AdrianWint said:

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 :)

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.

Z.

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

https://www.youtube.com/watch?v=9tNo-JZdvMA

Z.

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

https://www.youtube.com/watch?v=9tNo-JZdvMA

Z.

There are numerous Youtube videos showing how to reset the safety secondary cut-off on immersion heater 'stats. The problem must be a common one. 

https://www.youtube.com/watch?v=td3SuGeTefM

Z.