Replacing two thermostats

[TJH2]

I am planning to purchase the Nest Learning Thermostat, but would love to see a wiring diagram for replacing the two thermostats I have now.  From what I have read the Learning Thermostat is compatible.   

Presently, I have one thermostat controlling the boiler heat for the house with the other thermostat controlling a SpacePak AC system, consequently I have two transformers.  

For the boiler, there are only two wires coming out of the wall.  One is connected to the terminal block labeled "R".  The other wire is connected to the terminal block labeled "G"

The AC thermostat has 3 wires coming out of the wall.   Red wire is connected to "RC".  The Yellow wire is connected to "Y",  The green wire is connected to "G".  

Before purchasing I would love some wiring advise.   Seems is shouldn't be rocket science 🙂

[TJH2]

I need to slightly amend the boiler wires above.  The red wire is attached to both “R & RH” on the terminal block.  The other wire is attached to the “W”    The terminal block & lettering are not clearly lined up. 

[zoeuvre]

Hi TJH2, 

Thanks for posting here in the Community forum and for showing interest in our Nest Thermostats. Are you located in North America or Europe? Also, could you send us a photo of boiler and AC thermostat wiring configuration so we can take a closer look at its compatibility with our Nest Thermostat? You may also use our compatibility checker to determine if a Nest Thermostat is compatible with your heating and cooling system.

Keep us posted.

Best,

Zoe