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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: 6,127
    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: 169
    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