How to correctly test if a Cotherm TSE or TSR plug in type immersion thermostat is working or not?

** DO NOT EVEN BEGIN ANY OF WHAT IS CONTAINED IN THIS BLOG UNLESS YOU ARE SUFFICIENTLY QUALIFIED OR COMPETENT **

Safety first: Always turn off the power to the immersion heater at the breaker before performing any tests.

Gather your tools: You will need an 8mm spanner to access the thermostat, you will then need a screwdriver and a multimeter to test the thermostat.

Step 1: Check and reset the safety cutout

Before any electrical testing, check if the thermostat's built-in overtemp safety has been tripped. This is a common and easy-to-fix issue.

1.  Turn off the power to the immersion heater.

2.  Remove the plastic cover from the immersion element by undoing the M5 nut. The thermostat will be plugged directly into the heating element.

3.  On the Cotherm thermostat (white or blue), look for a small reset button, marked "Safety". It may be black or red and is found near the temperature dial.

4.  If the button has popped out, press it firmly with a non metalic probe until you feel and hear a click. This resets the thermal cutout.

5.  Replace the cover, restore power, and see if the water begins to heat.  If the button keeps tripping, there may be a serious issue, and the thermostat should be replaced. To find out if it is the thermostat or another issue, carry out a continuity test described next.

6.  Finally, look for any burn or scorch marks.  If there are any signs visible, the thermostat should be replaced.

Step 2: Test for continuity with a multimeter

If resetting the cutout does not work, use a multimeter to check the thermostat's internal switch.

You may find it easier to remove the thermostat for testing.

• Although the thermostat may seem fixed to the element, it is removeable separately.
• The thermostat is fixed to the heating element via two spade connectors.
• The units are designed to have a really tight fit, so it may need some force.

Test preparation

1.  Isolate the power. Turn off the immersion heater circuit breaker at your home's main fuse board.

2.  Verify isolation. Use a voltage tester to ensure no power is reaching the heating element before you touch any wiring.

3.  Access the thermostat. Remove the heating element cover to expose the thermostat and wiring.

4.  Disconnect the wires. Carefully disconnect the wires that run to the immersion heater element from the thermostat terminals. It can be helpful to take a photo of the wiring first.

5.  You can then remove the thermostat from the heating element.

6.  Set your multimeter to the lowest ohms setting (Ω) or the continuity setting, if it has one.

7.  Test your meter by touching the probes together. It should show a reading of zero or beep if set to continuity, confirming it works correctly.

Performing the electrical continuity test

For additional information on continuity tests, see the video at the end of this blog.

•  The thermostat is a temperature activated switch.

•  You will need to test that the unit is both making and breaking contact.

•  The TSE & TSR thermostats are single pole on the regulation. 

•  Set your electrical tester to continuity, with one probe on terminal A.

•  The second probe on the LIVE spade connector.

•  Now  turn the temperature dial from 1 to 5 and back again.

•  Your tester will buzz when contacts are made and be silent when they are broken.

** BE AWARE IF THE AIR TEMPERATURE IS BELOW 16°C, THE ROD SECTION MAY NEED TO BE WARMED **

Alternatively test method

•  You can link the spade connectors and test across terminal A & B.

•  This method allows you to be confident there isn’t an issue with current flow.

•  Combined live & neutral test.

•  Confirm the over temp safety has not activated and/or reset correctly.

When to replace a faulty thermostat.

•  If there are any signs of burning or scorching, always replace the thermostat. 

•  Damage to the outer plastic case or thermostat stem, always replace the thermostat.

•  If there is no continuity, always replace the thermostat.

•  Circuit won’t break when turning the temperature dial, even after warming the stem, always replace the thermostat.

To achieve an EXACT like for like replacement you’ll need the information printed on the side of the thermostat.

No problems found after testing

•  If the visual and electrical test were fine, then the issue will be elsewhere in the system. 

•  Future testing blogs will include wire in thermostats, capillary units as well as the heating element itself.