Use an External Transformer to power up Nest thermostats without C wire

[Misrata]

Hello,
I'm a tenant trying to install a Nest thermostat. The old thermostat has a jumper between Rc and Rh, and there's no C wire. Because I'm not allowed to touch any wiring inside the furnace, I'm using an external 24V transformer for power.
My question is simple: Do I need to connect the R wire from my furnace to the Nest, or should I only use the R wire from the new external transformer?
I want to avoid any damage and ensure I'm wiring this correctly since I can't use a Nest Power Connector or modify anything in the furnace.
Thanks for the help!

[kcruzgonzalez]

Hello @Misrata,

Thank you for posting in the community. I understand your question about the wiring of your thermostat in order to properly set up the device in a safe manner. I'm here to help you get it set up correctly and safely.

To help me give you the safest and most accurate advice for your specific setup, could you please provide a little more information?

1. Do you happen to have a photo of the old thermostat's wiring before you disconnected anything? This is the most helpful way for us to see the original setup.
2. Could you please confirm the specific model of your thermostat? (Please visit our Help Center to learn more about how to tell which Nest thermostat you have).
3. Could you also tell me the model or type of your HVAC system? For example, is it a conventional system (forced air), a heat pump, or something else?

By getting these details, we can ensure we provide the safest and most effective wiring instructions for your home.

Regards,

Kevin

[Misrata]

https://drive.google.com/file/d/1NU0wm0Igri_OJ6b2r8TYI0iTtvwIUA8T/view?usp=drivesdk 

It’s a regular forced air system and I have Nest Thermostat (The one that has finished mirror screen and no dial)   That has R,C, W, Y, G , and *OB input

[kcruzgonzalez]

Hello @Misrata,

Thank you for providing the photo and the details about your HVAC system. That is very helpful, and it gives us a much clearer picture of your situation.

A great idea is to try to set up the thermostat using the wires you have and see what the device says during installation. The Nest Thermostat's software is designed to guide you through the process and will perform a check to see if your system needs any additional wires.

Important: Before you touch any wires, be sure to turn off the power to your HVAC system at the breaker to avoid any damage or injury.

Carefully disconnect the wires from your old thermostat. Remember to keep a copy of your photo handy.

Once the wires are connected, push the thermostat display onto the base and turn the power back on at the breaker. The thermostat should power on and begin the setup process. The Google Home app will guide you through the installation and let you know if a common wire (C wire) is needed for a stable connection.

Please let me know how the installation goes and what the app says during the process. We'll be here to help you with the next step.

Regards,

Kevin

[Misrata]

Thank you for attempting to answer my question. however, my question was clear. The thermstat requires a C wire. I want to know how to use an external transformer to provide a constant power to the thermostat.

[kcruzgonzalez]

Hello @Misrata,

Thank you for your patience. I understand that you'd like to get this installation done correctly and safely, and considering the complexities of your specific setup, a professional technician is the best way to ensure everything works as it should.

Because we cannot physically inspect the wiring at your furnace or confirm the installation of the external transformer, it's difficult for us to provide further guidance without seeing everything in person. Hiring a professional is the best way to avoid any potential damage to your home or your new thermostat.

A qualified technician can:

• Safely inspect your existing wiring.
• Properly install and confirm the voltage from your external transformer.
• Complete the entire setup and configuration of your Nest Thermostat.

Please let me know if you have any questions.

Regards,

Kevin