4kw Immersion keeps failing.

[tvrosam]

Hi, hope someone can help.

We have a 260L thermal store. It has 3X3kw and 1x4kw immersions.

The 4kw is the current problem. This and one of the 3's comes on for 7 hours each night (economy 7)

See the image here

As you can see the wire coming from the thermostat into the element is rather worse for wear. It came off when i poked it (all power off of course). And there's lots of moisture in there. The other odd thing is the earth nut has gone a rather attractive share of orange. I suspect some kind of chemical reaction.

Also note that i stripped all the wire back about 10cm last week due to the thermostat failing. I replaced the thermostat with one 'borrowed' from one of the other immersions that we dont usually use.

So all this has happened in a week!

Anyone care to hazard a guess as to the cause of all this?

 

[tamz]

You have a leak somewhere directly above the immersion, you can see the water mark on the top edge.

Wipe around everything with kitchen roll to make sure it is completely dry, wait a wee while and wipe around with kitchen roll again.
Any leak will show on the paper.
It may be that the leak occurs when HWC is cold. Small leaks can seal due to expansion when hot.

You may have to replace that heater by the looks of it.

 

[Agile]

It looks as if the live to the element has shorted to earth and arced thus burning the in sulation.

That connection to the element is welded and will mean a new element unless you make a bodged coinnection to the remaining 20mm.

I would guess the problem actually originated from a faulty connection at the output of the stat.

When running high currents every connection has to be 100% to be low enough resistance to avoid overheating on terminals. Just this evening I saw a 63A MCB which had seriously overheated at the outlet connection. It was only a year old and must have been badly fitted.

Tony

 

[tvrosam]

You have a leak somewhere directly above the immersion, you can see the water mark on the top edge.

Wipe around everything with kitchen roll to make sure it is completely dry, wait a wee while and wipe around with kitchen roll again.
Any leak will show on the paper.
It may be that the leak occurs when HWC is cold. Small leaks can seal due to expansion when hot.

You may have to replace that heater by the looks of it.

thanks. ill give that a go. there certainly isnt going to be any heat from that one any time soon.

 

[tvrosam]

It looks as if the live to the element has shorted to earth and arced thus burning the in sulation.

That connection to the element is welded and will mean a new element unless you make a bodged coinnection to the remaining 20mm.

I would guess the problem actually originated from a faulty connection at the output of the stat.

When running high currents every connection has to be 100% to be low enough resistance to avoid overheating on terminals. Just this evening I saw a 63A MCB which had seriously overheated at the outlet connection. It was only a year old and must have been badly fitted.

Tony

hmm..

I was planning on replacing the element anyway. I realised that the previous fault was the thermostat and so replaced that to save draining the tank etc. Looks like ill have to do it properly

You say you think the connection on the stat output was faulty. If I describe what I did, perhaps you can comment on my technique, as I dont want to have to do this job twice.

First off I noticed that the wire on the neutral connection was discoloured (both the actual wire and the insulation). So I cut back about 100mm or so, stripped 10mm or so of insulation, gave the strands a twist and inserted in the relevant terminals. Screwed down as tight as I dared.

Could I have done any more?
Clean contacts etc with something (wet and dry/some abrasive)?
Tin the wires with solder?

I'm expecting that replacing the element should be straight forward. i.e. Drain tank, unscrew old element with appropriate tool. Put in new element and connect. Im assuming the new one will come with a washer or something to seal it up?


Thanks very much for your help.

 

[Agile]

Once the tinned copper wire has overheated it will never make a good connection on a screw down type suitable for high current.

These things are hardly thick enough when new and the slightest problem and they overheat.

Industrial wire for 16 Amps current are usually about 4 mm² as a minimum and usually solid copper with crimped or soldered connection tags.

Tony

 

[FDJ]

Tin the wires with solder?

No, don't tin stranded wires that are held in screw terminals.
The solder is soft and will flow under pressure and cause a poor contact eventually.

Frank

 

[tvrosam]

Tin the wires with solder?

No, don't tin stranded wires that are held in screw terminals.
The solder is soft and will flow under pressure and cause a poor contact eventually.

Frank

okay. i didnt think that was a good idea. worth checking though.

 

[tvrosam]

Once the tinned copper wire has overheated it will never make a good connection on a screw down type suitable for high current.

These things are hardly thick enough when new and the slightest problem and they overheat.

Industrial wire for 16 Amps current are usually about 4 mm² as a minimum and usually solid copper with crimped or soldered connection tags.

Tony

Would I be better off replacing the wire back to the mct with solid core?
Its only about a meter so no issue on cost.
Not sure if 4mm² would fit into the terminals on the thermostat, but worth a look.