboard a storage cistern can create annoyance from noise and trouble can result from condensation of moisture from the warm damp air of the airing cupboard on its cold surface. Then too, a reduction in height inevitably means a reduction in water pressure. Flushing cisterns will refill more slowly, pressure at hot and cold bathroom taps will be reduced and the provision of a conventional shower on the same floor as the storage cistern will become impossible.

Within the roof space the cistern should be sited well away from the eaves, preferably near a partition wall that will help to support the weight of the cistern and the water it contains and, if possible, immediately above the hot water storage cylinder. However well the latter may be lagged some warmth will seep upwards to help to protect the cistern from frost. If it is intended to install a shower on the floor below it is wise to raise the level of the storage cistern. This can be done by constructing a substantial wooden platform for it, so as to raise it three feet or so above the roof timbers.

In the past it was usual to install cisterns of relatively low capacity—25 gal where a hot water system only was to be supplied and 40 gal where the cistern was also intended to serve bathroom cold taps and the w.c. flushing cistern.

Domestic demands for water have increased in recent years as a result of the popularity of washing machines, dish washers and more frequent bathing. Nowadays domestic cold water storage cisterns are standardised at a capacity of 227 litres though smaller sizes are available. It should be noted that this refers to actual capacity; that is, to capacity when filled to normal water level, about 4Viin from the cistern’s rim. The accompanying table gives a guide to the capacity of existing galvanised steel cold water storage cisterns. When faced with the need to replace an existing storage cistern with one of a more appropriate capacity, the dimensions of the trap door giving access to the roof space may appear to present an insoluble problem. A full sized cistern will not pass through many existing trap doors and to enlarge the access makes a substantial increase to the size of the replacement job.

Round polythene cisterns can often be flexed through relatively small openings but, if all else fails, storage capacity can be increased by linking two smaller cisterns together by means of a 28mm pipe 2in above the bases of the cisterns. Where this is done it is important to ensure that the distribution pipes are taken from one cistern and that the ball-valve inlet is connected to the other. This will ensure a steady flow of water through both cisterns and avoid any risk of stagnation.

The traditional cold water storage cistern is constructed of galvanised steel. Cisterns of this material are tough, generally long lasting and offer good support to the water supply pipe. They are however heavy, not too easy to clean thoroughly and-above all- are subject to corrosion.

Members of the older generation of plumbers frequently express surprise and disgust at the fact that in a modern home, a galvanised steel cistern may show serious evidence of corrosion within four or five years of installation whereas they have known similar cisterns, installed in pre-war houses, to remain untouched by rust for half a century. This is not entirely due, as they probably imagine, to the inferiority of modern manufacturing methods. It is explained by the almost universal post-war substitution of copper for lead or iron supply and distributing pipes.

If connected rods of zinc and copper are immersed in a weak acid a simple electric cell is produced. Electric current passes from one rod to the other. Bubbles of hydrogen gas form in the electrolyte and the zinc dissolves away. Similar conditions are produced when copper tubing is connected to a galvanised steel storage cistern. The water in the cistern will, if slightly acid, act as the electrolyte. The zinc coating of the galvanised steel will dissolve away and permit water to attack the steel underneath. This process is called electrolytic corrosion.

One way of preventing this process is to ensure, by protective internal painting, that water does not come into direct contact with the galvanised steel. When installing a new cistern cut the holes for the pipe connections and apply the treatment before making the actual connections. Roughen the entire internal surface of the cistern with abrasive paper to form a key and then apply two coats of a tasteless and odourless bitumastic paint. Several manufacturers produce a paint suitable for this purpose. Make sure that the entire internal surface is coated, paying particular attention to the areas in the immediate vicinity of the holes cut for the pipe connections. Install the cistern when the paint has dried thoroughly.

This method of protection can also be used to prolong indefinitely the life of a cistern that is already showing serious signs of corrosion. Drain and dry the cistern thoroughly and preferably disconnect the supply and distributing pipes. Remove every trace of existing rust with abrasive paper or by wire brushing—use goggles to protect the eyes when using a wire brush. This process may well leave deep pit marks in the metal, possibly even holes penetrating right through it. Such holes and pitmarks may be filled with one of the many epoxy resin fillers now on the market used in accordance with the manufacturer’s instructions.

When the filler has set apply two coats of bitumastic paint as suggested for a new cistern. It is not generally necessary to roughen the internal surface of an old cistern to form a key. This treatment will prolong the useful life of the cistern for several years and can be repeated when it becomes necessary.

Galvanised steel cisterns which are not already showing signs of corrosion can also be protected by means of a sacrificial anode. This method makes use of the same principle that is responsible for electrolytic corrosion. All metals have a fixed electric potential and, when electrolytic action takes place, it is the metal with the higher potential that dissolves away. Anodic protection takes advantage of the fact that magnesium has a considerably higher potential than either zinc or copper.

