Nest Thermostat not working

[RJUK]

Hi folks, I've had two Nest Thermostats installed in my house for a few years now, one for upstairs and another for downstairs and hot water.

Today it's -1C here and I noticed it was feeling chilly, so I looked at my phone and the Nest app said the heating was on for downstairs.  I couldn't hear the boiler running, so felt the radiators and noticed they were cold.

I've since done some fiddling, but can't quite work out what's going on.  The upstairs Nest still functions fine and makes the boiler turn on and heat upstairs.  The downstairs thermostat says it is heating, but will not cause the boiler to fire.  I have restarted the boiler, the white Nest box, the Nest wall thermostat and even my iPhone.  However, it still doesn't work.  If I stand next to the white Nest box I can hear a relay click inside when I turn the temperature up and down past the actual temperature. So it appears to be working as it should in terms of a connection between the wall thermostat /my phone and the Nest box.  However, despite that click, the boiler never fires.

But, if I press the button on the Nest box itself, the boiler then does fire.  So again, it doesn't seem that the Nest box is unable to control the boiler, because it does so manually and the radiators get hot.

Obviously this isn't ideal because manual makes it heat both water and heating, which I don't want. I just need heating.  Also, it means I'm unable to sxmchedule the heating or allow it to automatically click on and off with the thermostat.

Any ideas what the problem could be please?

[Frustratedddd]

Have you checked your motorised valves, as mentioned on this forum dozens of times?

[RJUK]

I'm not a plumber, so don't quite understand fully how they work, but I moved the downstairs heating valve to be manually fixed open.  As such, the downstairs got warm this morning when the upstairs heating clicked on.  However, once the upstairs got up to temperature, the heating clicked off, even though downstairs was still supposed to be heating.

I did consider whether the valves send a signal back to the control system to allow the boiler to fire, but as I say, I'm not a plumber, so I don't really understand how it all works.

Are you suggesting that the valve sends a signal back to the control system to say that it's open, but without that signal the boiler will not even try to fire?

Interestingly, the hot water still works via remote control from my phone and on the schedule. (Of course, it uses a separate valve from the downstairs heating, so this would agree with this hypothesis as well.)

[Frustratedddd]

The Heat Link box you mention is nothing more than a switch with 2 contacts inside. This takes a signal from the display, and when calling for heat from the display the H/L switches on. That's the click you hear. When you manual overide the H/L by pressing the button it does just that, overides everything.

I am not saying anything about motorised valves as your system could be anything, probably underfloor for example. 9/10 when an issue like this happens it's the valves, but users think it's the thermostat as mentioned on here as I said.

I would suggest calling a good plumber, and they will get the system working.

[RJUK]

Thanks for your help. I texted my boiler service guy with everything I'd checked and he confirmed it was the motorised valve.  At this time of year he's super busy so he just told me what to do.  I ordered a valve head on Amazon and swapped it out yesterday.

All working again now.

At least I know for future that the boiler won't even fire if it's not getting a signal back from the valve head.  House is only 8 years old, so they don't seem to last very long.  I'm sure it's only a matter of time before one of the other two fail. 🙄

[Frustratedddd]

Sadly as expected. Most users assume it's the thermostat at fault, but again this shows there could be other things.

Good to see you back running