Immersion heaters

Summary: Learn how to replace an element, replace a single or twin element and set a thermostat.

Immersion heaters are designed to heat water. They are made up of a cylindrical copper tank with either a single or twin element inside and are fitted with a thermostat control.


Single and twin elements

Single elements are available in lengths ranging from 280mm (11in) to 800mm (2ft 7 ½ in). The longer elements screw into the top of the cylinder (top-entry) while the shorter elements are fitted as a pair through the side of the cylinder (side-entry). Both elements will have their own thermostat.

Twin elements have a short element at the top for heating small volumes of water quickly and a long element for heating the whole cylinder. Twin elements are controlled by a single thermostat.


Replacing an element

If you notice a lack of hot water or that the water is slow to heat up, this could indicate a burnt out element or a build-up of limescale around the element.

  • Inspect your immersion heater to see if you have a top entry or side entry element and buy the correct replacement.
  • Switch off the boiler.
  • Turn off the electricity supply at the consumer unit.
  • Cut off the water supply to the cylinder. If there is no isolating valve on the cylinder supply pipe, you will need to drain the water tank.
  • To do this, turn off the water to the supply cistern. If you cannot find a stop-tap or it is too stiff to undo, you can prevent water flowing into the cistern by placing a length of wood across the top of the cistern and tying the ballvalve to it. This will prevent water entering the supply tank.
  • Drain the supply pipes by turning on the bathroom tap.
  • Locate the drain valve which will be situated either on the cylinder or on the supply pipe at its base.
  • Using a jubilee clip attach the end of a garden hose to the drain valve, then place the other end of the hose in the bath. Drain the cylinder by opening the drain valve. A top-entry immersion heater only requires 4.5 litres of water to be drained from it. A cylinder with low side entry elements will need to be drained completely. When the cylinder is drained close the drain valve.
  • Unscrew the immersion heater cover. Make a note of which cores are connected to which terminals and using a screwdriver disconnect them.
  • Unscrew the heater using an immersion heater spanner or ring spanner. For a lagged cylinder you may require a box-type immersion heater spanner which is turned with a tommy bar. If the heater is stiff, spray penetrating oil around the joint or heat the area with a hairdryer.
  • Remove the immersion heater.
  • Before inserting the new immersion heater into the cylinder ensure there is a fibre sealing washer on the thread of the tail of new immersion heater. If there is no washer wind PTFE tape around the thread instead.
  • Insert the new heater and screw it securely into position with the appropriate spanner.
  • Turn the water back on to check for leaks around the heater joint. If there is a leak tighten the immersion heater slightly.
  • Unscrew the cover of the new immersion heater and reconnect the flex, connecting the cores to the appropriate terminals in the heater.
  • Set the thermostat and replace the heater cap and restore power to the circuit.

Unscrew the immersion heater cover.

Make a note of which cores are connected to which terminals and use a screwdriver to disconnect them.

Setting a thermostat

To control the temperature generated by the immersion heater use a screwdriver to turn the thermostat dial to the desired setting. In an area with hard water, the maximum temperature should be 60°C or up to 70°C in a soft water area.

If you have two thermostats, the one controlling the daytime temperature should be set between 50°C and 55°C.

Immersion heaters with two elements are sometimes fitted with a switch on the thermostat with two settings ‘sink' and ‘bath', but this type of unit does not allow for any variation of temperature for the two settings.

Use a screwdriver to turn the thermostat dial to the desired setting.