Change the immersion heater thermostat: how to guide

If you have no hot water from your hot water cylinder, or the hot water from it is scalding hot, it may be time to change the immersion heater thermostat. This is actually quite a straightforward task, and new thermostats are easily available from your local DIY store or plumbers’ merchant. In fact, you should be able to complete this task in a matter of minutes. Plus, you won’t even need to drain the hot water cylinder.

Change the immersion heater thermostat

Immersion heaters operate on mains voltage power. Mistakes with electricity can cause serious injury or death. You should never attempt this task if you can’t safely work with electrical fittings. If you’re in any doubt, leave it to a qualified professional. Don’t take any risks.

The first thing to do is to isolate the power supply to the immersion heater. You should always do this at the consumer unit. That’s because some electrical central heating fittings can carry live current, even when they are switched off at the control panel. Let other household members know that you’ve intentionally shut off the power supply so that they don’t turn it back on while you’re working. Before working on electrical fittings, it’s also good practice to verify that there is no current flowing through them with a voltage tester.

Thermostat sizes

Before you complete this task, you should know that immersion heaters come in different lengths, and so do their thermostats: 7, 11, 18 and 27 inches. You’ll obviously need to replace the thermostat with one that’s the right length.

Opening the immersion heater cap

Once you’ve isolated the electrical supply, you need to remove the plastic or metal cap over the immersion heater. You’ll need a spanner or a screwdriver to remove the small bolt or screw which holds it in place.

Removing the old thermostat

First of all, make a note of the existing temperature setting on the old thermostat. Next, disconnect the old thermostat by unscrewing the two screws which hold the live and neutral wires in place. Once you’ve disconnected it, lift it out from the immersion heater.

Installing the new one

The next steps are basically the reverse of the previous ones. Slide the new thermostat into place, and connect the live and neutral electrical connections in accordance with the manufacturer’s instructions.

Use a blade-edge screwdriver to set the temperature on the new thermostat. This is usually 55-60 degrees.

Finally, replace the cap over the immersion heater, and restore the power supply. Congratulations, job done.

Testing the new thermostat

It’s easy to test the new thermostat. First of all, turn off your boiler and run off a few litres of hot water from the hot tap. Then, activate the hot water boost function on your immersion heater. This could be a case of flicking a switch in your airing cupboard or selecting the relevant option from your hot water control panel. If you hear a hissing or fizzing noise, the immersion heater is working. Turn the boiler back on and remember to switch the hot water boost function off on the immersion heater, as heating the cylinder with electricity will be expensive.

If you have an Economy 7 cylinder, you obviously won’t have a boiler. If it’s the top thermostat on the ‘boost” heater that you’ve replaced, it should come on automatically.

However, if you’ve replaced the one on the bottom immersion heater, you’ll have to wait until nighttime for the timer to activate the heater when the cheaper electricity is available.

Did you know?

Thermostats manufactured after 2004 must have a safety cut-out feature. This means that if they fail, they simply don’t work. The immersion heater won’t come on. However, many thermostats made before this time don’t have this feature. In contrast, they will fail in the “on” position, allowing the immersion heater to heat up the water indefinitely.

This is a really dangerous fault that can overwhelm both the hot water cylinder and the cold water tank in the loft with boiling water. If the tank in the loft isn’t supported properly, it can split and dump its contents on people in the bedrooms below.

If your cylinder thermostats originate from before 2004, it could be a really good idea to replace them with modern ones that have a safety cut-out feature.

For more information on the symptoms of this problem when it occurs, check out the article here.