This is the same issue I have.
-using 2 iPhones to determine if we are home or away
-worked perfectly from new in 2017 until a few months after Google bought Nest and implemented Google Home and made use of Google Home a must. Since then
2. NEST will reliably recognize when we leave and switch to Away
3. Nest sometimes recognizes when we return home. This error has become more frequent, to a point that I have been shopping for a new smart thermostat.
4. Nest has recently been deciding we are Away when home, including while we are sleeping. Now we are highly motivated to replace Nest
5. Nest has stopped recognizing my phone, although it does recognize my wife's.
6. We have done a full Nest reset. No change.
7. While shopping for other brands we found extended conversations about the C (common) connection on the thermostat. This provides a return path for the 24 volt power. Without it, most smart thermostats will not work. Nest uses a clever but questionable method of stealing power from the other wires to keep the Nest battery charged. However, the Nest without C connected is prone to battery failures and erratic operation.
8. Based on the discussions, I purchased an Add a Wire kit that allowed me to connect from furnace to the C of the thermostat. Venstar Add a Wire, $25 on Amazon. Delivery 2 days. Easy installation.
Other options:
a. Pull C wire from furnace to thermostat. Lots of effort, possible dry wall repair if existing passage is too small to pull the new wire
b. Use fan wire for C. Lose manual operation of fan
c. Add 110-24 volt transformer, plug into nearby outlet and run c wire from transformer to c on thermostat. Ugly.
d. Add Internal battery. Expect short life
e. Manually recharge Nest battery when needed/regular schedule to minimize erratic performance. I am not clear how this is done-through C terminal on thermostat? Sounds like a pain
f. Ecobee provides a device they name PEK to work around no C connection. Maybe same as Add a Wire?
9. Installed Add a Wire today. Confirmed HVAC works. Nest switched us to Away a few minutes ago while we were home, so maybe not a fix. No matter-this will allow us to buy an Ecobee or Honeywell with similar functionality.
We will see what happens for a week before taking further action
