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Taco ZVC 405 Intermittent Failure

Steve Garson_2
Steve Garson_2 Member Posts: 708
edited March 2021 in Thermostats and Controls
I have a ZVC 405 control on my boiler. This morning, I awoke to no heat and headed to the boiler, to see what was happening. I've had this control for three months and this is the second occurance of this problem: Four T-stat LEDs were illuminated but the boiler was cold. I power-cycled the switch to the boiler and then everything worked properly again: only the Zone-5 HW was operating, since the boiler had been down for perhaps six hours. I would hate to experience this in the dead of winter when away from home for a vacation!

I called Taco support and they said that unless the problem can be reproduced, there is nothing that they can do.

Any suggestions would be appreciated!

No heat LEDs illuminated:

After power cycling:

Steve Garson
Denver, CO
Steve from Denver, CO


  • If both the yellow and red lights are on to any of the zones, the end switch in the Taco ZVC should close and tell the boiler to come on. Either the end switch isn't closing or the boiler is not responding to the ZVC's call for heat.
    If it happens again, I would remove one of the end switch wires from the ZVC and check for continuity between the two terminals. If there is continuity, then something is up with the boiler or the wiring from the ZVC to the boiler and not an issue with the Taco ZVC.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,618
    edited March 2021
    I have never had this problem with the Taco ZVC controls and I have installed many. So the next time you have that condition you need to determine if the ZVC is actually the problem or if it is happening somewhere else.

    One scenario I can think of is that the ZVC dry contacts that are connected to the boiler operating control (there is more than one way to wire this) are in fact closing (as they should) and the boiler control has failed to bring on the burner. In order to determine if that is the case, you would remove one of the connecting wires at the boiler control and reconnect that wire to see if the boiler will start. You will want to make the test as indicated in RED on the following illustration. If the problem is in the TACO Control, you don't want to distribute the TACO in the event the issue is just a loose connection and by jarring the control the boiler starts. That
    will make the test invalid. If there is no change in the boiler operation, then you can use a continuity tester to see if the wires at the boiler control have an open or closed circuit. If the wires at the boiler are open circuit (they should be closed whenever any of the RED lights on the ZVC are illuminated) then either the wire or the ZVC is defective and should be replaced under warranty.

    If this is a DYI you will need to test this. If there is a contractor involved, then you need to call for service and leave the condition for the service tech to witness. Advise the contractor to conduct this test before cleaning the fault by cycling the power as you have already done in the past. It may be as simple as a loose wire.

    Respectfully submitted,
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • Steve Garson_2
    Steve Garson_2 Member Posts: 708
    Ed, thanks for taking the time to share your knowledge. I really appreciate it!
    Steve from Denver, CO
  • Zman
    Zman Member Posts: 7,554
    What zone valves do you have. It sounds like they are sometimes not closing their end switches.
    Are the switches worn, motors underpowered, mechanism worn?
    How is it piped? Do you need end switches? In some systems it is easier to just jump them.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    EBEBRATT-Ed Member Posts: 15,353
    If it is a gas boiler cycling the power will in most case reset the burner control. That could be your issue
  • Joe Mattiello
    Joe Mattiello Member Posts: 703
    Hi Steve
    thanks for using Taco ZVC405 control. It's a pretty simple facilitator for your low voltage zonevalves. Love the diagnostic lights. for trouble shooting purposes, you can jump out thermostat terminals for a validated demand. Also jump out terminals 3, and 4 at the bottom of the zone you placed a jumper for validated demand; the boiler should fire. If it does, then the zone-valve end-switch is problematic. You can leave the jumper in place at 3 and 4 if through your testing found the zonevalve endswitch to be root cause. the only problem with that, is if the zone valve does not open on demand, you could have a perpetual demand. It's safe to do it while you figure out what to do with the zone valve end switch that works intermittingly.
    Joe Mattiello
    N. E. Regional Manger, Commercial Products
    Taco Comfort Solutions
  • HomerJSmith
    HomerJSmith Member Posts: 2,407
    edited March 2021
    The ZVC is not the only way to activate the boiler TT. End switches on the zone valve can also activate the TT contacts on the boiler if the end switches are connected directly to the boiler. The ZVC only activates the zone motors. If your setup is connected per Ed's Taco diagram, then I am wondering why zone 4 & 5 shows the thermostat closed and the valve not getting power. How is it wired? Is #5 on priority mode?

    You might try moving the priority switch back and forth several times. You might check the 5 amp fuses or switch them out, they may be intermittent. An Ohm meter set to Ohms would confirm that or a volt meter set to 24 V across the fuse. If you get 24 V the fuse is bad, if 0 V the fuse is good.