Technical FAQs

How do I measure the BSP thread size of my immersion heater?

Measure the BSP thread size of your immersion heater using the guide below:

Once you have measured the thread, use the table below to select the correct BSP size:

I cannot find the product code for my immersion heater. What information do I need to purchase the correct replacement?

You will need to know the length of your immersion heater, the power rating in watts, the size of the thread of the brass boss and whether your hot water system is vented or unvented.

How do I tell if my hot water system is vented or unvented?

Unvented cylinders work directly from the mains supply of water, and are essentially a hot water storage cylinder. Whereas vented cylinders are connected via a vent pipe, to the cold water storage tank, which is usually located within the loft space of most properties.

How do I find out the length of my immersion heater?

The only way to be certain of the length of your immersion heater is to measure from the underside of the brass boss, to the end of the heating element. However if your immersion heater is fitted in the side wall of your hot water cylinder (horizontally) then it will usually have a length of between 11 and 16 inches. If it is fitted to the top of your hot water cylinder (vertically) then it usually has a length of 18 to 36 inches dependant upon the size of your hot water cylinder.

My Immersion Heater is not working. How do I know what's wrong?
There are five possible reasons for the immersion heater to stop working:
There is no electricity supply to the immersion heater.
The over temperature trip in your immersion heater's thermostat has cut out.
The thermostat in your immersion heater is faulty.
The immersion heater has failed to earth.
The immersion heater has gone open circuit.
Please see our video here on how to diagnose and fix these issues.

Do all domestic immersion heaters require a second level of protection against over temperature operation?
Yes, this can take three different forms:
Single dual funtion thermostat.
Single control thermostat and separate over temperature manual reset button cut out.
Single control thermostat with a second manual reset over temperature thermostat.

Can a thermostat be changed without draining the complete cylinder?
Yes. Please follow the steps below:
Ensure the power supply is isolated before removing the immersion heater cover.
Disconnect the electrical connections from the thermostat body.
Carefully withdraw the thermostat and replace with a suitable new thermostat.
Reconnect the electrical connections, ensuring these are adequately tightened.
Note: with plug in thermostats, resistance will be felt when removing the faulty thermostat. Care should be taken not to twist the thermostat from side to side during it's removal. When replacing the plug thermostat with a replacement, care must be taken to ensure correct location of the male tag into the tighteed female tag.

Is there a reset button on a Thermowatt RTS3 Thermostat?

Yes it is situated in a small recess above the temperature control dial.

What are the temperature settings on a Thermowatt RTS3 Thermostat?

Operating Trip - Single Dot is the lower limit and Double Dot is the higher limit.
Red Dial 52° - 60°C (75°C safety cut-out)
Blue Dial 63° - 71°C (85°C safety cut-out)
Yellow Dial 52° - 60°C (70°C safety cut-out)
RTS Plus 52° - 60°C (90°C safety cut-out)

Is there a reset button on a Cotherm TSE00187 Thermostat?

Yes. It is situated above the number 1 on the temperature scale and is marked 'Safety'.

How do I connect the wiring to a Cotherm TSE00187 Thermostat?

A = Live (Brown) | B = Neutral (Blue)

What are the temperature settings on a Cotherm TSE00187 Thermostat?

1 = 10°C, 2 = 25°C, 3 = 40°C, 4 = 55°C & 5 = 70°C