The most common type of hot water system is a vented system especially in older properties.
The cold water is supplied from a head tank usually in the loft, into the bottom of the cylinder and as it heats up, the hot water rises to the top of the cylinder where it is used to supply the hot taps. As the hot water is used, it is replaced with cold water from the head tank and so the cycle continues.
The head tank also doubles up as an expansion tank, as the water in the cylinder heats up due to expansion some of it will be forced back in to the head tank. There is also a vent on the hot water outlet pipe which allows any trapped air to escape which also prevents a build up of pressure.
There are two types of vented hot water cylinders.
Indirect Heated Cylinder
The water in an indirect heated cylinder is heated by a coil or an immersion heater. The water in the coil is heated by the central heating boiler and pumped around the system, the heat from the coil is transferred to the stored water. In the event of the the central heating not working or in the summer months, the stored water can be heated by the immersion heater.
Direct Heated Cylinder
The water in a direct heated cylinder is heated by immersion heaters, these can be single or dual element and there can be one or two separate immersion points. Separate immersion heaters are used when on economy 7, one for heating off peak and a boost. Another way to heat the water in a direct cylinder is to use a separate boiler specifically designed for that purpose, in this case the stored water in the cylinder is fed to the boiler where it is heated and returned to the cylinder.
Almost all newly built properties use an unvented system, this is due to easier installation and because an unvented system is fed directly from the rising main, there is no need for a head tank (cistern). This makes them ideal for new style houses and apartments with little or no loft space.
An unvented hot water cylinder is connected directly to the rising main which means it can supply hot water around the property at mains pressure, the system needs to incorporate an expansion vessel non return valves and many safety features in case of overheating and pressurising.
Unvented hot water cylinders can be heated both directly via an immersion heater or indirectly via a boiler. The advantages are they are can be fitted where there is no facility to incorporate a cold water storage tank (head tank) and it supplies hot water at mains pressure.
The installation of an unvented hot water cylinder needs to comply with local water authority regulations and building regulations, before installation you must inform the water authority and the local building department.