Why might my hot water tank not be working?

[jamieled]

This will probably end in me calling a plumber anyway but I though it might be worth giving it a go here first. I have a relatively new OSO unvented cylinder with two x 3kw immersions. The top immersion is connected to a wall switch and wired to the CU which allows you to boost for 0.5, 1, 2 hrs. The bottom element is connected to a myenergi eddi which diverts from our PV and controls a programme boost to top up. 

 

We normally just use the bottom immersion. Today it seems to have stopped working. I first noticed when the eddi was exporting all our PV which is unusual. It seems to think the bottom thermostat is at temperature but the tank water is tepid at best. I'm guessing a broken element or thermostat - anyone like to hazard a guess? The OSO tanks/elements seem to have safety cut outs and reset buttons but they don't seem to do anything suggesting that is not the problem.

 

The top immersion doesn't seem to work either - when I try and boost it from the switch it looks like it's on but it's not drawing power from the grid. To be honest I can't remember when (if)I've ever used it so it might have gone phut a while back. But the fact they're both apparently not working seems an unlikely coincidence to me.

[ProDave]

Do you have any test gear like even a simple multimeter?

 

[jamieled]

Yep, Have a multimeter.

[Marvin]

Have the fuses to the immersion heaters tripped?

 

What type of fuses are they? MCB, RCBO?

 

How long ago was the immersion installed?

 

Edited May 20, 2022 by Marvin

[jamieled]

Pretty sure it'd rcbo but it's not something I know much about. New house, new CU about 1.5 years old. Same with tank/immersions.

 

Nothings tripped.

[ProDave]

38 minutes ago, jamieled said:

Yep, Have a multimeter.

Turn the power off, there is usually a double pole isolating switch where the flex from the immersion meets the wall.  Then with the multimeter on ohms range measure resistance between brown and blue where the flex joins onto each heater and post the result.

 

Post a photo as well so we can see what thermostat you have on the heater.

[jamieled]

Both immersions gave a reading of about 18.

 

Top:

 

bottom:

[dpmiller]

Both elements are fine then.

[Marvin]

Possibly the relay is either not working on the timer system is not working.

Er or the thermostats. In my book less likely as both don't work.

 

Either way you don't need a plumber..😂

Edited May 20, 2022 by Marvin
Add info

[TerryE]

13 minutes ago, Marvin said:

Either way you don't need a plumber..

 

You need an electrian, though TBH is you own a multimeter, then this is probably something that you could buzz out yourself -- so lonf as you understand and are confortable with all of the 240V safe working pactices.

[jamieled]

My multimeter capabilities are pretty limited and usually used for fixing vehicles. Otherwise my safe working practices in the house usually start and end with switching off the thing I'm working on at the CU, though I realise that has limits in what you can check.

[jamieled]

Electrician it is then. Is this likely to be related? Press the test button on the CU and it shows a yellow window which I believe is an earth fault?

[ProDave]

When you did the resistance measrements, was that at the 2 terminals actually on each of the heaters, or between the blue and brown of the incoming flex which will probably have landed at one of the thermostats?

 

If the latter then the fault is not at the tank.  Try and think of something common to both heaters, it would be unusual to have 2 faults together.

 

It's not related to the RCD assuming the RCD turns on again okay?

[jamieled]

Thanks @ProDave. For the top immersion I can test on both the element and the incoming L and N attached to the thermostat. 18 ohms on the element and I get this on the incoming L and N:

 

 

 

Bottom element just 18. Can't access the L and N as the ends are terminated inside the grey thermostat box.

[ProDave]

That is showing open circuit so the top thermostat is open.

 

That red item in the first picture I think is the thermostat.  Is that a black button at the top between those 4 terminals?  If you press it does it go click?  Is there any mention in the manual of how to reset the over heat safety thermostat?

[jamieled]

Yes, that is the reset button for the top thermostat.  It doesn't do anything  - the manual suggests there should be an audible click when resetting if the safety cutout has been activated.

[ProDave]

That's probably exhausted all I can think of by way of remote diagnosis.  If the thermostat is open but it's not because the water is too hot and it is not because of the safety thermostat tripping then it sounds like it is broken.  I think it really needs someone there to have a better look and really work out what is going on.  Do you have an electrician nearby?

[jamieled]

Many thanks for the ideas, one of the local boys is going to pop round over the weekend.

[ProDave]

Do please let us know what he finds.

[jamieled]

Embarrassing update no.1.

 

The top immersion was not wired into the boost switch on the wall (the live cable end was terminated into a block inside the switch). I think this is from a miscommunication when we had a bit of damage from a line fault a few months ago-plumber replaced the damaged  immersion element but the switch was never reconnected).

 

Bottom thermostat is broken,  but at least we have some working hot water now.

[ProDave]

You tried the reset button on the bottom thermostat?

 

Odd that one cylinder uses 2 completely different forms of thermostat for the 2 heating elements.

[jamieled]

Yep, got the electrician to double check in case I was being dippy. The bottom thermostat is supposed to be some sort of 'smart' thermostat - changing the temp of the water as your usage changes but frankly I've never been able to notice it do anything other than a standard immersion with a fixed temperature.