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What are the parts of an immersion heater element?

What are the parts of an
immersion heater element?

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Labelled diagram to show the location of the key components of an immersion heater element

When attached to the cylinder…

Image showing the casing end of an immersion heater element When attached to a hot water cylinder, the only part of an immersion heater element that is visible is its casing. The element coil and thermostat extend inside the cylinder.

Immersion heater fibre washer

Image of the fabric washer that fits around the base of your heating element and keept the hot water cylinder from leaking A fibre washer (also known as a fabric seal) prevents leaks by plugging the gap between the immersion heater element and the copper boss or flange it has been screwed into.
Image to illustrate that a fibre washer expands when it gets wet The material from which the washer is made expands when it gets wet, which means it has a built in anti-leaking mechanism!

The washer will conform to the surfaces it is sandwiched between – in other words, it changes shape to mould around any minor lumps and bumps.

Wonkee Donkee say that fabric seals work better when you don't use jointing compound or PTFE tape

Immersion heater element coil

Image showing an immersion heater element coil The element coil is the part of the immersion heater element that heats the water.
Image showing a kettle to reinforce the message that an immersion heater element works in the same way as an element in a kettle When switched on, the element coil works in exactly the same way as a kettle. Electricity passes through the wire in the coil, which becomes hot. This heat is then transferred to the water in the cylinder.

Immersion heater thermostat

Image of immersion heater element thermostat showing both the part that extends into the tank and the part that is visible under the case The thermostat is the only component in the immersion heater element that extends into the hot water cylinder but is visible beneath the casing.
Image to show the part of the thermostat that fits inside the hot water cylinder The long rod at the back of the thermostat allows the device to take the temperature of the water inside the tank.

The rod of  the thermostat fits into a sleeve so that it can be removed easily and without draining the cylinder.

Image of the part of a thermostat that sits outside the cylinder and underneath the immersion heater element casing The head of the thermostat is usually plastic and contains the control dial that allows the user to select the temperature of the water in the tank.
Image showing a thermometer reading at 60-65 degrees This is usually set between 60˚ and 65˚ centigrade.
Image of a two for one logo to illustrate that there is a second thermostat inside an immersion heater element An immersion heater element thermostat unit actually contains two thermostats. One allows you to manually adjust the temperature of the water in your hot water cylinder. The other is a limit thermostat that is built into the unit as a fail safe.
Image of a steaming teapot to illustrate boiling water, which results from a broken limit thermostat If the main thermostat fails, the limit thermostat will ensure that the heater cuts out at around 85˚ – 90˚ centigrade in order to prevent the water in your cylinder from boiling. It does this by cutting power to the element. When this happens, the immersion is said to have tripped.
Image showing where the reset for the limit thermostat is on an immersion heater element thermostat component This is a safety device and will not reset automatically. Manually reset the trip by inserting a small object, such as a paper clip, into the reset hole and push to reset.

If the thermostat trips again, or on a regular basis, call a qualified plumber for advice.

Image showing a mountain of clothes to show a possible reason why a limit thermostat might keep tripping Sometimes this limit thermostat can trip accidentally (particularly if you cover the immersion heater casing with clothes) which can cause heat to build up and fool the thermostat into thinking the water in the tank is hotter than it actually is.

So make sure you don’t bury your immersion heater element in laundry! Never cover the thermostat. keep clothes and insulation well away to avoid nuscience tripping.

Immersion heater element base

Images showing the base of an immersion heater element including the screw thread The base of the element has a screw thread that helps the immersion heater element attach securely to the hot water cylinder without leaks.

Inside the immersion heater casing

Image of the inside of an immersion heater element - what it looks like beneath the outer case WARNING – removing the casing will reveal live parts.
When replacing an immersion heater element, you will need to remove the casing by removing the nut at the top. This will then expose the wiring and the thermostat control.
Labelled diagram of the inside of an immersion heater element showing the location of the live, neutral and earth posts and thermostat There are three posts beneath the immersion heater element casing. A red or brown insulated wire connects the Live post of the element to the thermostat. The mains cable will have three wires, a brown (Live) wire connected to the other side of the thermostat, a blue (Neutral) wire connected to the Neutral post of the element, and a green and yellow earth wire attached to an extended screw which is usually the same screw that is used to secure the element casing.

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