immersion heater with thermostat

Immersion Heater Thermostat Replacement Guide


This is an immersion heater thermostat replacement guide for anyone who is looking to replace their immersion heater thermostat and the heating element.

I am a professional plumber and heating engineer – I get called out to boiler repairs and jobs like this regularly.

Table of Contents

The Electrics

Multimeter for Immersion Heater Thermostat
Multimeter

First, I used a multimeter to test the immersion heater thermostat and found it was not getting the correct reading.

This confirmed it was in fact the thermostat that needed to be replaced.

Isolate The Power Supply

The next step was to isolate the electric supply at the main consumer unit (fuse board) where all the fuses were.

You should only isolate the electric circuit (or fuse) that the immersion heater thermostat is connected to.

Consumer unit
Modern Consumer Unit

It’s best not to turn off all the electric in the house because you don’t really want to turn the freezer off if you don’t need to.

Which Electric Circuit?

It may have its own electric circuit  with just the immersion heater thermostat connected.

Or, you may find it is connected to the ring main circuit, which has all the plug sockets connected.

Sometimes, it may even be connected to the light circuit.

If it’s not labelled at the consumer unit, keep turning one fuse off at a time until you have confirmed which one it is.

You need to keep trying until you find which circuit has the immersion heater thermostat connected.

You can do this by using a no contact volt pen, or sometimes you will have a light on the fused switch (on the wall) that is connected to the immersion heater thermostat cable that will turn off.

Drain Your Cylinder

The next step is to drain down the copper hot water cylinder. You will need to find the isolating valve, or stop tap, on the pipe that supplies the water tank that supplies the hot water cylinder. It’s sometimes above the cylinder but could be in the loft with the supply tank.

If you can not find one, then you will need to turn off the main stopcock that will turn off all the water to the property. Then, open all the hot taps to drain down the water from the header tank.

Brass drain off valve
Drain Off Valve

Then, you will need to find the drain off valve. This will be a little brass valve connected to a pipe at the bottom of the hot water cylinder. Once you have found this, connect a hose and run the hose outside to a drain ready to drain down. You will need to use an adjustable spanner, or pliers, to turn the square nut on the drain valve anti-clockwise until the water comes out.

Draining down the cylinder fully will take quite a while, so you may as well do something else while it’s draining down. When it’s fully drained down, you should be able to shake and slightly move the cylinder easily. You will be able to tell that it’s empty.

Remove Old Immersion Heater Thermostat

First, you need to remove the cover off the immersion heater thermostat. You can do this using an adjustable spanner to remove the small nut.

Then, you will need to remove the cable using a small flathead screwdriver. I recommend getting a VDE insulated screwdriver that will protect you from any electric shocks.

To remove the actual heating element from the hot water cylinder, you will need a blowtorch and an immersion heater box spanner. I don’t recommend using a normal immersion heater spanner, make sure it’s the box spanner type.

Immersion heater thermostat box spanner
Immersion heater thermostat box spanner

Some people like to leave the water in the cylinder to keep it more solid and easier to turn the spanner, just to get crack it open and start the process off. But you risk getting a leak doing this. You might rip the sealing washer, which can prevent you from tightening it back up if it leaks. Or worse still, tear the copper cylinder and ruin your day.

You will need to heat the brass heating element using a blowtorch. This is because the heating element seizes on the cylinder and you can easily damage the cylinder if you don’t use a blowtorch to heat it up first. Just keep the flame on the brass inside the heating element for a few minutes. Keep moving it around so it heats all the brass up evenly.

Immersion Heater Thermostat and Copper CylinderThen, you put your box spanner over the heating element and use the bar that comes with it to turn it anti-clockwise. You might need to use a hammer to give it a couple of taps to get going. You should then be able to turn the rest with your hands.

Always use caution when removing the immersion heater thermostat heating element. You can easily damage the copper cylinder, and you might end up having to replace the full cylinder.

That’s the hard part done, now for the easy part.

With the old heating element removed from the hot water cylinder, try to clean up any dirt and debris from inside and around the opening.

Once this is done, time to fit the new immersion heater element and thermostat. You should get a sealing washer with it, I would also recommend using jointing compound on the thread to further stop any leaks.

Next, you need to screw in the new element. Tighten it up as much as you can with your hands, then use your spanner to tighten it.

Immersion Heater Thermostat Wiring

New Immersion Heater Thermostat in Old Cylinder

Then you can insert the thermostat and wire it up using the instructions supplied with the thermostat.

  1. The live wire goes to the thermostat
  2. The neutral wire goes to the heating element
  3. The neutral wire from the element goes to the thermostat
  4. The earth wire goes under the nut on the threaded rod

Then put the cover on and tighten the small nut up.

All done.

Now just close the drain off valve where the hose is connected, close all the hot water taps, and turn the water supply back on. Check for leaks and if all is OK, turn the electric supply back on.


6 thoughts on “Immersion Heater Thermostat Replacement Guide”

  1. Yes Richard, you should definitely get a plumber if you haven’t done anything like this before.

  2. Bought a new thermostat, {Backer18}. Won’t push into the hole that comes out of the element. If it just sits there it leaks.

    Anyone got any experience of this problem and how did you overcome it?

    Any help would be most appreciated.

  3. Hi Chris, I would have to have a look at it to give you any advice. You can send me a photo if you like

  4. Um, sorry to be electrically declined but I found this side but it doesn’t say which way is which on the thermostat! Which side, left or right, does the live brown wire connect to? There are two holes on the thermostat, one on the left and one on the right. ( This is with the blue curved bit of the thermostat pointing upwards as per your picture… Your diagram appears to show the black little neutral connector wire going to the left… but I’m sure mine came out from the right of the old one, so I’m confused, help!

  5. Hi Nick, if you give me the model number of your new thermostat, I should be able to help no problem.

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