I am trying to setup a Nest Thermostat and I am getting a few errors when trying to set it up. I am getting errors that no wires are detected on the Nest even though the old thermostat is detecting the wires and working fine. I read online that I need to add a C-Wire. My current thermostat needs 4 wires (G-R-Y-W).

When looking at the zone control board, I only see those 4 terminals and no C-wire terminal. Do I connect the C wire to the com D+ port?

I have a ZTECH ZCV2 2 Zone HVAC Control Unit.

I would like to replace the two thermostats with two Nests. Meaning I would like to connect two Nests to the zone control unit.

Wiring schematic

  • Which part of this diagram/system are you trying to replace with the Nest? One of the Zone displays/controls in the upper left? Or the thermostat in the lower right (which I suspect is really "connect to HVAC like a thermostat would", but not clear)? Commented Sep 6, 2022 at 0:47
  • 1
    Ah good question, I would like to have the nest connected to the zone displays on the upper left. I have 2 nest thermostats and would just like to replace the thermostat unit I currently use with the nest.
    – anijjar
    Commented Sep 6, 2022 at 3:07
  • In that case, ThreePhaseEel's answer is the correct one. Those Zone displays communicate with a different (far more sophisticated) protocol than standard thermostats and Nests. Commented Sep 6, 2022 at 3:39

1 Answer 1


Your zone board doesn't use standard North American thermostats

Unlike many zone boards out there, yours uses proprietary control heads that communicate with the zone board over a digital data link, instead of interpreting the on/off control signals from a standard 24VAC North American thermostat into a suitable set of calls for the zone. As a result, your system is not compatible with the Nest, or any other smart thermostat for that matter.

  • I see, are there any workarounds I can do besides replacing the zone board? are there any converters I can have in between the nest and the zone board?
    – anijjar
    Commented Sep 6, 2022 at 3:10
  • @anijjar your best bet will be to replace the zone board -- you'd have to ask its manufacturer if they make an interface device for it Commented Sep 6, 2022 at 4:09
  • Got it, thank you very much for all of the information!
    – anijjar
    Commented Sep 6, 2022 at 4:14
  • Wow... actually a new "where does the 'c' wire go?" question!
    – FreeMan
    Commented Sep 6, 2022 at 12:53

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