I posted this three weeks ago. The specialist didn't even read my post carefully before answering my question. Those Nest thermostat worked fine without a C wire. I have used this way for more than 6 years and I still have two working without any issue. Please use some logic before giving an answer.
My Nest thermostat can only be charged via USB after a thunderstorm with power outage. This is the second one died in this way. My heating system doesn't have a C wire. I know the wiring works because I have 4 Nest thermostats, one for each heating zone. All the other work except this one which is connected to both AC and Heating. The rests are heating only. Every time, when this problem happened, I replaced the display unit, then it works.
Now I ended up with two dead Nest 3rd gen displays. But they work fine if I charge them with USB port, they just don't charge on the base anymore.
My question is whether the hardware was damaged on those displays? Can I still save them by using an external C wire adapter?
