Immersion tank thermostats blowing or burning out every 4 or

[alec brown66]

My immersion tank (its the type with the header tank on top of it and all covered in green insulation foam) keeps blowing or burning out its thermostats or elements every 4 or 5 weeks! it has a timer clock next to it on the wall, that regulates the economy 7 to just 4 hours a night for the main bottom element. however i have been going round and round with new thermostats not heating the water properly at first, then a visit by the plumber/electrician who re set the thermostat higher, then the thing runs fine for a few weeks, then suddenly either the water starts to get boiling hot, or we are woken up by the tank making a loud bang. also the amount of electric it uses goes right through the roof at the same time.
now i have had the wiring from the rcd fusebox to the control/timer box and on to the booster and main elements replaced, both daytime and economy 7 circuits. i have had the tank drained 4 or 5 times in the last 2 years, it flushed twice as we are in a hard water area, and have had 6 new elements for the main economy 7 element, and are averaging a new thermostat on the economy 7 main element, and sometimes the booster, every 4 or 5 weeks!
nothing my plumbers/electricians are doing seems to work, and i wonder if there is a fault with the timer unit - a horstmann economy 7 quartz- or a major earth fault with the system, power surges, or is it something to do with a fault inside the tank itself, thats causing the thermostats to start not cutting out properly and then not cutting out at all, then blowing?
it is becoming a nightmare, expensive and frustrating!
any one out there who can help?
cheers

 

[meldrew's_mate]

If you had a proper sparky change the supply components he would/should have checked for earth faults so this may be discounted. In any case the RCD should trip at 30mA to earth, compared to 13A for the normal heater load.
When you had the system flushed what did this consist of? Normally I will use a modified spaghetti spoon to remove as much of the scale from the bottom of the cylinder as I can, did your plumber?

All I can suggest at this stage is that you buy a good quality incolloy heater (for aggressive water) and a new thermostat with 80C safety trip built-in. Depending on the diameter of cylinder a side mounted heater is either 12" or 14".
I know how frustrating it can be to spend over an hour draining down, fitting a new heater, and filling up, only to have to wait until the next morning to see if the thermostat is to the user's liking, but start at 60C, and don't go higher than 70C.

 

[alec brown66]

the tank was drained and scooped out, then 5 months ago had a chemical flush as well!
interesting stuff about the arrgessive water type thermostats. would make sense if the plumber was just putting stock standard, but overall cheap and nasty thermostats in, and replacing with the same when they blow.
as for the gas quote, i am on all electric here.

 

[ChrisR]

Chap near me had 3 immersions in a year and was annoyed. Then I put in a bes.ltd.uk one "for aggressive water" and it's still fine 18 months later.
I couldn't see anything wrong with the previous installation (no 3) and I might have just got lucky , but they are a cheap supplier so might be worth a go.

 

[JohnD]

If it is indeed the thermostat failing, then the quality of the water is irrelevant, since the water doesn't touch it.

Perhaps the timer (or some other part of the supply) is worn out or there is a loose connection giving an erratic sparking supply, in the way that light bulbs fail more often when there is a poor contact.