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Old Taco ZVC 404-EXP, how to make sure which is the correct Common/C terminal?

I've an old Taco ZVC 404-EXP (photo and wiring diagram attached) with no obvious COM terminal to attached to. Yesterday I read on Reddit that https://www.reddit.com/r/Nest/comments/hts29v/6zone_radiant_heating_thermostat_advice/ someone has a wiring diagram for the common wire for ZVC 405. I was unable to find the documentation anywhere else besides that screenshot. I want to make sure that image matches my Taco setup since someone else with an even older Taco seems to suggest Terminal 2 is the Common.

How do I make sure I have found the right Common since both terminal 1 and terminal 2 seems to provide correct V measurement (see image)?

  • Left T <-> Right T : 26V
  • Left T <-> Zone 1 Terminal 1 : 0V
  • Right T <-> Zone 1 Terminal 1 : 26V
  • Left T <-> Zone 1 Terminal 2 : 0V
  • Right T <-> Zone 1 Terminal 2 : 26V
My conclusion:
  • Right T must be R (Power)
  • Left T must be W (Heat)
  • Either Terminal 1 or 2 is C ??? Which one? How do I know for sure?


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,506
    If you wire it that way you will smoke it. Then you can buy a newer one that has the common terminals on it.
  • enzo4
    enzo4 Member Posts: 4
    Can you help explain why I will smoke it by wiring it that way? The image uploaded in another forum definitely seems like an official Taco documentation (correct font, printed, etc.)

    I actually did wire it that way and the Nest 2020 appears to work fine until I accidentally R + C together which fried the fuse. I still want reassurance and make sure things are ok/not ok. Would like to understand a bit more as to why that wiring is bad, just trying to understand more. Thanks!
  • SteveSan
    SteveSan Member Posts: 234
    Enzo, please feel free to contact Taco Tech Support @ 401-942-8000 Mon-Fri 8am-5pm EST just ask for Tech. The "com" connection on the older zvc's is the #1 terminal down at the zone valve connection as in the above diagram you posted.

    Erin Holohan Haskell
  • nhgeordie
    nhgeordie Member Posts: 1
    This is correct. I added two Nest thermostats following these instructions today. I hooked the left contact of the zone 1 valve control terminal block to the C connection on the two Google Nest Power Connectors and followed the rest of the standard instructions. Afterwards the Nest thermostats verified they had contact with their respective power injectors, that power was being supplied, and heating control worked as normal.
    EBEBRATT-Ed Member Posts: 15,517
    Looks right to me