My thermostat is turned low (or off) but the heat won’t shut off

[Ant6]

So my learning nest thermostat is set low (or to heating off) but my boiler ignores it and keeps on heating everything. The only way to stop it is to turn the boiler off at the mains. This is the second Nest I’ve had behave this way. The first one was out of warranty and also hadn’t connected to the WiFi for over a year. So I just replaced it. This new one has been fine for only a few months. 

I can’t find anything about this particular problem online. Any advice is greatly welcomed. 

[CoolingWizard]

@Ant6 , are you in the United States or in the UK?

 Do you have a Combi Boiler or a straight Heating Boiler?

Do you have a Volt Ohm Multimeter?

You need to investigate the boiler control system and its configuration and capabilities.  If the Nest Thermostat has the temperature satisfied, it will break the connection between R and W.  If the boiler continues to run,  need to check the boiler controller and understand the boiler heating cycle.  

AC Cooling Wizard

[Ant6]

@CoolingWizard Thanks for the reply. I’m in the UK, I have a boiler that deals with the radiators and it heats a water tank. It was working fine for a few months and now it’s just stopped clicking off.  I’d assume a fault but my last nest did the exact same thing, as I mentioned. 

[CoolingWizard]

@Ant6 , I am not familiar with HeatLink models sold in the UK. 

AC Cooling Wizard

[AlexD]

Thank you for all the information provided @Ant6 . I am sorry to hear you're having difficulties with your device. I would be happy to try and assist. 

We want to do some tests to see how the relays in the Heat Link behave. Go next to the Heat Link and open the Nest app on a mobile device:

1. Select your thermostat to access the heating controls and make sure the screen is black, heating is off.
2. Turn up the temperature until the display goes orange, heating on, and listen for a clicking noise.
3. Turn the temperature back down until the display is black again, heating off, and listen for a clicking noise again.

Let me know what the results are.
 

[AlexD]

Hi, I wanted to check in. Did you see my last message? Keep me posted.