Hot water only works in manual mode

[Tonywest44]

In UK. 3rd Gen Nest Thermostat driving central heating (radiators) and hot water. No problems until last week when hot water suddenly started misbehaving. Nothing changed in system.

Heating working perfectly via either thermostat or app. Hot water only working with manual push on heatlink.

Heatlink connected to WiFi and thermostat, all software up to date.

When app or thermostat "ask" for hot water there is a click from the heatlink but it doesn't fire the boiler (although fine with manual).

Have done quick reset and factory reset for heatlink but still no good for hot water via thermostat or app.

thermostat is set to System boiler. Have done a test on the wiring and sometimes it sends power for the hot water and other times it doesn’t. 

very frustrating. 

had a boiler specialist come in and test everything and he says the only possible problem is either in the thermostat or the heatlink. Everything else is ok. 

[Emnu]

I had exactly the same problem with my hot water not working and Gen 3. Vaillant engineer diagnosed the problem caused by the heatlink (it shows no error or anything so everything looks fine which of course is not). We removed the heatlink and everything works fine. This is getting a common issue which Google online support does not acknowledge nor knows what to do with it. I won't install the Google thermostat back so I'm looking into an alternative reliable product.

[Frustratedddd]

You say.....''Have done a test on the wiring and sometimes it sends power for the hot water and other times it doesn’t.''

What test exactly have you done?

Have you tested terminals 4 - 5 & 6 at the heat link, if so what happens?

[Tonywest44]

I’m an electrician so I have tested using a volt meter to see if there is power when the click happens. Sometimes it sends power and we have hot water other times it click but there is no power. I’d say I have a faulty heat link which will be the third one in three years. 

[Frustratedddd]

What do you get between those terminal connections when calling and not calling for hot water? You say H/W is working with manual push, does that mean it only fails with the app and display in that case?

[Tonywest44]

When calling there is power. When not calling there is no power. Which is how it should be. What I am saying is when you go manual everything works fine. When using the app or thermostat sometimes it works and sometimes it doesn’t. Had enough of nest now. Going to another brand which has better tech support. 

[Frustratedddd]

This may be a silly question, but do you have motorised valves, and if so I assume you have checked them?

[Tonywest44]

Yea good question. We do. I e checked both and both seem to be working absolutely fine. Yesterday we had hot water for the first time in two days. Today cold again. I have managed to narrow it down to the thermostat itself. Everything else works fine. 

[Frustratedddd · Accepted solution]

Things like this usually point to the valve, hence asking.

Reason being, you say in manual mode all working fine, hence the relays must be switching. You say you have tested the terminals I mentioned and they are ok, which again doesn’t point to the heat link.

Unless I’m missing something from your tests, then the heat link is fine. A simple neon tester can test the contacts at the heat kink, and if that’s switching I would assume all is working. I would probably concentrate more on the system and motorised valve personally.

[Tonywest44]

Update. So it WAS the hot water valve and not the heatlink or the thermostats. I replaced the spring loaded valve a week ago and have had no problems since.