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Nest thermostat installation

Volcar21
Community Member

Hi Folks, I recently acquired a Nest thermostat to replace my existing Honeywell model #THX9421R02 that has two wires labeled R and C and when I connect the NEST unit using the connection labeled R and C I receive an error message stating that it needs to be corrected via W and Y and when I make the chang I receive another error message stating that the voltage is low. As a consequence I am seeking assistance on how to address this issue. Ane whether the NEST is a suitable replacement for the Honeywell unit. All replies and suggestions will be greatly appreciated. @Link to members 

4 REPLIES 4

0whitedevil0
Community Member

The Honeywell THX9421R02 and Nest thermostat have some differences, notably in the wiring. For your specific setup, try connecting the Nest thermostat with the following wires:

- C: Common wire
- R: Power wire
- W: Heating wire

You can leave the Y wire unconnected since the Nest thermostat doesn't use it.

Regarding the low voltage error, it's crucial to ensure a secure connection of the C and R wires to both the Nest thermostat and your HVAC system. Also, check the voltage at the R and C terminals at the HVAC system; it should ideally be between 24 and 28 volts AC.

If, after these checks, the issue persists, I recommend getting in touch with a qualified HVAC technician for further assistance. They can provide on-site support to troubleshoot the problem.

Now, regarding whether the Nest thermostat is a suitable replacement for your Honeywell unit — absolutely! The Nest thermostat comes with some fantastic features, including a learning algorithm to adapt to your habits, a convenient remote control app, and a self-diagnosing system for troubleshooting.

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 Hi Owhitedevil0, thanks for your response to my inquiry it is greatly appreciated, however I have a follow-up question. I have attached a photo of both my existing baseplate wiring and the NEST baseplate. I will be extremely grateful if you could provide me with a sketch showing how the NEST baseplate should be wired. A timely response will be greatly appreciated.

 

I understand. Based on the image you sent, your baseplate wiring only has two wires: R and C. This means that you have a single-stage heating system.

The Nest thermostat is a multi-stage thermostat, so it will not be compatible with your existing wiring.

If you want to use a Nest thermostat, you will need to upgrade your wiring to include a W wire. This can be done by running a new wire from the Nest thermostat to your HVAC system.

Once you have upgraded your wiring, you can connect the Nest thermostat as follows:

  • R: R wire
  • C: C wire
  • W: W wire

If you are not comfortable upgrading your wiring yourself, you can contact a qualified HVAC technician to do it for you.

I hope this helps!

EdmondB
Community Specialist
Community Specialist

Hello Volcar21, 

I'd like to jump in here and check if you've seen 0whitedevil0’s response. Let us know if you need more help by replying to this thread.

I appreciate the help, @0whitedevil0

Best,
Edmond