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Hi guys - could do with a bit of help if possible please.
Our immersion heater has failed. I was new the June - a 3Kw Tesla incoly from Screwfix. Tank is a pressurised system with biomass so the immersion is the backup.
I've watched 20 mins of John Ward on YouTube and done the following tests:
1. Power on to immersion - voltage across the live and neutral contacts 230V
2. Power off to immersion:
a) Resistance across the element is 20 ohm
b) Resistance across the thermostat is 20 ohm when the thermostat is set to 70 (tank temp is 40 at the moment) and is infinite when the thermostat is set to 20.
c) Resistance across live immersion contact to earth is 20 ohm
d) Resistance across neutral immersion contact to earth bounces about and settles at 1.3 ohm
I think this means it has shorted to earth (whatever that means!) and that I need a new element but I would love a guide if someone can explain/advise!
I had thought it may be the thermostat so got one from screwfix today but that does not have the 2 tabs on the back of it that my current one does so I think is very much the wrong type!!
.. I can fix people and soldier. Not good at this though!
If it's shorted to earth you'd get an impressive bang and flash before it either trips a ground fault protection device or the MCB, so not that. You're measuring neutral to earth, which are often tied together where your supply enters your home.
If it's from Screwfix just take it back. Or call the plumber that fitted it and get him back to replace. Incidentally you shouldn't be doing this work yourself - have you ever seen what happens if an immersion thermostat welds closed and the overpressure relief on the tank fails?
OK thanks Flaperon - it has not tripped the overheat trip on the thermostat itself nor the MCB.
I am planning to call a plumber but had thought if it was the thermostat it would be an easy fix. Replacing the whole thing I most certainly cannot do.
20 ohms is the typical resistance of an immersion heater element. It can range form 18 to 22.
The strange reading (1.3ohms) between earth and neutral is probably as you have a TN-C-S earthing system - the earth is connected to the neutral at the cutout.
If it had broken, and one end is touching earth, then the reading will not be the same as between L+N on the element terminals.
Start again. Disconnect the immersion heater completely.
Measure between L+N on the large terminals.
Leave one probe on the neutral connection, set the thermostat to 65 degrees, test again,on the live wiring going into the thermostat, the reading should be the same (so long as the tank is not above 65 degrees).
If the reading is the same - roughly 20 ohms, then that shows the thermostat and element are working.
Are there 2 cutout switches?
There is one on the thermostat, usually a tiny red pip, and possibly one screwed onto the immersion body.
Newer ones are now coming with this extra safety feature, a small cut out to prevent boiling - pic on the link below:
Immersion cut out
If you had a break in the element, it would trip the RCD immediately - you have got that circuit RCD protected?
If all the above turns out OK, then you need to check the input voltage, it could be a faulty switch.
alanl - thank you so much for the step by step. I will have to do it in the morning as the tank etc is literally just by my daughters bedroom and she is now asleep.
There is only one cutout on the immersion - a small pip on the thermostat and that has not tripped.
The immersion is on its own RCD on the main board and was off when I did the tests (excepting the first one measuring the voltage. The resistances all measured with the RCD flicked to off)
The thermostat and element is this one - tesla incoly 14
The thermostat has two tabs on the back which slot into to the L and N on the immersion coil and then have the screws on them for the brown and blue wires.
I have double checked the earth is secure.
When I have the RCD set to on and power to the thermostat, with the thermostat set to 60, voltage across the L and N is 230.
It sounds like you are the exact sort of expert I need - if you are anywhere near Brum/Stourbridge might you be free for a job?
I will do the other tests in the morning.
Have you actually had it working if it's a backup?
Resistance reading is ok(P=V.I, I=V/R 240x240/20 =2880W, close enough to 3KW as there will be some reactance I there as well).
Your reading over the th thermostat is the same as the element when temp set to 70 as the thermostat is closed (i.e calling for heat). So you're doing the same reading.
N-earth, is your meter auto ranging? If o the bouncing around might be it just working out the range to use. N earth will be tied together somewhere.
I think there is some silly wiring mistake as the components sound ok from your description
Good point about thermal trip reset made above, but I wouldn't expect the 20ohm reading.
Take a look at the pic 6 posts in. Black dot with wire either side. Push to reset once cool.
https://images.app.goo.gl/cvdQecz3ZEouBAKN6
The Brick - a good question. Yes I think so but it's hard to he sure as you say.
Thermal trip seems OK.
If any of you are Brum-ish do please let me know!!
Leicester, so a bit far for me.
It sounds to me that the element is OK.
Occasionally I have had an element fail, that does still give a ~20 ohm reading across the element, then there is no reading between live and earth indicating that it is good, but, the small meter I use for initial testing doesnt recognise readings over around 100 ohms. When tested with the big meter, it showed a live to earth fault. However, if RCD protected, it should trip pretty quickly if that has happened.
I've emailed a chap near me to ask if he does electrics and plumbing as it seems I need both!!
Thanks again team.
Update for you lovely experts - the element had failed. Resistance from both terminals of it to earth was 20 ohm at one side and 1 ohm at the other. RCD had not tripped though nor the thermal cut out on the thermostat. Removed and replaced with a new unit which is working perfectly...
Cannot explain why it went so soon (was only installed in June) nor why no tripping but...