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Unique Dual Fuel situation and Nest wiring/compatability question.

Community Member

Trying to determine Nest compatibility with my setup at my home. I have two separate systems in my home.  I have an oil burner with baseboard heat.  I also have a carrier AC unit added years later that is only for cooling. I have two zones in my home with an upstairs and downstairs thermostats so I realize I would need to purchase two nest thermostats. The heating system should work with no issues. Each thermostat has wiring that runs down to the oil burner ( 2 wires to each zone valve Red/White ) to an individual zone valve powered by a 24V transformer with an end switch to turn on the oil burner. Nest should do that with ease. The issue comes with the wiring difference between the upstairs and downstairs thermostats.  The upstairs has wires connected to R, RC, W, Y, G (no C )  The downstairs has R, RC, O, W, Y, G (no C).  When the Carrier AC was installed the tech told me that the downstairs was the master thermostat for cooling and that in order for the AC to work upstairs that the downstairs needed to be set to cooling mode which I assume is the extra wire on the O connector on the downstairs thermostat to set it as the master to send signal to the Air handler that cool mode is active. Once the downstairs is set to cooling they work completely independent.  ( AC runs upstairs without the downstairs needing to be actively cooling at the same time.  Just cool mode set on the thermostat ) Since my O connection is related to the cooling and not heat where would the O wire connect on the downstairs thermostat to enable the cooling for both zones when installing a Nest learning thermostat ?  From all my research Nest uses the O port for heating only.  Any ideas or help would be greatly appreciated. Just don't want to spend the money and purchase the Nest thermostats if they won't work. 


Community Specialist
Community Specialist

Hi HarveyW,


Thanks for posting here in the Google Nest Community. We'd be happy to take a look into this for you and see if your system is compatible with our Nest Thermostat. We’ll need a couple of pieces of information before we can confirm if it’s compatible or not. Please answer the following:


  1. Are you from North America or Europe?
  2. What type of Nest Thermostat are you planning to have?
  3. What is the make and model of your AC and heating system?
  4. Send us Photo of your HVAC wiring. 
  5. Send us a photo of your current thermostat wiring, both upstairs and downstairs.

I look forward to your response.