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(Have C wire) Nest Learning 3rd Gen works, Nest 2020 does not?

enzo4 Member Posts: 4
edited December 2020 in Thermostats and Controls
I've hookup additional C wire for my system based on http://www.taco-hvac.com/tmp/ExternalxfrmerZVC-4Powerrobbingthermostat.pdf . Nest Learning 3rd Gen works (technical info power section verified) and Nest 2020 kept saying "the C wire isn't getting power". What is happening?

- R<->C measures about 26V
- R<->W measures about 26V
- W<->C measures about 56V (weird check, but I checked anyway, is this value weird?)
- Nest Learning 3rd Gen works > Technical Info > Power indicates Vin and Voc at 32V
- I've tried multiple different wiring in different locations (I have 6 zones)
- Verified the wire connections multiple times

Nest Learning Thermostat (3rd Gen):

Nest Thermostat 2020:


  • SuperTech
    SuperTech Member Posts: 2,165
    edited December 2020
    Personally I would ditch the Nest for a better thermostat.  But if you want to use it I would look into why you have 56 volts at R and W. Are you using a separate transformer to power the Nest? It looks like you are. That transformer needs to be wired to terminals RC and common or else its voltage will be combined with the voltage at the R terminal. 
  • ratio
    ratio Member Posts: 3,627
    If I were reading 50+ volts somewhere, the first thing I'd look for is another transformer. Do you have cooling controlled by these stats as well?
  • Zman
    Zman Member Posts: 7,569
    With that voltage, you have 2 transformers on different phases being combined.
    Do you have a picture or a drawing of your wiring?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein