I had a Nest 2nd generation learning thermostat installed in July 2020, and it just went offline a couple of days ago and we noticed the heating wasn't working. Thermostat is in standby mode as battery has been drained. On observation, I noticed the heat link has no power though there is 240V supply into the heat link - it's just gone dead!
As a result, there is no 12V supply to the thermostat and also no way to control the central heating. Please can I get some urgent help with this, as I have a baby at home and unable to keep the house at the right temperature for the baby.
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I have the same problem, but I have temporarily solved the heating issue by taking of the thermostat and charging it via its USB port, and then replacing it. After half an hour there was enough charge in the battery to make the thermostat work, and it has now switched on the heating.
Now I will be calling a specialist to find out why the thermostat thinks there is no power and having to rely on the battery.
Hope that gets you some heating for the baby