Wiring – Do I need another relay

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:

1. Zone 1 R to Nest Rh (24VAC Hot)
2. Zone 1 C to Nest C (24VAC Common)
3. Zone 1 W to Nest W1 (Heat)
4. Zone 1 Y to Nest Y1 (Cool)
5. 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..

The Nest can only control one zone at a time, but that's ok, because it sounds like there's only one "control zone" in your system.

Your heating system may work better if its split into three sections, but since the will all come on at the same time, the Nest should be able to control all three at the same time. What the Nest can do is control each zone separately.

Typical control systems have a "hot" wire and a "common" wire. Just tie all the commons together and all the hots together and then run a single common and hot to the Nest for control.