Your cold water supply arrives in your home via an underground pipe connected to the water main beneath the road. The pipe is usually buried at a depth of about 750 mm (2 ft 6 in) to protect it from frost, and somewhere between the road and your building there is an underground stopvalve belonging to the water company that allows the home to be shut off from the mains supply. In towns this is probably in a small chamber underneath a hinged cover plate set in the pavement, but in the country it may be found anywhere within the property boundaries - it depends on the siting of the nearest water main. Make sure you know where yours is. You are responsible for the pipe as soon as it leaves the water company's underground stopvalve.
After the supply pipe enters the home, it will run to another stopvalve. This allows you to cut off the supply within the home in an emergency or to make alterations to the system. Again, find out where yours is, if you don't already know. From here, the route the supply pipe takes depends on whether your home has an indirect or direct plumbing system.
An Indirect Plumbing System
With this system, found in the majority of homes in the UK, the supply pipe runs up to a cold water storage cistern (tank) in the loft, which fills via a float-operated valve (ballvalve) that shuts off the water flow when the cistern is full. A branch off this so-called rising main supplies mains-pressure, fresh, cold water direct to the kitchen sink for drinking and cooking. Other branches may also be taken off the rising main to supply a washing machine or dishwasher, a garden tap, or an instantaneous gas or electric shower or water heater. An outlet pipe from the storage cistern supplies all the other cold taps in the home. A second outlet pipe feeds the hot water cylinder, which in turn supplies the hot taps. Each of these two outlet pipes should be fitted with a shut-off valve called a gatevalve.
In an indirect system, the hot water comes from the hot water cylinder. This may be heated by an electric element called an immersion heater, but it is more common for it to be heated by the boiler that operates the central heating system. The cylinder contains a copper coil called a heat exchanger, and this is connected to the boiler by flow-and-return pipes that make up the hot water (primary) circuit for the central heating. This does not mix with the water in the hot water cylinder (secondary circuit).
As hot water from the boiler is pumped through this coil, the coil heats the cold water in the cylinder and the (cooled) water from the coil returns to the boiler to be heated up again. When a hot tap is opened, cold water from the storage cistern in the loft flows into the base of the cylinder, and the pressure drives hot water out of the cylinder via a pipe at the top.
This pipe (from the top of the hot water cylinder) divides into two. One part goes to the hot taps, while the other rises up into the loft and is looped over the main cold water cistern. This pipe - called the open vent pipe - is a safety device and allows air (or steam if the system were to overheat) to escape from the system.
A Direct Plumbing System
With a direct system, each cold tap and WC is fed directly from the rising main, so there is no main cold water cistern. Hot water may be supplied either from a hot water cylinder with its own small cold water cistern (sometimes on top of the cylinder), or from an instantaneous gas or electric water heater or a 'combination' gas boiler also connected directly to the rising main. Direct systems using combination boilers are more common in flats and conversions than in houses, although they are increasingly used to replace indirect systems in smaller houses.
The advantages are that they require less pipework, do not require space-guzzling cylinders and storage cisterns, the amount of available hot water is not dependent on the amount in the cylinder, and they are cheaper to run. Disadvantages are that the boiler itself is more expensive at the outset than the boiler in an indirect system and there is no stored water in reserve if supplies are interrupted for any reason.
