Immersion Heater Rod Thermostats Tripping Off.

Has anyone had incidences of rod 'stats tripping off by the back-up safety cut off with no apparent reason. The copper cylinders had no secondary heating that could overheat the water such as a coil heated by gas or oil.


  • First guess would be that the immersion's normal thermostat isn't quite right - either stuck in the 'on' position or a bit slow in switching off when max temperature is reached.

    Second guess would be the safety-cut out is a bit over sensitive and is cutting out at too low a temperature.

    Third guess would be that inappropriate parts have been used - occasionally (especially if 'top' entry) if the 'stats have the wrong lengths they can be sensing different parts of the tank - e.g. the overheat looking at the very top but the main thermostat sensing somewhat lower (and thus usually cooler) part.

      - Andy.

  • only on Heatrae Sadia, but because they are garbage

  • Yes! I had alot of trouble with this happening after I replaced our immersion heater last winter.

    In trying to solve this turned down the thermostat to the min setting - it still randomly tripped.

    I replaced the thermostat with a (identical) new unit - it still randomly tripped.

    I replaced the entire immersion heater again - it still randomly tripped with both thermostats.

    During this debugging process I modified the outlet on the hot water cylinder to allow me to insert a 300mm K-type thermocouple into the water. By logging this thermocouple I was able to prove that the water temperature never reached anyway even close to the 90C cout-out point of the over temperature trip - yet it still tripped!

    Just when I was reaching the point of despair - it stopped tripping & has behaved itself since the spring (no, this doesn't coincide with the central heating being turned off!). The logging is still in place & have have noticed that the thermostat switch off point is slowly creeping upwards. We are in hard water area & i'm wondering if this is happening as things are slowly being covered in limescale.

  • Yes, had one a couple of weeks ago - cct tested out fine, and the stat was cutting in and out by turning the temp setting at least, so no apparent reason.


  • where in the water is the thermostat measuring,  and orientation of this relative to heater element ? just wondering if it matters if the heater and stat are one above the other in terms of convection - it may not be stirring itself as well as you think.

  • We are in hard water area & i'm wondering if this is happening as things are slowly being covered in limescale.

    I have stripped out quite a few cylinders in London and you always get lumps of greenish limescale in the bottom of them. When replacing the lower immersion heaters, the level of this sediment can often cover the immersion heater (and thermostat) shortening the lifespan due to overheating.

    Fortunately, scrappys pay good money for that stuff, if inadvertantly :)

  • I did think about that... I tried rotating the the immersion such that the element was above the 'stat (this is a immersion mount on the side of the dome of the cylinder so it projects through the water at an angle) as well as above it - it didnt make any difference.

  • This is a top, dome mounted immersion so, although there is some limescale debris clearly visible in the bottom of the cylinder (view through the boss when the heater was removed), it cant be touched by the element.

  • I have had tripping off of the secondary safety cut-offs with 27 inch top entry element 'stats and short side entry heater 'stats. There seems no rhyme nor reason for this. It must be down to bad manufacturing or poor design of the 'stats.


  • The ones that I have had trouble with are the Backer type as shown here.