Something I have not hidden is that I am a big fan of the Nest learning thermostat. I purchased one on the first day they were available in the UK and soon after became an authorised installer. Up until recently all had been good, I was saving money and my house was more comfortable to be in but late last year they issued a software update and for a short while all was good. Soon I noticed that on occasion one or more of my nests were dropping off the network and showing off line. Sometimes after a few days they would come back on there own, other time I would have to restart them. On a few occasions even this did not work so I had to reset the networking on them or even in two extreme cases restore the unit to factory defaults.
Out of exasperation I finally made contact with Nest support and as always they tried to say the issue was with my router. Finally after a lot of backwards and forwards with support they have admitted that there is an issue with wifi, on these devices. Solution? Well at the moment there isn't one although they have suggested they are working on one.
In the mean time there is a work around, remove the nest from the stand or wall and reset it by holding in the button (or to be more accurate push in the back) for 10 seconds or until the screen goes black and then put it back on the stand. It appears to get them back online for a while but they are still dropping off every so often.
I'm not sure if this issue is affecting many other users having spoken to friend with Nests most seem to of had them drop off occasional but not nearly as often as mine do. Hopefully they will come up with a long term solution soon, in the mean time I'm retting it as and when needed.