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Taco ZVC403 zone controler

I have a taco zvc403 zone control on a Veismann boiler with air handler. The zone 3 valve has decided open without the thermostat asking for it. Green and red lights on. Was able to reset by turning off the air handler then turning back on. Now the valve stays open and the fan remains on continuous. Cannot control with the thermostat. I have taken the thermostats out, changed them around (I have two, one for the infloor heating and one for the air handler). I flipped the breaker on the air handler to turn it off. Green and red lights are off now, by to is my heating system. Is this a zvc403 issue or control bird on the air handler?


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,240
    What type of zone valve?
    I'd start from the top of the control.
    Take the wires off of R-W. Power on. If the zone valve doesn't open, it's the thermostat or the wires from thermostat to control are shorted. If the zone valve remains open, and the light indicates the zone is calling, continue checking the control.
    If not, take wires off the 403 itself to the 3rd zone. The zone valve should close.
    Take the wires off going to the zone valve, put your meter across the end switch wires (2 & 3 for a 3 wire zone valve and 3 & 4 for a 4 wire zone valve). If there's continuity the zone valve is bad or it's wiring is shorted.
    Switch the meter to volts and read terminals 1 & 2. With the control on, and the thermostat wires disconnected, there should be no power on 1 & 2. If there is, the control is bad.