Help identify how thermostats are wired and how to wire Nest learning

[stevedud]

Just bought a new house and I’m not sure how the upstairs thermostat works.
It’s a two story home with a cathedral using forced hot air and AC, with a thermostat on the bottom floor and one on the second. The bottom floor thermostat works fine, controls heat, AC and fan. Upstairs doesn’t seem to do anything but it’s wired.

there is only 1 zone.

I am replacing these thermostats with Nest Learning ones.
help please with how this works.

here is the downstairs thermostat.
W, Y, G, R, C wires

here is the downstairs thermostat. W,Y, G, R, C wires

Here is the upper one.
R, Y, G

Here is the upper one. R, Y, G

[Patrick_Caezza]

Two separate thermostats usually mean you have a 2-zone system.

The upstairs thermostat is for cooling only by the wiring with the downstairs thermostat controlling both heat and cool.

Check to see if there is a zone controller mounted close to the furnace/air handler.

Post a picture of the terminal block on the control board in the furnace/air handler,

[stevedud]

I cannot find a way to post a picture in the reply but the upstairs wiring at the furnace is as follows:

red wire goes to Y

white wire goes C

green wire is not connected to anything

how should a nest learning be connected upstairs to control the AC only? 

[Patrick_Caezza]

That sounds like the control wiring for the outdoor unit.

Look to see if there is a smaller control box next to or near the furnace. Try tracing the thermostat cable from the furnace and see where it goes.

Zone controllers have a Master thermostat and then slave thermostats. The downstairs thermostat should be the master and the upstairs thermostat should be a slave

[stevedud]

After looking at all the wiring, the upstairs 3 wire cable is not connected to the furnace. So with that, I can splice into it. Please suggest how I would use the nest upstairs and which connections to make. 

[Ryan_G]

Hey folks,

Thanks for visiting the Google Nest Community. 

Since this thread hasn't had activity in a while, we're going to close it to keep content fresh.

If you have additional questions, feel free to submit another post and provide as many details as possible so that others can lend a hand. 

Hope this helps!

Kind regards,

Ryan