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Topic Title: Immersion Heater - High Limit Stat Keeps Tripping
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Created On: 16 March 2010 11:15 PM
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 16 March 2010 11:15 PM
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andy4748

Posts: 130
Joined: 08 June 2006

The customer has told me that their immersion heater element (top entry 30inch with 18inch rod thermostat) was replaced because it went defective recently, as it was scaled up and took the RCD out.

Since the element and thermostat was replaced together, the high level cut out keeps operating. I was called to check it out, and replaced the rod stat with another type, just in case it was defective, and has the high temperature cut-out fitted - as all do now.

The problem is still there with tripping on high temperature. The temperature to the water is set about 60 degrees.

Can anyone shed ideas a possible reason why the cut out keeps opertating! I will try turning down the stat, but not conviced this will resolve the problem.

FYI - The immersion is full, and is vented. This is the only form of heating for the water, so cannot consider any thing else. The tank is well insulated.

Has anyone else had a similar problem, and how did they resolve it?

-------------------------
Andy Dunn. MIET

http://www.inspectrix.co.uk/
 16 March 2010 11:51 PM
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typiod

Posts: 200
Joined: 09 February 2008

Sounds to me that the cylinder has a big build up of scale inside it and the element is in the scale.
 16 March 2010 11:51 PM
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NozSpark

Posts: 47
Joined: 14 May 2007

Could it be that the HW coil from the boiler is overheating the tank and tripping the over temp stat???

I've had that before!
 17 March 2010 12:16 AM
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FizzleBang

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A few weeks ago I went and connected a brand new Element and stat installed by the plumber.

It worked nicely until I left. The customer got one tank of hot water then nothing.
I Went back and re-set the over temp cut-out on the element and it worked until the water reached a certain temeperature.

Decided that the new element had a duff cut-out and sent the plumber round to replace it. Job done.

The cut-out was simply operating way too early and before the thermostat operated. Just bad luck on picking the wrong one off the shelf!

Paul

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To me, to you
 17 March 2010 07:42 AM
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andy4748

Posts: 130
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I will check with the plumber regarding scale build up within the immersion and also carry out another visual in case I missed a secondary means of heating, but I am sure this was the primary means of heating.

-------------------------
Andy Dunn. MIET

http://www.inspectrix.co.uk/
 17 March 2010 08:31 AM
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FizzleBang

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On the immersion heaters I've see lately the cut-out is in contact with the water. So it can only be water temperature that is causing it to operate.

It's probably a defective cut-out (even if it is new) that is tripping below the set temperature of the thermostat.

If the plumber is going to take out the element again to check for scale he'd do well to fit a different element while he's at it. Save himself a third visit.


Paul

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To me, to you
 08 April 2010 01:52 PM
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primo

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Andy,

Did you ever resolve this?

I've got exactly the same problem at the moment and I can only think that it's the tank that's causing the TCO to keep tripping? Someone else has replaced the element already (so I'm told, and it looks new!)

I've replaced the stat already.
 10 August 2010 09:02 AM
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unshockable

Posts: 853
Joined: 18 May 2007

Me too! Brand new Element and tank; scale can't be an issue after three months. I have only had to reset once so far.

Two questions; do all systems need this fail safe cut out or are you able to leave it out in some circumstances? Also, why have the water stats lost the temperature marking? 1-4 seems the only indication; what should it be set on?

Thanks

Simon
 10 August 2010 09:30 PM
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Igot3ears

Posts: 364
Joined: 24 December 2004

Guys

I had exactly this problem a couple of months ago with a rented property. Tenant had no hot water. Found the high limit stat had cut out. Re-set it and checked with a tong tester that the element was drawing 11.8 amps. 4-days later was told tenant has no hot water again although it had worked for 4-days. Attended again and re-set the stat and turned the normal stat down to about 50 degrees. 4-days later another call from the now disgruntled landlord to say the tenant had no hot water again. I told him I would have to bite the bullet and drain the tank and change the element so that's what I did. Have heard nothing since so I guess that solved the problem.

3ears
 10 August 2010 11:02 PM
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moorhouj

Posts: 60
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I have experienced similar problem - I suspect this is a design issue - in the case I was looking at the cut out was screwed to the immersion boss, so I wonder if there is heat transfer from the element to the cutout
Jack
 10 August 2010 11:14 PM
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peteTLM

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Ive swapped a few thermostats before- they sit within a metal jacket and dont touch the water so can be just pulled out and replaced.
Personally i hate changing the heaters themselves as ive ripped a tank before due to it being stupid tight!
They (cutouts) are there as the tank is an atom bomb waiting to go off. 2 years ago there was an incident where a child was killed by an overheating tank.
Ive been out to a heater stuck on. The water was so hot it was streaming out over the cold tank via the vent at an unbelievable temperature. The whole loft was thick with steam, and all the cold taps were at 100'c.
Scale is formed when the water goes above 55', so its always a good idea to keep immersions set at that to prolong their life a bit.


