Understanding Plumbing and Water systems

The two common types of domestic plumbing system are the gravity storage system and the direct system in which everything is directly connected to the mains.

Gravity storage

With the gravity storage system, water from the mains enters the house and is forced up to a storage tank situated high in the house (usually in the attic or loft). From the cold storage tank, feed pipes pass throughout the house to the various points where cold water is required — the cold taps at baths and washbasins and to lavatory cisterns — and to the heater that supplies hot water.

Water stored and distributed in this way cannot be guaranteed to be pure. Apart from spores in the air, it can be infected by dust or dirt particles in the loft dropping into the storage tank. Because of this, water from a cold storage tank cannot be used for drinking or the cooking and preparation of food. So where the mains enter the house there is a branch line to the sink to ensure that the water at the kitchen cold tap is pure. Beyond the kitchen tap branch line the pipe is referred to by plumbers as the rising main.


Water passes up to the loft because of the pressure from the mains. Its entry into the cold storage tank is controlled by means of a ball valve. This device consists of a ball floating on the surface of the water and connected to a sort of tap by means of a lever arm. As the water in the tank rises, the ball goes with it, taking the arm along too. When the correct water level is reached, the arm shuts off the tap and thus prevents any more water from entering.

Should anything go wrong with this arrangement an overflow pipe carries away excess water safely to the outside of the house, preventing it from cascading down into the house. So if you see water gushing out of an overflow pipe, you will know that the ball valve at the cistern to which it is connected requires attention.


The feed from the cold tank to the rest of the house is purely by gravity. The water runs “downhill”, as it were, to the taps. The action of opening the taps to release water draws cold water from the storage tank. As the level there drops, so does the ball, which causes the lever arm to open the valve to admit more water until the correct level is again reached in the tank.

The higher the cold storage tank is above the outlet, the greater the pressure of water at the outlet. This is an important point to observe if you are thinking of installing a shower. The difference in height between tank and outlet is known to plumbers as the “head of water”.

Water is fed, too, to the cisterns of the W.C’s. These work in a similar way to the cold storage tank. Each is controlled by a ball valve and there is an overflow pipe.

Hot water.

Finally, cold water must be fed to the hot water system. The usual “go-between” is the hot water cylinder, supplied with water from the cold storage tank. Pipes convey hot water from the cylinder to the hot taps around the house. When you open a hot water tap, hot water flows from the cylinder, causing the level in it to drop. This is made good by water flowing in from the cold cylinder to replace it.

Note that, although the hot taps are gravity fed, too, it is the pressure caused by the height of the cold tank that forces water to them. The head of water between the hot cylinder and the hot taps does not matter. The hot cylinder is merely a small reservoir that lies between the cold tank and the taps.

Heating the water.

The hot water cylinder, as well as supplying hot water for washing, often plays a key role in central heating by hot water radiation. This heating system is powered by a boiler that might be solid fuel, gas or oil-fired. The system works by what is known as thermo-syphonic action — another way of saying that hot water rises and cold water falls. Because of this, the hot cylinder must be placed on a higher level than the boiler.

As water is heated in the boiler, it passes up to the hot water cylinder to be stored ready for use. If you put your hand on the cylinder (take care — it might be very hot) you can feel that the water lies in layers at different temperatures. At the top layer and hottest is the newly arrived water from the boiler. On arrival it forces the coolest water down to the boiler so this can be warmed up in turn. To enable this arrangement to work well, the pipe taking the heated water to the cylinder joins it somewhere near its top and the pipe by which the cooler water leaves it is near its bottom. These two pipes are known as the flow and return. The feed from the cold tank will also be near the bottom of the cylinder.

Modern automatic boilers are thermostatically controlled, so that the water in them should not get excessively hot. But as a precaution against an over-heated boiler dangerously overheating the water in the hot water cylinder, a pipe rises from the hot cylinder to the cold storage tank and curves over the top of it, so that steam and boiling water can be harmlessly discharged into it. Even so. Should you ever hear water in a hot cylinder boiling, run a hot tap so that the overheated water is drawn off and replaced by cold.

Water in the cylinder can also be heated electrically by means of a thermostatically controlled immersion heater, either as the main source of heat or as an auxiliary in providing hot water if the central heating boiler cuts out or is switched off in the summer.

The hot water cylinder is usually situated in an airing cupboard, and any excess heat emitted from the cylinder keeps clothes and linen gently warm. However, the cylinder’s function is to store hot water with minimum heat loss, so it should be lagged with a heat-retaining insulating jacket.

Direct system

The alternative to a gravity system of domestic plumbing is to have everything (e.g. all cold taps, lavatory cisterns, water heaters) connected directly to the mains.

A possible risk with a direct system is that infected water could be sucked back into the supply, thus fouling it, and appliances that are directly linked to the mains require built-in safety devices to stop this happening. Different water authorities impose varying regulations. Instantaneous heater. One way in which water can be heated in a direct system is by means of an instantaneous heater. The heater can be single point (i.e. designed to supply one outlet, such as the sink, bath or washbasin, although a single point heater with a swivelling arm over, for example, the bath could also deliver water to an adjacent basin). Alternatively, the heater can be multipoint, powerful enough to give hot water throughout the house. Such heaters can be either gas or electric.

The action of opening the tap to draw off water automatically switches on an instantaneous heater. In the case of an electric heater, a switch is operated. On a gas model, jets are opened up to be ignited by a pilot light.

Storage heater.

This is an alternative to an instantaneous heater. It fills the role that the hot cylinder fills in a gravity system but contains its own integral heater (usually electric). It can be sited anywhere in the house, but it is usually positioned near the main draw-off point (e.g. the kitchen sink). Where the kitchen and bathroom are far apart, it can pay to have two smaller heaters — one near the sink, the other near the bath.

Knowing your system

Whether your domestic plumbing system is gravity storage or direct, understanding how it works will save you money, time and anxiety should anything go wrong.

At various points there is a form of tap, known as a stopcock. When you turn this off, the flow of water is stopped so that sections of the system can be isolated to allow you to carry out repairs and alterations. You should find a stopcock at the point where the mains first enter your property. There should be one, too, on the rising main just inside your house and before the branch line to the kitchen sink. By turning these off you can shut off the water supply to the whole house. There may be other stopcocks throughout the house. Likely points to find them are where the various feeds leave the cold storage tank (if you have one), beside lavatories, heaters etc.

Get to know which stopcock turns off what. Turn off each in turn and then go round your house opening the taps to see which go on giving water and which gurgle to a stop after a while. In the case of a cistern, listen to find out if water is running into it.

Warning. It can be dangerous to draw off water from a heater or storage cylinder if you have already turned off the cold supply that replenishes it. So before turning off the cold supply, switch off the heating.

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