HW Stopped Working

[OliMortimer]

It seems like the heating for our hot water has stopped working for some reason, so hoping someone could point me in the right direction to determine what's failed.

If I request HW on the controller, the boiler doesn't fire up (ie the request for heat light doesn't light up, and the boiler doesn't kick in). If I request CH at the thermostat, the boiler fires up fine, and everything starts whirring away as usual.

Here's how our system is wired in currently:

Final Nest Wiring

• OliMortimer
• 14 May 2015
• 1

 

[Agile]

Sounds as if you have wrongly connected a Nest thermostat!

Tony

 

[OliMortimer]

The Nest has been wired in since December last year, so I don't think it's that, especially as it doesn't interfere with the HW circuit (as far as I'm aware).

 

[Dean744PiN5]

gonna need a multimeter for this one.

 

[OliMortimer]

gonna need a multimeter for this one.

Just popped home and tested the thermostat. I think it's bust. Set it to 25C (lowest setting), and checked the current across the in and out - looks like no current is going out. Typically it doesn't seem to be an off the shelf thermostat (ie, Screwfix, Wickes etc don't sell them), as it has two copper prongs that go inside a pocket on the tank:

Codice Dual Combined High Limit Thermostat and Control 542794

 

[FiremanT]

Did you check current or voltage? Check on switched side: should be 240V between that terminal and earth.
If you can do it safely, check V at HW motorised valve. If 240V at valve, then, normally, motor is goosed

 

[OliMortimer]

Did you check current or voltage? Check on switched side: should be 240V between that terminal and earth.
If you can do it safely, check V at HW motorised valve. If 240V at valve, then, normally, motor is goosed

Sorry, voltage, not current.

These are the readings from the thermostat:

Brown / Earth
0V 25C no call for heat
0V 65C no call for heat
240V 25C call for heat
240V 65C call for heat

Blue / Black
0V 25C no call for heat
0V 65C no call for heat
240V 25C call for heat
0V 65C call for heat

 

[Johnmdc]

I don't understand what readings you have put here.
Do you have 240 volts between the brown wire and earth when the clock is on?
If you do then the clock is okay, next test between the black and earth if you don't have 240 volts then the overheat stat has tripped.
To reset it push the little white pin sticking out.
If there is 240 volts between black and earth but not between blue and earth the cylinder stat is knackered.
If there is 240 volts between blue and earth then your motorised valve is faulty.
Hope that long winded explanation helps.

 

[Dan Robinson]

 

[Johnmdc]

What's that supposed to mean Dan?

 

[ScottishGasMan]

Is that thermostat from an unvented cylinder? if so, I would highly recomend putting it back and getting someone with their Unvended qualification out for a look, these thermostats have saftey systems built in you dont want to be tampering with

 

[Dan Robinson]

Pretty much that I think we are flogging a dead horse trying to the help the OP - he is working on an unvented cylinder with no clue how an S plan works. As SGM has said the risks outweigh hte benefits for the sake of a cheapish repair from a qualified engineer.

 

[Johnmdc]

Okay no probs.

 

[OliMortimer]

Pretty much that I think we are flogging a dead horse trying to the help the OP - he is working on an unvented cylinder with no clue how an S plan works. As SGM has said the risks outweigh hte benefits for the sake of a cheapish repair from a qualified engineer.

You're making a lot of assumptions about someone you don't know.

Considering the tag line for this site is "Why not Do It Yourself?", I thought these forums were for people to ask questions about DIY queries, and for "experts" to offer assistance. Was I wrong? Obviously you're just here to turn your nose up at those who are less experienced than you in DIY, and offer no assistance to those who wish to learn. Hopefully I stumble across you asking for advice about my areas of expertise, so I can also sit on my high horse and take pleasure in telling others that we're just flogging a dead one.

I'm simply asking for advice on diagnosing a faulty thermostat, and to determine whether the voltage readings point to a fault. Advice that will save me from waiting on a heating engineer to visit; for them to do the exact steps I'm doing; for them to order a replacement; for them to wait for said replacement; and for them to revisit to install the replacement. If I'm able to determine that the thermostat is faulty, it will take me far less time to order and fit a replacement myself, around my schedule, than it will for them to fit around theirs.

Now please, if you're not going to offer assistance, take your upturned nose elsewhere, because I'm tired of seeing people like you, belittling others on forums designed for help and advice.

 

[Dan Robinson]

I'm simply asking for advice on diagnosing a faulty thermostat, and to determine whether the voltage readings point to a fault. Advice that will save me from waiting on a heating engineer to visit; for them to do the exact steps I'm doing; for them to order a replacement; for them to wait for said replacement; and for them to revisit to install the replacement. If I'm able to determine that the thermostat is faulty, it will take me far less time to order and fit a replacement myself, around my schedule, than it will for them to fit around theirs.

No professional worth his salt would work that way.

If you need the help of a DIY forum to diagnose a faulty thermostat on an unvented cylinder then you are far from qualified to be working on one. Simples.

You might think I have an upturned nose, but you obviously don't know with what you are meddling and the fact is your cylinder is as dangerous, if not more so than a gas boiler. Call a professional and stop meddling. Believe it or not I do thin about you and your's safety in these matters.


As I don't know what your area of expertise is, I couldn't care less how much you hope to stumble upon my postings elsewhere. I wouldn't hold my breath if I were you.