I have 5 Nests installed. All have common connection (from the AC side). I had to buy 2 spare units and replace 2 base plates due to a failure that starts as a chatter in one heating circuit (W1 and W2, both chater but not at the same time, all at random). 3rd gen. The last replacement lasted 2 months before chattering started.
Google support was great this week and a new base is on the way. But as I can not leave without heat, I had to go to Lowe's and buy a new spare Nest even though I have two perfectly good "heads". I makes not sense that one can not buy the bases separately.
I have had other zones chatter with increasing frequency. These thermostats are out of warranty. I am willing to pay for spare bases but can not find these.