Wiring thermostats on a 3-wire without a common

[randydutton]

How can I get a Nest Gen 3 to work with three wires, R,G,W on a five-zone system with heat pump (for cooling and heating), aux heat? In the tech page, it says "The Nest Learning Thermostat is designed to work even if your home doesn't have a common C wire, but installing a new C wire or the Nest Power Connector may be required in some cases." but it doesn't say under which circumstances a C wire isn't needed. I really don't want to run new cables.

[sporfrog]

The specific answer of "under which certain circumstances" is ...  if it doesn't work on your specific system. The most reasonable way to know is to try it, but understand it may not work. You should have a plan for C.  If C is required you can:

- utilize an unused wire if there is one, connect to C at both thermostat and HVAC unit.

- Install the Nest Power Connector, which does not require running new thermostat cable.

If you have a chance send an image of the wiring for your current thermostat.

[randydutton]

It's only a three wire setup. failing control panelSample zone

[Patrick_Caezza]

Your current system uses a proprietary interface for each thermostat and none of the Nest thermostats are compatible.

[randydutton]

Thank you!

[MelbaDT]

Hey folks, 

Appreciate your responses, Patrick_Caezza and sporfrog. 

randydutton, I'm chiming in to check if everything is good to go here. Let us know if you still have some questions in mind. 

Best, 

Melba

[randydutton]

No questions. I know Nest requires 4 wires which won't work in my case.

[MelbaDT]

Hi randydutton, 

Gotcha — I'll be locking this thread now. If you have any new issues, updates or just a discussion topic, feel free to start a new thread in the community.

Best, 

Melba