1 year old immersion heater keeps tripping thermal cut out, any suggestions?

[SparkySy]

Hello good people.

I have an ongoing issue with a top entry immersion heater over heat stat that keeps Tripping out. (Built into stat) On a 24hr mechanical timer.
The tank was replaced just over a year ago and with it a new immersion heater and stat.
The original stat worked fine for about 3 months then the O/H stat tripped. I did some tests and all came back good so I reset it and went on my way.
About a month later it did it again so I returned and did the same again. This time it worked for about a week and tripped again.
So I replaced the stat and all was good for about another 3 months or so, then got called to it again.
Tested it again and the circuit and all was the same as the previous test, I put it down to a bad stat and swapped it under guarantee.
Again it was fine for a while about 4 months or so and it did the same again.
I went back with a plumber and we spent a good couple of hours playing around and couldn't get I to trip out, so we left and all was fine until about a week ago when I got called to it.
Again it was tested and all results were good so I replaced it once more with a different make.
Got called to it this evening again thermal overload tripped!
I have run out of ideas, usual things checked, stat set to 55 and is cutting out at that.
Insulation resistance test came back 999m ohms on the circuit and heater hot or cold, element resistance 21 ohms, earth loop 0.33 ohms, voltage 241v.
All connections checked for tightness.
Any other ideas guys?
Cheers Sy

 

[littlespark]

Is the tank being heated by any other means? Gas boiler, fire back boiler etc?

 

[SparkySy]

Is the tank being heated by any other means? Gas boiler, fire back boiler etc?

No other heat source. All electric property.

 

[7029 dave]

Mmm the resistance of the element seems about right, may do an IR test on the element down to to the copper cylinder.

 

[brianmoooore]

There will be a label on the immersion head cover, "Do not cover". Is this heeded by the customer?

 

[bill01803]

What make is the stat?

 

[DefyG]

stat set to 55 and is cutting out at that.

But is the main stat cutting out at 55? does the water seem excessively hot? Can you measure temp?
Maybe the stat tube was bent on installation and is touching the element tube?

 

[Avo Mk8]

No other heat source. All electric property.

Is there a Surrey flange as the top cylinder connection, and/or could there be something up with the pipework that allows air to collect at the top of the cylinder? Apparently air at the top of a cylinder is a known cause of the effect you have.

 

[timhoward]

Tricky one.
Regarding air in the cylinder - if there's a kitchen mixer tap that's mains cold, it could be worth turning both taps on with a towel wrapped around it to force the cold back up the hot....and getting them to do that every few days to see if it changes anything.

 

[brianmoooore]

Tricky one.
Regarding air in the cylinder - if there's a kitchen mixer tap that's mains cold, it could be worth turning both taps on with a towel wrapped around it to force the cold back up the hot....and getting them to do that every few days to see if it changes anything.

Might if it's the type with dual flow, but a lot of modern taps are single flow with non return valves in the feed.

 

[timhoward]

I'll be watching this thread with interest as I've had two of these.
One was bottom entry IH, and we eventually put it down to limescale on an impressive scale.
The other top entry one...well...we tried 2 or 3 new stats, even in desperation tried changing the IH unit, and then the famous John Prescott winter occurred where many people got a subsidised condensing combi including my customer, and we never did solve it.

 

[SparkySy]

Thanks all for the suggestions!

Customer doesn't store anything else in the cupboard with the tank, not even towels or sheets.

Heatrae stat is the newest one, old one was stieble.

Water at the tap doesn't seem over hot.
Possibly bent stat tube but that would need the plumber to come back and remove it. I may suggest that!

No mixer taps in the property.
I can't see any way that air could be getting trapped in the top of the tank as it's nearly flat and there isn't any gurgling when the taps are in use.
I am at a loss.
Contacted the customer this morning and it was fine last night.