Is my 2-wire TH8321 system compatible with 3rd gen Nest Thermostat?

[leftlane]

I'm trying to replace a Honeywell TH8321R1001 thermostat with a 3rd gen Nest.

The old TH8321 thermostat has one red wire going to R with a jumper from R to Rc, and one white wire to C. (Photo of TH8321 wiring )

My HVAC system is a Trane XV80, which provides both AC and gas-powered heat.

When I went through the installation process via the Nest app, it prompted me to contact support to verify compatibility. I did, and the support tech told me to connect the red wire to Rh and the white wire to W1. 

That resulted in the Nest only controlling the heat. 

Switching the white wire to Y1 results in the Nest only controlling the AC.

The EIM in the attic only appears to have support for two wires to the thermostat, so I don't think I can pull an additional wire. (Photo of EIM wiring )

Is my system just incompatible with the Nest thermostat?

[Patrick_Caezza · Accepted solution]

@leftlane 

You need to verify how the red and white wires are connected to the furnace/air handler.

Take a look at the terminal block on the control board in furnace/air handler and post a picture of how it's wired

[leftlane]

Thank you for responding. The second photo I linked to in my original post shows how the red and white wires are connected to the EIM. From there, the EIM connects to the AC and furnace, as shown in the photo. 

[Patrick_Caezza]

@leftlane 

None of the Nest thermostats will work with the EIM as it uses a wireless interface. the red and white wires are just for powering the Honeywell thermostat.

The Nest thermostat will work if you remove the EIM and wire it directly to the Furnace/air handler, but you might lose any propitary features that the Honeywell thermostat provides