Boiler power switch melted again

P

pauldk

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This is the second time this has happened in 10 years, I believe caused by me using the immersion heater today. Can you identify the switch for me so I can buy a new one. My best guess is a 45w cooker switch:

www.toolstation.com

Axiom 45A DP Cooker Switch 2 Gang Upright Neon | Toolstation

BS3676 AC only BS EN 60669. Licence No. KM73110.
www.toolstation.com www.toolstation.com

20230611_231603[1].jpg
 
You say "boiler" but I think you mean "hot water cylinder"

In your photograph I can't actually see a switch. I don't recognise what the black thing used to be. Stand back and take a wider pic. Both sides.

The cables have been severely heat damaged and need to be replaced.

Once copper has overheated it softens and will no longer hold a good connection in a terminal

The sort of damage you show is often seen where the connections were poor.
 
I doubt a cooker switch would be used, perhaps an Immersion Switch? The black thing @JohnD can’t seem to recogni what it used to be is the terminal connector. Wires have been overheated as John says and again where he states often a poor connection, sometimes where the insulation has been screwed onto.
 
CBW said:
I doubt a cooker switch would be used, perhaps an Immersion Switch? The black thing @JohnD can’t seem to recogni what it used to be is the terminal connector. Wires have been overheated as John says and again where he states often a poor connection, sometimes where the insulation has been screwed onto.
Click to expand...
If you look at the back box, it appears to be a rectangular box, the type used for cooker switches.
 
JohnD said:
In your photograph I can't actually see a switch. I don't recognise what the black thing used to be. Stand back and take a wider pic.
Click to expand...
It appears to be exactly as the OP has linked to - the back box is cooker sized and there is a neon above.
I suspect that this may switch an electric boiler rather than an immersion.

Aah, beaten to it! :)
 
It's an electric heat pump boiler with a backup 9KW immersion heater, A Danfoss Maxi AQ.
 
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39 to 37.5 A heater approx heater using voltage of 230 or 240, should not burn out. Although continuous use of boiler as opposed to a shower means the good connections of suitable conductors are more vital. What is the total max load at any one time with this set up? i.e. can pump and heater be used at same time
 
I've bought one of these MK Cooker Switch from Toolstation and turns out it's exactly the same as the burnt one. I've bought some wire wool as well to clean the wires. I'll avoid using the immersion heater setting in future, I never use it much anyway just the heatpump compressor. https://www.toolstation.com/mk-45a-switch/p74426
20230612_163117.jpg
 
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The 100Amp DP switch I linked (there are many reputable brands) will withstand it, and has good big terminals.

I always have a few knocking about

But you need to replace the cables as well.

We don't yet know the total combined load.

With large cables, you need to tighten them at installation

Then again a few days later

Then again a month later

With the correct size screwdriver

For best results

As copper creeps under pressure and after thermal cycling

I feel sure the problem was poor connections. Not a faulty switch.
 
I'm happy to report it took 15 minutes to replace the switch and the boiler is working again. I will of course monitor it, and tighten the wire clamps up again at intervals. I must compliment JohnD on his fastidiousness.
 
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pauldk said:
I've bought some wire wool as well to clean the wires.
Click to expand...

The problem is, heat damage is more than surface corrosion (although there might be some oxidisation, or contamnation from PVC that will clean up), the damage goes further, as far as understand it, the copper gets annealed and its physical properties change, and its not so adept at forming a sound connection in a screw terminal. You have to cut back to bright copper, or if you cant do that, joint or replace the cables.

Going back and tightening it at intervals is not something that can generally be done, putting asside DIYs (and i suppose on site factory maintenance guys), But if you terminate it properly in the first time its not an issue. Dress your lives and neutrals in and tighten as you put them away, then give them all a bit off a wiggle and re-tighten. Put your earths away, and then offer upto the backbox and push home, then pull forward a little and tweak them all up a little more including the earths. Loose connections result from tightening once and either the stands settling over time, or settling when its first moved
 
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