Learning thermostat works only when C-wire is disconnected

[RSH]

My thermostat worked for 1.5 years and then went offline...

Then it would come online after a day, and then offline again.

This is in a rental property that I manage remotely.

The tenants said that it would just go dark and be non-responsive, and then magically just come online! we also caught the message "no power on Rc wire" and "need to shutdown to charge the battery".

So we tried installing a new similar thermostat but couldn't even fully set it up - it couldn't be  connected to the app (maybe again not enough battery charge? but there was no explicit errors), but in equipment view of the new the thermostat also had red mark at the Rc showing some problem with it.

The HVAC technician came a said that everything looked ok in terms of wiring at the thermostat and air handler but still switched power from Rc to R terminal in the thermostat (I assume this didn't fix) Then he disconnected C-wire and said that the thermostat booted up.

It worked well right away except wouldn't connect to internet saying "battery to low". So it still didn't connect within the next 7-8 hours (battery was charging very slowly?). So we took it off and charged for a couple of hours using USB. After putting back - it booted up and connected to internet. In about 6 hours it went offline and in 3 hours came back online and was working perfectly for 4 days.

But I was concerned that it is relying on a battery now, so I wanted the C-wire to be restored.

The HVAC tech came and connected it back - the thermostat recognized it and seemingly was functioning properly...

6 hours later it went offline and this is current situation.

Can anyone tell why such behavior? And if this is a problem with a thermostat or wiring?

[aperezdeleon]

Hi there RSH

Thanks for reaching out to our community!

Nest thermostats can typically charge their batteries using your heating and cooling wires. However, in your case, it seems your system requires a C wire for stable current flow and constant power delivery to the thermostat.

This behavior likely means the C wire in your system isn't grounded and might need inspection.

I recommend contacting a technician to assist you with checking your C wire's current.
If you prefer not to use a C wire, you can install a Nest Power Connector. Doing so will eliminate the need for a C wire and allow your system's heating wires to provide the necessary current and stability for your thermostat.

Let me know how it goes.

Regards,
Angel.