Nest Installation Wiring [duplicate]

Question

Hello trying to wire a Google Nest Learning Thermostat and my old one used only VG and V+ and now I need to know what I should hook it to.Also I have 3 others wires that weren't hooked to anything in case we need those, they were in the wall already. Also I have three wires already running into my C terminal and I wouldn't be able to put a fourth one in there. Any help would be appreciated and can provide pictures. Thanks

Answer 1

Google has tons of instructions online.

Here is basic for all Thermostats.

Unfortunately the industry did not standardize on markings or color codes.

First you have 2 wires that bring 24 Volt (AC) to the thermostat.

Then you have mostly 3 more wires.

Those are for Cooling On, or heat ON, or Just Fan.

You might have more wires depending on your HVAC, like if you have separate heating.

Those 3 are interchangeable.

What that means is just stick one in turn on Nest to Heat/Cool/Fan and find out what goes on.

(Cooling, Heating, Fan).

You can not do anything wrong with just trying.

Answer 2

OK, so you may notice that thermostats have a lot of wires. Why do they have so many wires? Can't they used digital tech to cram it all into 2 wires, sort of the way USB works?

After all, this is a huge inconvenience for someone with 2-wire thermostat cable who wants to add an air conditioner or modern multistage heating system or heat pump.

Somebody at Carrier looked at this question and came up with a split system. The thermostat actually has two "halves": The display unit and the control panel.

The control panel (oddly named since it has no controls to operate) sits in utility space near the furnace, and takes the usual wires: R, W, G, Y, and C, and possibly others depending on your system. It then sends V+ and Vg to the display unit.

The display unit, which you are incorrectly calling the "thermostat", sits in an attractive location and you interact with it. As you see it only connects V+ and Vg.

This would be easy if you wanted the Nest where the "control panel" is. However I gather you want it where the display unit now is.

That means you must find the control panel, identify all the wires in your system, figure out what kind of system you have, figure out which wires you really need, and then figure out a way to bring them up to the Nest.

It sounds like you have 5 wires in your wall, so you have a chance of success if it's a basic heat+A/C system.

If it's a heat pump, this will take a lot more design thought - aw heck, the simplest way to resolve that is to install 18/8 thermostat wire between the control panel and where you want the Nest.