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15 volts out of zone controller?!

HVAC_GUY Member Posts: 1
edited November 2019 in Radiant Heating
I need help please. I’m a service tech with a Taco zone valve controller, 6 zones.
5 zones give 24v to the Honeywell valves for end switch but one only gets 15v and valve will not open. I checked both transformers and they are good. Do I have a bad board? What am I missing?



  • STEVEusaPA
    STEVEusaPA Member Posts: 6,506
    Sounds bad to me. What kind of zone valves?
  • Jamie Hall
    Jamie Hall Member Posts: 22,941
    Before you give up on the board, check all your connections very carefully. It's perfectly possible that one of them has high resistance (particularly plugs, such as Molex) and that will give exactly that symptom.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,506
    I’m assuming he checked the voltages at the control.
    But checking all connections, swapping zones, or ohming the low voltage wires for shorts are always good first troubleshooting methods.
  • mattmia2
    mattmia2 Member Posts: 9,467
    did you disconnect the vale that has the low voltage or otherwise rule out a bad valve motor or jammed valve pulling the voltage down?
  • Jolly Bodger
    Jolly Bodger Member Posts: 209
    @mattmia2 is right. Did you check load on and load off? Sounds like a bad relay in the board. Or I have had the solder connections crack on some circuit boards.
  • HomerJSmith
    HomerJSmith Member Posts: 2,403
    The End Switch is controlled by the boiler circuit board. The Transformers on the Taco ZVC powers the thermostats and Honeywell zone motors. Look for a wiring error in the circuit.

    Take an adjacent zone valve and connect the bad wiring to it and see if the zone valve functions perfectly. If so, the problem is the zone valve. Or with the thermostat calling for heat, disconnect one of the wires to the ZVC on the thermostat and measure the voltage, if you have 24V the problem is the zone valve. If 15V the problem is the ZVC or thermostat.

    I have had Honeywell zone valves pole out and not work.