Understanding – Immersion heaters – hotwater cylinders

Immersion heaters, otherwise known as electric water heaters or hot water cylinders, double as water heaters and hot water storage systems. Most combine a number of heat sources to maximise the flexibility of the system.

The typical hot water storage cylinder has two electric elements and one or more hot water heat exchangers. These electric elements and heat exchangers heat water in the cylinder to be used for hot water in the house, operating in an order which best provides the hot water as efficiently as possible.

Firstly, most immersion heaters are heated by the main boiler system. A heat exchanger inside the cylinder is piped from the boiler, though is often on a different circuit than the heating system (so the heating can be switched off without switching off the hot water from the immersion). As electricity is more expensive than other fuels in most countries it is cheaper to run than the electric elements, and so is the default option.

Next, a small electrical element located towards the top of the cylinder is used to heat small quantities of hot water. In many switching arrangements this the control for this element is marked “sink”, though it is generally sufficient to provide water for a shower.

A higher rated electrical element is located lower down in the cylinder and can be used to heat the full cylinder (often marked “bath”). Electrical heating is generally quite easy to use and for this reason it is quite popular, but it is by far the most inefficient from an environmental point of view, and also the most expensive.

In many cases renewable heating systems such as solar panels or geothermal systems are also piped into the immersion heater. Heat exchangers reject heat from these systems into the immersion heater, heating in a similar fashion to the central heating system. In some cases these renewable systems can not heat the water to a sufficient level, and so require conventional methods to heat the water the final amount. While this is not an ideal situation, the energy required from the conventional methods to heat the water the last bit is significantly reduced.