A year ago I bought and installed two Nest thermostats. If you are not familiar with the Nest they are modern “smart” devices that are wifi accessible, have tablet/phone apps that enable remote access/control functions and are able to learn your habits or patterns and adjust temperature control to accomodate them. I have gas heat on the main floor in my house and a heat pump on the second level. The Nests have proven to be useful and interesting from the perspective of control and awareness (they include an energy usage function) and I do like having them. The gas heat thermostat installation was very easy but the heat pump was a bit more challenging. That being said the heat pump air handler in the attic began exhibiting a problem recently that required a maintenance call. The potential problem was an unusual sound in the attic air handler when the fan started. The technician that came (with an assistant) decided that something was causing a chatter in the fan circuit in the air handler. He replaced one of the two things and the problem appeared to move to the other one. He replaced that one and then while trying to determine what caused the issue found a low voltage being supplied to the circuit. He then replaced the breaker for that circuit when he found that one side of it was not correct. At this point some problem still existed so he bypassed the Nest with a standard thermostat and the problem appeared to go away. He pointed at the Nest as the root cause but he was not familiar with Nest so I was not certain that it was the real cause. The end result was replacing the heat pump Nest with a standard thermostat and the system is now working again. This leads to one drawback with the Nest approach in that maintenance technicians may not be familiar enough with them to trouble shoot and therefore likely to want to remove it from the system. There is also the possibility that the Nest is the issue either due to an issue with it or the firmware update that was automatically applied to it a couple of weeks ago. It is also possible that when the technician was replacing one or more parts in the air handler that he changed some circuit wiring that impacted the Nest (he did not have a wiring diagram).
I would like to re-install the Nest at some point but I also know that the second generation Nest does support the heat pump that I have more completely than the first generation unit that I installed. The dilemma then becomes putting back the old Nest, buying a new second generation one or just leaving the whole thing alone. I am not sure which course I will take at this point but will update this site if I do attempt something else.
My recommendations include having HVAC service from someone already familiar with the Nest because they are likely to assume that it is the crux of the issue when it might actually be somewhere else. When you are in the middle of winter and it is cold outside you might not have the opportunity to trouble shoot the issue yourself making you very dependent on a contractor to help.