A sacrificial anode is a lump of magnesium suspended in the water of the cistern and maintained in electrical contact with the metal of the cistern. The magnesium dissolves away and the zinc coating of the galvanised steel is protected. Anodic protection has been found to be most effective in hard water areas. Unlike internal painting it can be adapted to protect galvanised steel hot water storage tanks as well as cold water storage cisterns. Although, as we have seen, galvanised steel cisterns can be protected from corrosion, cisterns made of non-corrodible materials have obvious advantages.

Non-metallic cisterns

Asbestos cement cisterns come into this category but they are not without their drawbacks. They are heavy-a cistern with a capacity of 50 gal weighs 104 lb. They are liable to accidental damage and to damage from frost. They must be handled with care. Holes should not be cut nearer than 4in to the base of the cistern and, since the material of which they are made is lMn thick, hole cutting can present difficulties.

Cisterns made of modern plastic materials have the advantages of asbestos cement cisterns without their disadvantages. They are, of course, quite non-corrodible and have smooth and easily cleaned internal surfaces. They are light in weight so that one-man installation is a practical possibility. Since the plastic of which they are made is a poor conductor of heat they offer a measure of built-in frost protection.

Round, flexible polythene cisterns have already been mentioned. There are as well a number of rectangular plastic cisterns on the market, some reinforced with fibreglass for extra strength. The author’s home has a plastic/fibreglass storage cistern that was installed more than fifteen years ago and is, quite literally, ‘as new’.

Plastic cisterns are easily installed but there are a number of important points that must be borne in mind. Plastic cisterns must rest on a flat, level base-never just on the roof timbers. A piece of chipboard or two or three pieces of floor board spiked to the roof timbers are quite satisfactory. Cut the holes for the tappings with a saw hole cutter fitted into a drill or brace. Cut from inside with a block of wood placed under the cistern wall for support.

Make sure that all pipe connections are taken squarely to the cistern walls so as to avoid stressing the plastic. For each connection use two large washers-one metal and one plastic-on each side of the cistern wall. The plastic washer should be in direct contact with the cistern wall, followed by the metal washer and back nut. Never use boss white or other waterproofing material in direct contact with the walls of a plastic cistern. It is quite unnecessary and could damage the plastic.

Remember that a plastic cistern does not offer the same support to the rising main as does a galvanised steel or asbestos cement cistern. When fitting a ball-valve to a round polythene cistern always use the metal supporting plate supplied with the cistern. Secure the rising main firmly to the roof timbers. Failure to observe these final points can lead to intolerable noise and vibration as the cistern fills with water.

Whatever the material the cistern is made from, it should be fitted with a dust and vermin-proof but not airtight cover. This is both a frost precaution and a safeguard against contamination. The water from the cistern will not be used for drinking but it will be used for teeth cleaning!

The walls, but not the base, of the cistern should be lagged with fibreglass tank lagging material or-in the case of rectangular cisterns-with purpose made expanded polystyrene or strawboard lagging units. This precaution is less essential with plastic cisterns than with those of other materials.

The base of the cistern should not be lagged and if, to conserve warmth in the rooms below, a fibreglass blanket or loose fill lagging material, has been laid between the joists, this should be omitted from the area immediately beneath the cistern. There is, in fact, something to be said for extending the lagging material downwards from the base of the cistern to the ceiling below, to provide a channel via which slightly warmer air can be funnelled up towards the base of the cistern.

One other vital frost precaution relates to the one connection to the cold water storage cistern that has not so far been mentioned—the overflow or warning pipe. This pipe, which should be at least 22mm in diameter, connects to the cistern below the level of the ball-valve inlet and about lin above normal water level. Its purpose is not to give indefinite protection against overflow in the event of a ball-valve failure but to give warning that this has occurred. It should therefore discharge in plain view in the open air. A steady drip-much less a full-bore flow-from this pipe should never be ignored.

The warning pipe obviously provides a route by which icy draughts can penetrate to the vulnerable plumbing fittings of the roof space unless proper precautions are taken. The traditional method of protection is to provide a hinged copper flap at the external end of the overflow pipe.

Wind blowing against the end of the pipe will close the flap and will be prevented from entering the pipe. Unfortunately, through disuse, these flaps often jam open and fail to function when required.

A better and more modern method of protection is to continue the warning pipe inside the cistern and to bend it over so that its open end is a couple of inches or so below the water surface. A trap is thus formed which prevents cold air from blowing up the pipe. Plastic screw-on extension pieces are available to enable this form of protection to be given to existing warning pipes.


Strictly speaking the word ‘cistern’ should be used to describe water storage vessels open to atmospheric pressure—the main cold water storage cistern, w.c. flushing cisterns and the small feed and expansion cistern that supplies the primary circuit of an indirect hot water system. ‘Tanks’ are enclosed vessels and the use of this word, in domestic plumbing, is restricted by the purists to galvanised steel hot water tanks sometimes used for hot water supply.

