I have two separate zoned systems in my home baseboard hydronic heating and a separate air handler added many years later for cooling only AC. Heating works totally independent and Downstairs thermostat was set as master for cooling only. In my current setup if the downstairs is not set to cool mode then the upstairs will not turn on the AC system. I believe this was accomplished on the downstairs thermostat by connecting a wire to the O terminal ( I don’t have a heat pump). After doing some investigating I found the wire that leaves the O terminal on the downstairs thermostat connects to the RC terminal on the upstairs and basically sends needed power upstairs to allow the upstairs Tstat to turn on in cooling mode. For reference downstairs is wire R, RC, O, W, Y, G and upstairs is R, RC, W, Y, G. I was planning on purchasing two nest learning thermostats and spoofing a heat pump on the downstairs and configuring the upstairs without it . Will this work and I assume I will just have to play around with the O/B mode to send needed power when downstairs is set to cooling mode ? Just need to know if there is always power or not any power at the O/B terminal when the cooling mode is changed one way or another or it only energizes when there is a call for heating or cooling depending on the configuration of O and B
