When hot water is not being provided by a central heating boiler, it is normally provided by an immersion heater. This is screwed into a socket, or boss, in the top or side of the hot water cylinder and provides a central supply of hot water for the whole house.
The principle of operation is very similar to the electric kettle, but on a larger scale. The normal configuration is one element and one thermostat, both in the side of the cylinder. The element consists of a metal rod, normally with a copper sheath, although some have a titanium or incalloy sheath. These are more expensive but can prolong the life of the element, particularly in hard water areas. When the immersion heater is switched on, an electric current flows through the element, causing it to heat up. As it does so it heats up the water in the cylinder. The required water temperature is set on the thermostat. When the water reaches the desired temperature the thermostat will interrupt the electrical supply to the element, until the water temperature falls below the set level, when the current will again be allowed to flow.
Variations on this format are dual and twin element heaters. A dual element heater is normally screwed into the top of the cylinder, one element being shorter than the other. This is used for economy, so that when only half a tank full of hot water is required the short element is used, only heating the top half of the cylinder. The dual element type only has one thermostat. With the twin element type, the operation is similar but both elements have their own thermostat. A third configuration would be two elements screwed into the side of the cylinder, one above the other. The top one would be used in economy mode, again heating only the top half of the cylinder. If a full tank was needed both would be used.
Whilst many homes use the central heating system to heat the hot water, the immersion heater is used extensively during summer when heating is not required, to prevent running the boiler purely for hot water.
