Why won't my thermostat control the HVAC anymore after trying to switch thermostats?
Nest states their thermostats do not require a Common 24 volt leg of control power to be provided to the Nest, However there are some situations where the Nest will not stay charged after a bit thus requiring a Common leg of the 24 volt power sent to it,  they allege the Common leg back feeds to  the thermostat by unused circuits and it does in many if not most cases however there are many complaining after a week or so  of the Nest no longer operating or operating normally, this is due to no Common leg in most cases.