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Wiring Question - Two Systems, Identical Wires (R, W, G)

Community Member


Hi All --

I have a Nest learning thermostat that I am looking to wire two independent systems (heating and AC) into.

Each system has three wires -- red, white and green.

The reds I can put in Rc and Rh.

I assume I put the whites in W1 and W2, but might be wrong.

I have no idea with the greens.

Here is how it was wired previously on the E thermostat (and unlabeled by installer). The heat appeared to work, but not the AC:


We changed thermostats to accommodate two systems and here is what I am stating at:


Any help is much appreciated!




Platinum Product Expert
Platinum Product Expert

@mabond17 ,

You cannot connect more than one HVAC system to a single thermostat.  So, you can have a separate heating  system, and a separate air conditioning or a combined heating and air conditioning system.  So here is what the standard HVAC control wires do:

R terminals, (R, Rc,Rh) are for Power Input to thermostat. It is 24Volts AC power. Think of this as the LINE.

C terminal, this is called the Common, this is 24 VAC, think of this as the Neutral.

Y terminal, (Y, Y1, Y2) this is for cooling, it is the Compressor relay known as a Contactor.  
W terminal, (W, W1, W2) this is for heating.

G terminal, this is for the indoor blower fan relay.

When the thermostat is calling for cooling, it connects R/Rc to Y and G. This turns the blower fan on, turns the outdoor compressor on and thus initiates cooling.  
When the thermostat calls for heating, it connects R/Rh to W/W1. This initiates the heating process. 
The Y2 and W2 are for second stage cooling and heating respectively.

The HVAC industry typically uses thermostat cables with Red, Yellow, White, Green, Blue, Black, Orange Brown conductors.  There are 4, 5, 7, 10 conductor cables in our industry. These cables are 18-gauge solid copper conductors.  

AC Cooling Wizard

NestPro, Google Pro, Mechanical Engineer and HVAC service company owner.
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Community Specialist
Community Specialist

Hello mabond17,

I'd like to jump in here and check if you've seen CoolingWizard's response. Let us know if you need more help by replying to this thread.

Thanks for the input, @CoolingWizard.


Community Specialist
Community Specialist

Hi there, 


I want to check and see if you are still in need of any help. Please let me know, as I would be happy to assist and answer any questions you may have.



Community Member

Thanks, all! The replies were very helpful. 

Community Specialist
Community Specialist

Hi mabond17,

I'm glad to hear that you've found the answer you're looking for. If you're still in need of assistance, feel free to let us know or create a new thread to open a new topic.

Best regards,