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Q. How do I measure the BSP Thread Size of my Immersion Heater?



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

Q. Can a thermostat be changed without draining the complete cylinder?


A. Yes

    1. Ensure the power supply is isolated before removing the immersion heater cover.
    2. Disconnect the electrical connections from the thermostat body.
    3. Carefully withdraw the thermostat and replace with a suitable new thermostat.
    4. Reconnect the electrical connections, ensuring these are adequately tightened.

Immersion Thermostat Being Withdrawn Immersion Thermostat In Immersion Thermostat Withdrawn

Note – With Plug in thermostats resistance will be felt when with removing the faulty thermostat, care should be taken not to twist the thermostat from side to side during its removal. When replacing the plug thermostat with a replacement care must be taken to ensure correct location of the male tag into the tightened female tag.

Q. What are the temperature setting on a Cotherm TSE 00187 Thermostat?


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

Cotherm temperature setting

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


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

Cotherm TSE00187

Q. Is there a reset button on a Cotherm TSE00187 Thermostat?


A. Yes It is situated above the number 1 on the temperature scale and is marked (Safety)

Reset button on a Cotherm TSE00187 Thermostat

Q. What are the temperature settings on a Thermowatt RTS3 Thermostat?


A. Operating Trip – Single Dot is the lower limit and Double Dot is the higher limit.

    1. Red Dial 52° – 60°C (75°C Safety Cut-Out)
    2. Blue Dial 63° – 71°C (85°C Safety Cut-Out)
    3. Yellow Dial 52° – 60°C (70°C Safety Cut-Out)
    4. RTS Plus 52° – 60°C (90°C Safety Cut-Out)

Temperature settings on a Thermowatt RTS3 Thermostat

Q. Is there a reset Button on a Thermowatt RTS3 Thermostat?


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

Thermowatt reset button

Q. My Immersion Heater needs replacing how I know what to buy?


A. All our Immersion heaters have a product code either on the side wall of the cover or on the top of the cover. This code will be a series of letters and numbers please see images.

Immersion Heater LabelImmersion Heater Label

Q. Do all domestic Immersion Heaters require a second level of protection against over temperature operation?

A. Yes this can take 3 different forms.

    1. Single Dual function thermostat
    2. Single control thermostat and separate over temperature manual reset button cut out.
    3. Single control thermostat with a second manual reset over temperature thermostat.

Single Dual function thermostat.Single control thermostat and separate over temperature manual reset button cut out.Single control thermostat with a second manual reset over temperature thermostat.

Q. I can not find the product code for my immersion heater what information do I need to purchase the correct replacement?


A. 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

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


A. Unvented cylinders work directly from the mains supply of water and are essentially a hot water storage cylinder; where as 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.

Q. How do I know the Size of the thread on my immersion heater brass boss?


A. All domestic immersion heaters have a BSP thread size on the brass boss. The actual measurement across the diameter of this boss is 0.25 inches larger than the stated BSP thread size. For example a 2.25 inch BSP thread will have a diameter of approx 2.5 inches see attached illustration.

Brass boss head size

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


A. 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 on the size of your hot water cylinder.

Measuring Immersion Heater Length

Q. My Immersion heater is not working – How do I know what’s wrong?

A. There are 5 possible reasons for the immersion heater to stop working.

    1. There is no electricity supply to the immersion heater.
    2. The over temperature trip in your Immersion heater’s thermostat has cut out.
    3. The thermostat in your immersion heater is faulty.
    4. The immersion heater has failed to earth.
    5. The immersion heater has gone open circuit.

Got a question? ask the experts

If you have a question about any of our products simply submit it and we'll get back to you as soon as we can. Alternatively just call us on 0161 727 3713 and we'll do our best to make sure you get the right part.

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