Nest 3rd Gen suddenly not turning heating on but water ok

[AndrewQ]

Hi all

yesterday with colder weather coming I thought I’d test our central heating.  It’s not been on for some months due to the warmer weather here in the UK.

I turned on the thermostat via the app and whilst it turned orange and suggested it’ll warm up in X hours nothing came on.  Reading various posts I checked to hear a click when turning on - and I did.  I also heard a click when turning it off.  Just when I turn it on nothing happens, no pump or boiler kicking in.

When I need hot water, everything works fine (boiler comes on and pump to shift it into the tank comes on). We’ve been using hot water throughout so no issues there.

I tried to manually run the heating pressing the button on the heat link and everything turns on and runs fine - radiators heat up all as they should.  

I’ve tried a reset on both thermostat and heatlink hasn’t worked.

I’ve cut power to the heat link and factory reset the thermostat - hasn’t worked.

I thought it might be a stuck valve but this can’t be if all works as it should when I manually turn the heating on.

I presume thermostat and heat link are talking - I have no issues with hot water, I hear clicks for the heating, and it updates both thermostat and app when in manual mode (which I can successfully turn off from the app).

Not sure what else to do - I’m reluctant to look at wiring, it’s been working fine for the past few years and given the manual press kicks it all in suggests (to me anyway!) wiring should be ok.

my thermostat isn’t wall mounted just connected to power in a desk stand fyi…

thanks for any help / advice?! 

[Frustratedddd]

There is an easy way to prove the Heat Link contacts are switching, if you have a tester or just a simple neon screwdriver tester.

Do you have access to either?

[AndrewQ]

Thanks for replying - Ah no I don’t I’m afraid. I’m guessing things work ok with regards to water as no issues. When I press the heat link manual button all works ok? Would this still be a contacts issue? 

[Frustratedddd]

Yes it could be the contacts still. The heat is one set, and the hot water another set.

The only other way to prove it's the contact at fault is to do a temp swap of the connection, or the best a simple neon tester. Can't you ask anyone to borrow a neon screwdriver for a few minutes?

Hot water working, and no heat could well be the relay faulty. You need to prove they are switching.

[AndrewQ]

I’ve got one on loan on Saturday so will be able to check - what specifically am I looking for (is it obvious?).  Saving that I’m assuming swapping the wires over (powered off at the board!) would be one way to test for sure 

[Frustratedddd]

I assume you have a system boiler due to the information you talk about with hot water.

That should mean terminals 2&3 - 5&6 have cables terminated.

You need to make sure nothing is calling for heat or water, then put the neon onto term 1, you should see the neon glow bright. now put the neon on term 3, you might get a dull light, but that's OK. Now place the neon back on term 1 and press the Heat Link button, it should go from a bright light to a dull or no light. Should that be correct, go to term 3 and place the neon onto term 3. That terminal should now be a bright light.

What you are doing is proving the relay is switching between those two terminals 1&3. That proves the Heat Link is switching correctly for heating.

You can do that same for terms 4&6 to prove also, but that's the hot water contacts.

[AndrewQ]

Wow thank you I can follow that when my friend comes around - i did manage to speak to google support too, they said I need to get a plumber to run a continuity test which is what you’ve got above - however they feel because the heating works fine when manually activated on the heat link they feel it should be working ok and suspect more it could be a stuck valve.  But then again a stuck heating valve would it work even manually activated!?! I know zero about plumbing !!

[Frustratedddd]

Anything is possible, but by testing the Heat Link first, you remove one part from the equation.

[UanaC]

Hello there! @Frustratedddd Thanks so much for your help. Really appreciated as always. @AndrewQ Thanks to you too for bringing this to our attention. I am really sorry for the inconvenience created by this situation. As my colleagues mentioned to you over the phone and as explained by @Frustratedddd  here over the community posts. It might be a problem with the valves. If the heating and hot water turns on by activating manual mode and if the Heat Link passes the '' click test'' the only thing left to do is a ''Continuity test''. The continuity test is not mandatory, but you can do it in case you want to rule out the possibility of the unit to be the root cause of this issue. Is there anything else I can help you with or do you have any other questions I can answer you?

[AndrewQ]

Hi if the above is the continuity test I can do that today.  I was supposed to be sent instructions from the google help agent for a plumber to do but nothing arrived is this online also ? Weirdly the heating did come on today for some reason, so I’m starting to think it’s more likely a dodgy valve for the heating ….

[UanaC]

Gotcha. Is it okay if I sent you the steps via the email you're using here?

[Frustratedddd]

@UanaC

Over to you.

[AndrewQ]

Yes please. Got the tester so following the test above (hopefully) 🙂

[AndrewQ]

Ok so lots of wires!

L and N have brown and blue wires from one pair.

1- empty

2-yellow bridge wire from live

3-brown wire from second pair.

4-empty 

5-yellow bridge wire from 2

6-blue wire from second pair.

so when I pop the tester in, with nothing calling on heat link:

1=high voltage

3=low voltage

4=high voltage

6=low voltage 

when I turn on manual button

1 stays high voltage 

3 now has high voltage

4 stays high voltage

6 now has high voltage.

does this make sense?

[AndrewQ]

Here’s the wiring layout as a picture.  

as I say, 1 and 4 high, 3 and 6 low voltages.

turn manual button on…

1 and 4 stay high, 3 and 6 now high

this suggests the heat link is doing its thing correctly? 

[AndrewQ]

Sorry for completeness, when I turn the hot water on, 4 and 6 both on high voltage, 3 is not.

when I turn the heating on, after the click terminal 3 goes high voltage, and 6 stays as low (as water is off)

[AlexD]

Thanks for all the information. We do expect to see high voltage in the active terminals and low or no voltage in the ones that are not.

For example 1,2,3 are heating. Power is being provided in terminal 2 and when the heating is off the relay inside of it is bridged between 1 and 2 - so to an empty terminal. When heating is turned on the relay moves over and voltage is provided between 2 and 3 and the heating should activate. 

However there can be voltage leakage when checking for just voltage. Would you be able to check for only continuity? 

[LarisaL]

Hi there, just checking in! Did you see the message from my colleague?

[AndrewQ]

Hi all so plumber has been today - heat link was ok he did a test on it, swapped the heating valve and hey presto it works. All sorted now thanks for the help !

[LarisaL]

I'm glad to hear that! Is there anything else I might help you with?

[AndrewQ]

No I think we’re all back to normal 

[AlexD]

No worries. I'll go ahead and close this thread. If anything else comes up, make sure to open up a new one. Have a great weekend ahead!