Replacing old 2-wire Honeywell with a Nest

[tmeyer2000]

I have an old Honeywell 2-wire mercury switch thermostat (R & W wires) that is connected to 240V baseboard load via a relay. How do I get power to the Nest? There is a spare green wire disconnected at both ends. Can I use this to supply 24V to the Nest by using a simple transformer plugged into 110V in the basement where the relay is? What would the circuit diagram look like, i.e. where is the common terminal?

[zoeuvre]

Hi tmeyer2000, 

Thanks for posting here in the Community forum and for showing interest in our Nest Thermostats. If you have an old Honeywell 2-wire mercury switch thermostat with only R and W wires, and you want to power the Nest Thermostat without running new wiring, you can potentially use the spare green wire as a common (C) wire to supply 24V power. Here's a suggested circuit diagram:

1. In your basement where the relay is located, find the transformer that provides the 24V power for the relay. It's typically a small box mounted on a wall or near the electrical panel.
2. At both ends of the spare green wire, strip a small portion of the insulation from each end to expose the copper conductor.
3. At the relay end, connect one end of the spare green wire to the C terminal of the relay. This terminal is typically labeled "C" or "Common." Ensure it is securely connected.
4. At the Nest Thermostat end, connect the other end of the spare green wire to the C terminal on the Nest base. The C terminal is typically located in the wiring block on the Nest base. Ensure it is securely connected.
5. Connect the R wire from your existing wiring to the R terminal on the Nest base. Connect the W wire from your existing wiring to the W terminal on the Nest base. Ensure both connections are secure.
6. Attach the Nest base to the Nest display unit and mount it on the wall.
7. Find a nearby electrical outlet in the basement and plug in the 110V side of a 24V transformer.
8. Connect one wire from the 24V side of the transformer to the C terminal on the relay. Connect the other wire from the 24V side of the transformer to the R terminal on the relay.

By connecting the spare green wire as a common wire and using the transformer to provide the 24V power, you should be able to power the Nest Thermostat. However, it's important to note that every HVAC system is unique, and it's recommended to consult a professional electrician or HVAC technician to ensure proper wiring and compatibility with your specific setup. They can verify the circuit diagram and make any necessary adjustments or provide alternative solutions if needed.

Let us know if that helps.

Best,

Zoe