Unvented meltdown

[wads]

Corroded terminal

• wads
• 17 Jul 2015

Hi. I moved into a new house in feb 15.

The boiler was broken so we decommissioned it and the heating. We reverted to using the immersion heater of the Ariston 500 STI Unvented system for hot water only.

We ran out of hot water today so the immersion heater must have stopped working a couple of days ago.

I've investigated and had a look under the immersion thermostat cover and discovered a corroded/melted terminal. See the photo, the loose wire and terminal are below the red tape.

Now after more investigation I noticed the ProTech anti corrosion device was turned off. Working backwards I worked out the plumber switched it off 4 months ago when he decommissioned the heating and boiler (because it is on the same circuit as the heating water pump).

Could this have caused the electrical terminal problem or is it coincidence? Is the anti corrosion device to stop corrosion inside the cylinder or to stop this? I thought the former.

If the anti corrosion device is not to blame any idea what could have caused this?

Note we've been irregularly turning the immersion off and on to save some electricity.

Finally would the immersion still have worked with this broken wire? I'm wondering if this has been broken for a while and today's problem is something else?


Many thanks

 

[Newboy]

You might be better asking in the electrics forum. It looks like an poor connection has caused overheating. The brown sheathed wires connectors look unusual - spade terminal / barrel / conductor wrapped in tape and not capable of carrying the current.

 

[bernardgreen]

Those slide on connectors are fine for the first time they are slid on to the spade on the element. But if taken off and then fitted back on again then they may not make a good contact with the spade. They distort slightly when removed and the pressure on the spade is less than it needs to be when refitted on the spade. A poor contact will heat up, this further reduces the pressure and the contact fails and damage results

Possibly at some time the element was changed or the connections were dis-connected for some reason ( testing the element ) and the slide on connectors not replaced with new ones.

 

[MrTherm]

The Ariston 500 STI has a built-in sacrificial anode, it sits between the two looped heating elements and acts as a dummy load to prevent corrosion within the storage water vessel.

If it disconnected or bypassed, the electrolytic action would 'eat away' at some other convenient point yet this would have to be an area in contact with the stored water - an heating element or its connections for example as shown in the picture.

I've seen complete heating storage vessels eaten away to the point of complete porosity.
The anode, usually made of a magnesium alloy will periodically need replacing at service intervals.

wads, you are correct in your assumption that the anode is disconnected or bypassed.
regards

 

[simond]

The Ariston STI is a glass lined steel cylinder, and is made by rolling liquid glass inside a mild steel drum. The better the quality of cylinder, the more coats of glass it gets, as it is recognised that the glass lining never covers everything first time.

Smaller cylinders are usually made of stainless steel, but the costs start to become prohibitive at larger sizes to some of the market (especially new build).

The ProTech reduces the water corrosion but it should be noted that artificially softened water is incompatible.

We haven't fitted a 500STi for many years, but the last ones we did had a 3 phase compatible 6Kw immersion, which could be converted to single phase by changing some links, to give 3x2Kw immersions at 230v.

Yours looks like it is single phase but is probably 6Kw, so if this is confirmed by the rating plate there should be a 32A cable and a 32A fuseway/MCB on the circuit. Looking at the spade terminal connections with the larger brown cables, it looks to me like these have been hacked around with on site and the inserts from a 30A connector block used to join them to the (previously crimped) spade terminals. This is evidenced by the use of leccy tape around the two terminals.

So your problem is entirely unrelated to the Protech, but is a dangerous on site bodge by a central heating fitter which could have caused a fire.

The disconnection of the Protech will cause premature corrosion of the cylinder shell, so get that back on asap, but that isn't the cause of the problem in your picture.

 

[Agile]

As far as I can gather the cylinder EITHER has tao magnesium anodes or it has the Protec device.

Few of them that I have seem are actually switchen on and one had never been connected to the mains.

The Protec has been discontinued I think and hopefully the glass lined cylinders too as they usually had a pretty short life.

 

[flameport]

Now after more investigation I noticed the ProTech anti corrosion device was turned off.
...
Could this have caused the electrical terminal problem or is it coincidence?

No, the damaged wires have been caused by overheating, which in turn was caused by incorrect installation/repair.
The red tape should not be there - it's evidence of where someone has attempted to fix a previously damaged connection that should have been replaced.

Finally would the immersion still have worked with this broken wire?
I'm wondering if this has been broken for a while and today's problem is something else?

No, that wire has broken off recently and in doing so has caused it to stop working.
The connection has been overheating for a long time.

At the very least, both of the large brown wires will need to be completely replaced - the overheating will have damaged the copper inside making it impossible to reliably connect anything to them.
Part of the lug on the immersion heater has broken off. It may be possible to connect to the remaining half - but if not a new heater will be required.

Most of the damage could have been avoided if the previous 'repair' involving the red insulation tape had been done properly.
If you remove the red tape, you will probably find a screw connector underneath, similar to the other burnt one. Neither of them should be there.

 

[DP]

Overheating is due to high current passing through the screw terminal that was possibly not tight enough. Slightest termination resistance will lead to localised overheating leading to meltdown shown in picture

Instead of using proper crimping tool and Lucas connector, someone has used a chocky block screw link.

 

[wads]

Thank you all for your answers.

This makes sense. We have been turning the immersion on an off a couple of times a week so that may have accelerated the problem.

I've got some confidence around the issue and we might be able to fix it in the short term before replacing in a couple of months.

The previous owner certainly was prone to doing bodge jobs as I'm slowly finding out ...