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Yet another Nest intstall nightmare!

mattafactmattafact Member Posts: 4
Hello friends!

I recently decided to take the plunge, sell a pint of blood, and purchase a Nest Learning Thermostat.

My system is set up as follows...
- AC controlled by Unico system in attic (Rc-Y-G)
- Heat controlled by Taco ZVC404-4 in basement (Rh-W)

The issue is the Nest seems to struggling to pull power from my Taco. (Error on my Nest reads = N72 : No power to the Rh wire). When the heat zone is not active, the orange T-Stat call light flickers and is not solid. When the zone is active, is stays solid and eventually the valve opens.

I did not orignally have a c-wire installed, so I ran one from my Unico unit in the attic to the Nest, thinking that this would solve my problem, but the flickering orange light on my Taco persists.

Does the c-wire need to be wired to the Taco panel instead of the Unico? Or is this an issue with my Rh wire?

Pictures of both of my valve panels are attached.

Any words of wisdom would be much appreciated!


  • Dave HDave H Member Posts: 303
    I ran the c-wire from ZVC 404 and haven't had a problem in 4 years.
    For what its worth, I also have a Lyric on the same ZVC controller also with the c-wire from the controller.
    However, for my AC with a Lux Geo, the c was from the air handler.

    Dave H.
    Dave H
  • mstrbillmstrbill Member Posts: 22
    Add a relay at the heat zone panel and power it from the Nest AC side of the wiring . Must use one power source - at Nest use the AC -blue splice behind it , wire to common nest base and red of heat wire - , white goes to w - nest base - then powers the relay at heater.
  • mattafactmattafact Member Posts: 4
    Thanks Dave H. I ran the common down to my Taco unit and the same problem power to the Rh. So I hooked the Nest directly up to the Taco (just W-Rh) and it worked fine! I think there is an issue when the AC wires (Y-G-Rc-C) and the heater wires (Rh-W) are all hooked up together. They don't play nice.

    And thank you mstrbill for your input. Unfortunately I'm not sure I follow about how to add a relay at the heat zone panel. Any chance you could explain in detail?

    thank you!
  • ratioratio Member Posts: 1,529
    Here is a link to a schematic showing a Nest controlling two separate mini split heads via relays. Your application would be simpler, basically just the one relay marked "W1", the AC would connect to the Nest directly.
  • mstrbillmstrbill Member Posts: 22
    Here goes ... add a RIBUC1 to the Taco zone box - yellow - orange to R W of your zone - white/yellow and white/blue of rib to r w that used to be there from stat . Cap spare rib wires . Now at Nest twist together Blue (C) from AC with red (old heat Rh ) and add a 3rd blue color wire splice with tiny wirenut - tuck behind nest base - connect blue wire to C -nest base . Then, old white W back to W - nest base , reconnect Y, G , Rc . Try heat and AC . If Heat does not turn on , try Rc to Rh wire switch , if AC or heat does not then base is no good , order new base from nest. keep wire configuration as told - powered from the AC trans only . Go to settings - equipment - pro setup - turn all yellow lights green by selecting your new configuring , and all will be fine .
  • mattafactmattafact Member Posts: 4
    That is a HUGE help mstrbill! Thank you! One thing I'd like clarification on please. The spare RIb wires....can I cap them all together, or do they need to be individually capped?

    Thanks again!
  • Big EdBig Ed Member Posts: 1,068
    I have enough experience to know , that I dont know it all
  • mstrbillmstrbill Member Posts: 22
    yes individually
  • mattafactmattafact Member Posts: 4
    Worked like a charm! Thank you mstrbill!
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