With our below zero temperatures outside, my newly installed Nest just quit working this morning. As the house temp was down in the 50's, I was on hold for Nest support for 2 hours and 15 minutes (Happy New Year). This a big issue and we are starting to see the reviews stating the serious problem. If you do not have a functioning control wire (usually the blue wire) and the temps get below zero outside, some HVAC systems quit powering the Nest thermostat and it just turns off because the battery drains out. The temp solution is to disconnect the yellow wire which controls the air conditioning. That will get the furnace working again. It also means you need to remember in the spring to hook up the yellow wire again or you will have no AC. The long term solution is to get an HVAC repairman to hook up a common wire to the Control port of the Nest so the battery will always receive power. The only redeeming part of this review was the Nest support person I dealt with (Alexis) was very helpful and spent the time to answer all of my questions and wait patiently for about an hour as we fixed the problem. That good support person got it up to a 2 star review.