How To Install Immersion Heaters

One great advantage of an immersion heater fitted to your hot water cylinder is that it can be used to supply as much or as little hot water around the home as you need at any particular time. It can also supplement other heating systems such as gas, oil or solid fuel. With care, you can fit it yourself.

One of the most convenient methods of supplying hot water in the home is by installing an immersion heater in your hot water cylinder, although it is a fairly expensive form of heating to run if used constantly. Heaters are made in a range of lengths and loadings to suit the different types of cylinder and to give varying quantities of hot water. On some the heating element is coated with a titanium sheath; this is specially for use in hard water areas where corrosive substances in the water would adversely affect an ordinary element without a special coating.

The length of the heater can range from 245- 914mm (10-36in). The type most commonly fitted is the single-element one which will heat the whole cylinder. It is, however, more economical to have two elements, one fitted near the top and the other about 50mm (2in) from the bottom of the cylinder. The top element heats enough water for hand or dish washing and the bottom one heats the whole cylinder, when for example you want a bath. There is also a dual heater, with a short and long element, which operates on the same principle. Both systems are independently switched so you can have either or both elements on at any time to suit your needs.

Special long heaters are needed for indirect and self-priming cylinders and for rectangular tanks. Hot water cylinders designed to work on the electricity company’s White Meter tariff have either two heaters or a dual-element one to heat part or all of the water.

Wiring heaters

Common ratings for the heater are 1, 2 and 3kW, but because the immersion heater is considered to be a continuous load, whether you keep it switched on all the time or not, it must be supplied by its own circuit direct from the consumer unit using 2.5sq mm cable from a separate 20amp fuseway. The cable runs to a 20amp double pole switch (usually with a pilot light) which should be sited near the heater and close enough for anyone to operate if they are adjusting the thermostat. The wiring from the switch to the heater should be a 20amp rubber heat-resistant flex.

If you are installing two heaters in one cylinder, your double pole switch should incorporate a second switch which allows you to have either one or both heaters working. In this case a separate flex must run to each heater from the switch.

Installing heater

Connect the cable from the consumer unit to the IN terminals of the switch and connect one end of the flex to the heater, making sure the electricity is switched off at the mains. Turn off your water supply at the cold water storage cistern, drain the cylinder and remove the relevant boss, into which the heater will screw. The threads are sealed first by winding PTFE tape against the direction of turn, or by using hemp string and a non-toxic plumbing compound. Tighten the heater against its sealing washers, using a large wrench — but never over-tighten. Connect the free end of the flex to the OUT terminals of the switch, turn on the stopcock at the cistern and the electricity at the consumer unit and, after waiting for the cylinder to refill, switch on.


Be sure to clamp the flex at both ends in the cord grips fitted to the heater and switch, and use the correct flex grips to secure it to the walls. Otherwise the flex might become entangled in linen (if in an airing cupboard) and be pulled away.

Cutting boss

Most cylinders are now made with at least one boss fitting. However, if you have a direct copper cylinder, without a boss, in good condition you can cut a hole to take the heater. Mark out the required diameter hole and cut it with a hole saw fitted to an electric drill. Alternatively, drill a series of holes around the edge of the circle, knock out the centre and file the edge smooth. You can buy a patent fitting that includes the boss, a thread to take the heater thread, washers and a retaining nut. Don’t try to cut a boss in an indirect self-priming cylinder.

Adjusting thermostat

You must turn off the heater before attempting to adjust the thermostat setting. You reach it by unscrewing any screws holding the cap in place. Use a screwdriver to obtain the required setting, generally 60°, 71° (or 82°C (140°, . 160° or 180°F). In hard water areas scale tends to build up in cylinders at temperatures above 60°C (140’F) which is the lowest acceptable temperature for normal domestic purposes. The thermostat automatically turns off the power supply when the required temperature is reached.

Using time switch An immersion heater can be controlled by a special time switch, which offers two on and off periods in each 24 hours.

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