AC wiring does not work

[VTRick81]

Hello. I am desperate for help here and have scoured the internet for answers. I have a 3rd gen Nest learning thermostat. The system is baseboard hot water for heat, and a separate ducted AC. The wires connected to the old thermostat were R and W for heat, and Y, G, and RC for cooling. I have added a C wire connected to the 24v off the boiler control panel to power the thermostat. The problem I have is when I connect the RC wire, it gives an e79 error saying there is no power to common. Disconnecting the common gives me e80, no power to Y1. The heating system works fine with the AC wires disconnected. Nothing works with them connected. The AC won't run without the RC wire, which turns the system on when I touch it to G. Sorry for the long post. I've tried everything. Please help! 

[Markjosephp]

Hello VTRick81,

Thank you for taking the time to let us know about your inquiry. I'd be happy to take a look at this for you.

I appreciate all the steps you've taken. To check its configuration, could you please share with us a photo of your previous and current thermostats' wiring? Also, gather its power readings in the Technical Info settings.

I'll look forward to your response.

Regards,
Mark

[Markjosephp]

Hey there,

I'm checking back in. How's it going? Please let me know the details I need above so we can move forward. An incorrect configuration could cause a wiring report. Also, if possible, could you gather your HVAC system's make and model?

Best,

Mark

[Markjosephp]

Hi VTRick81,

I'm checking in to make sure that you've seen my response. Please let us know if you have any additional questions or concerns before we close this thread in 24 hours. 

Thanks,

Mark

[VTRick81]

Thank you for the reply. I did more research and discovered that with a dual transformer system, the 24v common power has to come off the cooling system, not the heating system. I'm not sure why this is, or why the Nest team does not publicize that more, as I had to dig for hours before I came across that solution. Rather than running even more thermostat wire up to the AC handler in the attic, I installed an outlet in a closet behind the thermostat and wired a 24v plug connected to C and RC. Everything works great now. It does give an error now saying there is no rh power, but that appears to be normal with this configuration and does not prohibit the heat from functioning normally. 

Again, thank you for your response. 

[Jenelyn_O]

Hi there,

Thanks for keeping us in the loop. I'm glad that everything is working perfectly fine now. Please feel free to create a new thread if you have any questions or concerns, as we'll be willing to assist you from there. Otherwise, I'll be locking this thread after 24 hours.

I appreciate your help, Mark.

Best,

Jenelyn