As however these tanks are increasingly being replaced by copper hot water storage cylinders and can therefore be regarded as obsolescent, there is a growing tendency to refer to the main cold water storage tank and, particularly, to the feed and expansion tank. Cisterns and open tanks are supplied with water by means of ball-valves or float-valves. Like many other plumbing fittings these valves have experienced revolutionary changes of design over the past half century. Depending upon the age of the installation the householder, or plumber, may find any one of five ball-valves of quite distinct design in current use—and there are a number of sub-species.

All ball-valves operate by means of a float fixed to the end of a rigid float arm of brass or plastic. As water is drawn off from the cistern the water level falls and the float falls with it. This movement is transmitted via the float arm to the valve itself which opens to allow water to flow into the cistern. When the water in the cistern-and the float-reach a predetermined level, the float arm closes the valve and flow of water ceases.

The oldest, simplest—and least satisfactory-kind of ball-valve likely to be encountered today is that of the Croydon pattern. As water level in the cistern falls the plug is pulled away from the valve seating and, noisily and with a great deal of splashing, water flows into the cistern via two channels constructed on either side of the valve body. The inherent, and incurable, noisiness of this kind of valve is its main disadvantage. It is quite suitable, and is often used, for cattle watering troughs and for static water storage cisterns on municipal allotments. Few people nowadays would wish to have one in the home.

The ball valve in commonest domestic use-certainly in installations dating from before the 1960s-is that of the Portsmouth pattern. These valves are quieter than Croydon valves if for no other reason than that water flows from the valve outlet in a single stream. A slot in this plug accommodates the angled end of the float arm and the plug therefore moves to and fro as water level rises or falls.

It used to be the practice to enhance the silent action of this valve by screwing a ‘silencer tube’ of metal or plastic into the valve outlet so that incoming water was discharged into the cistern below the level of water already there. Water Authorities now forbid the use of these silencer tubes because of the risk of back siphonage in the event of a failure of mains pressure, which could result in mains water being contaminated by water from a storage cistern. This prohibition will undoubtedly hasten the changeover from the Portsmouth valve to those of more inherently silent action.

Common faults to which both Croydon and Portsmouth valves are prone are as follows.

Plug sticking in valve body resulting in failure to open or close properly -This is common in hard water areas and is due to the formation of hard water scale on the plug and the interior surfaces of the valve body.

To remedy first cut off the water supply to the valve. Pull out the split pin on which the float arm pivots and remove the float arm. The plug of a Croydon valve will now fall out into the hand.

Some Portsmouth valves have a screw-on cap at the end of the valve body. This cap must be unscrewed and removed. Then insert the blade of a screw driver into the slot under the valve body from which the end of the float arm has been removed. Push the plug out of the open end of the valve body.

Clean the plug and the interior of the valve body with fine abrasive paper. Smear the plug lightly with petroleum jelly and reassemble.

Valve fails to close properly because of washer failure -This is indicated by a steady drip of water from the overflow or warning pipe of the cistern. To renew the valve washer dismantle the valve and remove the plug as indicated above.

The plug is in two parts—a body and a retaining cap. The body should be held securely in a vice and the retaining cap unscrewed with a pair of pliers. The old washer can then be replaced with a new one. The retaining cap of a long-installed valve can be extremely difficult to remove. Rather than risk damaging the plug it is better to pick the old washer out from under the rim of the retaining cap and to force the new washer under this rim. If you take this course of action you should make sure that the washer lies flat in its seating before reassembling.

Continued dripping from the overflow pipe after the ball valve washer has been renewed suggests that the valve seating is scored by grit from the main. Although it is possible, with a special tool, to reseat the ball valve it is usually as cheap to renew the valve.

Slow filling or leaking past the valve despite the fact that washer and seating are sound-Either of these faults suggests that a valve of the wrong pressure classification has been installed. Ball valves are classified as ‘high pressure’, ‘medium pressure’, ‘low pressure’ or ‘fullway’ according to the diameter of the nozzle orifice. They are usually stamped HP, LP etc. on the valve body to indicate the pressure for which they are intended.

Except where mains pressure is unusually low, ball valves serving cisterns supplied direct from the main should be high pressure. Those serving flushing cisterns supplied from a main storage cistern should be low pressure. Where a flushing cistern is supplied from a low level storage cistern—perhaps in the upper part of an airing cupboard only a few feet above the level of the w.c. suite-a fullway valve will ensure rapid recovery.

Noisy filling-Noise may arise both from the sound of inflowing water and from water hammer and vibration in the plumbing system. Water hammer and vibration arise from ripple formation on the surface of the water as water flows into an almost full cistern.

These ripples may make the valve bounce on its seating to produce water hammer. They may also shake the float arm up.