P

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----------------------------------------
Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry
 11 August 2010 10:02 AM
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AJJewsbury

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Scale is formed when the water goes above 55', so its always a good idea to keep immersions set at that to prolong their life a bit.

Although many recommend min 60 degrees to stop bacteria (e.g. legionella) growing in the DHW storage.
- Andy.
 11 August 2010 03:13 PM
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moorhouj

Posts: 60
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I assume we were talking about the safety cut-out - there seem to be two types, integral with the thermostat itself, or part of the element. The latter are screwed onto the immersion flange, and it is these I have experienced problems with.
Jack
 11 August 2010 05:30 PM
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chrishaworth

Posts: 247
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ESC guide to periodic codes suggests a code 2 if there is no cutout and the cold water tank is plastic, so I suppose that no cutout is acceptable if the cold water tank is metal. If the customer has a fire-back boiler and the tank is in the loft, what is the alternative?

Cheers
Chris
 12 August 2010 10:05 AM
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unshockable

Posts: 853
Joined: 18 May 2007

Originally posted by: AJJewsbury

Scale is formed when the water goes above 55', so its always a good idea to keep immersions set at that to prolong their life a bit.


Although many recommend min 60 degrees to stop bacteria (e.g. legionella) growing in the DHW storage.

- Andy.


That would be a bad enough if I had a temperature indication. 1-4; 4 marks between + and -. Why would these be more common now?

The client wants the cut out although the tank is metal. Thanks for that Chris. I am not sure why we need to step outside BS7671 in a PIR. What on earth will we be responsible for next? I' ll run out of room in the Limitations.

The last one I had a problem with was 2 * thermostats, in that case we changed both. Haven't been called back so all good.

Was this fever for cut outs rushed in before the technology? Would thermal fuse links work better? The false positives of these mechanical cut outs while expensive are not as expensive as a false negative of a less sensitive device. DISCUSS.

Thanks to all

Simon
 17 July 2012 10:11 AM
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solarfix

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Joined: 17 July 2012

On one occasion I was sent to trouble shoot immersion heater problems that had been ongoing for years and had defied best attempts by various plumbers and electricians.One tripping out on the high limit stat continuously.this was a flat topped cylinder top entry immersion heater.The cylinder was solar surrounded by 50mm of insulation .The immersion heater boss on the top was a 50mm shaft the depth of the insulation with the immersion screwed in the top .This formed a 50mm air bubble round the top of the element and thermostat that could not escape.Immersion heaters should be totally immersed.It kept overheating and tripping.
The 2nd case had destroyed elements and thermostats for 10 years.
It was a top entry immersion heater in a domed cylinder.I drained the
cylinder and took out the immersion heater element which although still heated uncontrolably was completely shot.When I looked through the immersion boss into the cylinder I could see that the plumber who installed the cylinder had pushed the hot water supply / vent pipe 100mm down through the top of the cylinder like a dip pipe causing a 100mm bubble in the top of the cylinder which cooked the element
and thermostat and high limit stat.I hope this helps people

Jacko
 17 July 2012 01:02 PM
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potential

Posts: 1286
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Trapped air in the cylinder is the first thing I'd suspect too.
 19 July 2012 01:17 AM
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sparkyaj

Posts: 57
Joined: 04 June 2012

I would agree with Potential. Either trapped air in the system or high resistive connections from your DP isolator maybe. Supply voltage lower, causing heat to build up exceeding the 85oC. or faulty stat.

Had a few showers recently with air pockets in the water supply casuing C/O to trip. Being careful becuase of H&S, no reason why you couldnt take the stat out and pop it into a mug of boiling water and see what it does.

On a solar thermal course last week as i understand, you can now buy a testing device which heats the stat up to test its function and cutout at 65oC and 85oC. Dont know if anyone here has one yet?
 19 July 2012 07:08 AM
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ebee

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Joined: 02 December 2004

I had exactly the same problem with new stats.
A right pain.

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Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 19 July 2012 08:17 PM
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kaichung

Posts: 370
Joined: 02 December 2003

Side entry elements are better.

Most of the heat tends to rise to the top of the cylinder, stressing the connections and thermostat etc. On my megaflow, the backup immersion element is a side-entry type, mounted just below the Central Heating Coil, which helps to ensure the immersion is not stressed when the boiler is powering the megaflow. The immersion, being at the base of the megaflow, tends to be relatively cool, due to the input of cold mains water at the bottom, when hot water is drawn off at the top of the megaflow.

I am surprised that people still tend to use top entry elements these days (plumbers merchant's window display)??
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