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Taco ZVC404 EXP - All valves open

Hi all,

New homeowner here having an issue with my Taco Zone Relay. I have 3 zones. I recently replaced a thermostat (Honeywell non-progammable) in one of the zones, and now I'm having issues.

Currently, all valves are stuck open. The relay still recognizes all 3 thermostats when they call for heat (yellow light) but the valve open (red light) now only comes on for the upstairs thermostat. The two downstairs thermostats (including the new one I just replaced) do not trigger any valve functions. And I know all valves are stuck open because all the pipes get hot when the upstairs zone is triggered.

Any troubleshooting tips here? Can I manually reset the valves somehow?



  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 4,999
    Some valves have a manual open lever. What valves do you have? Also What model thermostat was removed and what model thermostat was it replaced with? Posting a picture of the valve(s) may help.
    Edward Young Retired HVAC Contractor & HYDRONICIAN Services first oil burner at age 16 P/T trainer for EH-CC.org
  • Joe Mattiello
    Joe Mattiello Member Posts: 654
    edited November 2021
    if the only change was thermostat it could be a wiring issue. Remove the thermostat from RW terminals and see if board works normally.
    if not remove all three stats and jump out RW terminals one zone at a time. 
    My suspicion is your new stat is electronic and a resistor across W and C may be necessary try my diagnostic suggestion, and we’ll go to plan B if we have discovered issue. 
    you can contact Taco tech support for additional help. 
    Those technicians love this stuff!
    Joe Mattiello
    N. E. Regional Manger, Commercial Products
    Taco Comfort Solutions
    EBEBRATT-Ed Member Posts: 13,165

    Did you install a Nest thermostat?
    HVACNUT Member Posts: 4,873
    Were all three zones working correctly before you replaced the thermostat?
    Is it possible there were issues when you purchased the house?
    If so, is there $$ in escrow?
  • @EBEBRATT-Ed No I'm trying to install a Honeywell CT87K1004 (Heat only).

    @HVACNUT I'm actually not sure if this third zone was ever properly working. What's also different is this third zone has 4 wires (red,white,green,blue).

    @Joe Mattiello what do you mean by "..see if board works normally."

    EBEBRATT-Ed Member Posts: 13,165

    I would do what @Joe Mattiello suggested. Mark and disconnect all thermostat wires from the ZVC404.

    Check and see if all zone valves are closed

    Then get some jumpers or a scrap piece of wire. Jump out R & W for zone 1 and see if the valve drives open....take the jumper off the valve should close.

    Then do the same think for zone 2 & 3

    If they all work as they should reconnect zone 1 thermostat and see if it operates the valve open and closed.

    then repeat for the other zones

    If a zone works with the jumper or not with a stat then you may have found the issue

    As @Joe Mattiello said you may need a resistor with that electronic stat
  • @EBEBRATT-Ed thanks for the clarity, this is very helpful.

    How do I check if the zone valve is closed? Turning on the heat and checking if the pipe is cold?
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 3,517
    edited November 2021
    There's a slide lever on those Honeywell zone valves. It's just underneath where the wires go into the operator. If it slides all the way over to the other end very easily, it's open.

    It could be that the end switch on the two zone valves is no longer closing. Once the valve opens, it closes a micro switch in the zone valve that tells the zone valve control to turn on the boiler and the pump. It's what turns on the red light on the control. It's a common thing for end switches to fail and you have to replace the zone valve operator. You can buy them here.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hourTwo btu/ per sq ft for degree difference for a slab
  • Joe Mattiello
    Joe Mattiello Member Posts: 654
    what do you mean by "see if board works normally?
    When you've disconnected all thermostats and validate a demand using a jumper across RW on control one zone at a time, the board should open the valve, close the end switch interfaced with boiler RW terminals and fire the boiler. that's what I mean normal operating condition of ZVC control.
    HAs this been resolved?
    Joe Mattiello
    N. E. Regional Manger, Commercial Products
    Taco Comfort Solutions