Why is my Nest thermostat not working with A/C?
This has been idle a while, but my guess is you need to connect the Nest to the R terminal of thermostat 1 on the controller instead of R1.

It looks like your control system was designed and wired for a heatpump but you no longer have one (there are only 4 wires on the lower left block which I think is the output to the furnace/AC compressor).  If so the O and B wires are unneeded and can be removed from the control panel.

So what I would try:


Zone 1 R to Nest Rh  (24VAC Hot)
Zone 1 C to Nest C   (24VAC Common)
Zone 1 W to Nest W1  (Heat)
Zone 1 Y to Nest Y1  (Cool)
Zone 1 G to Nest G (Fan)


Only thing I don't understand is how the fan gets turned on by the second thermostat in Cool mode.  (In cool usually the thermostat has to tie both G and Y to R (24VAC Hot), in heat only W has to be tied to R (+24VAC Hot)... Since it works it must be something the controller does automatically.

By connecting the Nest to the C terminal it gets access to 24VAC without trying to power it self from "leakage current".

It you want to test, jumper Zone 1 R to Y for a minute and see what happens.  Does the compressor come on outside?  Does the interior blower fan?

Double check this against the manuals..