replace honeywell with EIC to nest learning 3rd gen

[lx450]

Hello I am trying to replace a honeywell TH9421C1004 thermostat that communicates to a honeywell THM5421C EIC. 

I have effectively bypassed the EIC and connected the nest directly to the C, R, RH, W1, W2, Y, G connections found on the EIC. We do have an AC unit just to mention. everything has booted up but the nest is giving me an error E79 (no power to C) it shows the c is connected but it is greyed out with the red warning. does anyone have any suggestions? 

[CoolingWizard]

You do not need the EIC at all. The wires that attach on the right side go to the HVAC equipment and the Rc/Rh and the C come to the EIC from the HVAC equipment and attach on the left side. The wires on the right side of the DIC should be the W, Y, and G.  You will need connect the Rc wire from the thermostat to the Rc and/or Rh wire removed for the EIC.  Connect the W1 from the Nest to the W1 wire removed from the EIC. Repeat this process for the W2, Y1, Y2, and G.

AC Cooling Wizard

[lx450]

• Hello thank you for responding. I had read similarly to what you described about bypassing the EIC but with all connections made i was getting an error for the C common wire. Upon further investigation I am seeing a few discrepancies between whats labeled in the EIC and what’s labeled on the hvac unit. On my EIC inhave two yellow wires attached to the Y terminal. One yellow wire appears to go out to the ac unit and the other yellow wire appears to connect to the Y2 terminal on the hvac unit. Im thinking i should connect my nest wire directly to the hvac y2 terminal along with moving the ac Y wire to the same terminal? Now as for my red R wire. The EIC has the R wire connected going to the external ac. The RH terminal on the EIC appears to be connected to the R terminal on the hvac unit. However this wore goes through a condensation pump as well. Should I combine the RH and R wires and connect to the RC terminal on the nest.
  thank you again for youre help 

[CoolingWizard · Accepted solution]

@lx450, lets discuss the yellow wires you described on Y2. The Y terminals are controlling the cooling.  Y activates the outdoor unit; basically turns on the compressor. When you see a Y2 that is for stage-2.  Some compressors have multiple stages and each stage increases compressor performance. I would need to know the manufacturer model number of the outdoor unit to verify its features.  
When you have multistage heating and cooling, the indoor control board must adjust the blower fan speed. Now, back the yellow wire. It is the thermostat that calls for cooling. The thermostat takes the incoming power on R/Rc and connects it to Y/Y1. This power goes on the Y wire to the indoor air handler and is also cross connected to a Y/Y1 wire going to the outdoor unit. When the indoor unit sees this incoming power on the Y/Y1 wire, it starts the blower fan. Now let’s add in the second stage Y2. 
When the thermostat detects the cooling is taking to long or that the room temperature is still raising, the thermostat will call for stage-2 cooling by connecting the  power from R to the Y2. This power arrives at the indoor air handler where it is cross connected to the Y2 wire going to the outdoor unit. The air handler will increase the  blower speed and the compressor will internally close the bypass flow valve thus increasing the refrigerant flowing. I hope this helps. 

AC Cooling Wizard