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Nest 3rd gen wiring for 2-stage heating and 1-stage cooling with 5 conductor thermostat wire

MrRyan
Community Member

We are replacing our furnace with a new 2-stage model (York TM8Y). We previously had a single-stage furnace. We also have a single-stage A/C unit.

It looks like the thermostat wire to the Nest is only 5 conductors. Based on the diagrams I have seen, it looks like I would need 6 conductors to accomplish this:

  1. G - Fan Relay
  2. Rh - 24VAC power from heating transformer
  3. C - 24VAC Common Wire
  4. Y1 - Compressor Relay (Stage 1)
  5. W1 - Heat Relay (Stage 1)
  6. W2/AUX - Heat Relay (Stage 2)

 

My Nest Backplate (v5.3) is currently wired up with 1-5 above. I noticed in the Nest Pro Installer Guide that it says:

Fan relay (G) is optional. However, without a G wire, Nest will not be able to control the fan independent of heating.
Common wire (C) not required in most cases, but strongly recommended.

Does this mean I could use the Common wire (C) conductor to add my W2 for my 2nd stage of heating?

I would rather not use the Fan relay (G) as sometimes I like to turn the fan on to circulate air. We have a tri-level and the temperature from the top to the bottom can be several degrees different.

How can I tell if I can use the Common wire?

1 Recommended Answer

EdwardT
Community Specialist
Community Specialist

Hi folks,

 

@Patrick_Caezza, thanks for the help!

 

@MrRyan, thanks for reaching out. Since you have a Nest Learning Thermostat, you could put your stage 2 heating on the W2 terminal. Also, if you're not experiencing any issues with your system without the C wire, that means that your system is fine with how your Nest Thermostat charges its batteries using your cooling and heating wires.

 

Thanks,

Edward

View Recommended Answer in original post

7 REPLIES 7

Patrick_Caezza
Platinum Product Expert
Platinum Product Expert

Post a picture of the original thermostat's wiring


To ensure that I see your reply, please tag me using @Patrick_Caezza

MrRyan
Community Member

I don't have it. I have had the Nest 3rd gen installed for about 6 years. The issue is we are replacing a single-stage furnace with a 2-stage furnace. I am looking for guidance on how to wire it.

EdwardT
Community Specialist
Community Specialist

Hi folks,

 

@Patrick_Caezza, thanks for the help!

 

@MrRyan, thanks for reaching out. Since you have a Nest Learning Thermostat, you could put your stage 2 heating on the W2 terminal. Also, if you're not experiencing any issues with your system without the C wire, that means that your system is fine with how your Nest Thermostat charges its batteries using your cooling and heating wires.

 

Thanks,

Edward

EdwardT
Community Specialist
Community Specialist

Hi MrRyan,

 

It's me again. I wanted to check back in to see if you have other questions and concerns. Feel free to let me know if you do. 

 

Thanks,

Edward

MrRyan
Community Member

I haven't tried re-purposing the C wire for my 2nd stage heating, but I understand that is my option. I wasn't sure if there is some kind of analog-to-digital interface conversion kit that could make it so my thermostat has all functions available, regardless of how many wires, and translates it at the furnace into however many wires are needed.

My HVAC contractor has suggested I don't tell Nest that I have a 2-stage heater and instead set the furnace so that the 2nd stage comes on after 20 minutes.

EdwardT
Community Specialist
Community Specialist

Hi MrRyan,

 

Thanks for the response. I'm not sure of an analog-to-digital interface converter either but please observe your Nest Thermostat for now and let us know if anything comes up.

 

Thanks,

Edward

EdwardT
Community Specialist
Community Specialist

Hi MrRyan,

 

I'm just checking in to make sure that you've seen our response. Please let us know if you have any other questions or concerns as I will be locking this in 24 hours.

 

Thanks,

Edward