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One zone calls for heat but boiler not firing without other zones

I have a three zone hot water baseboard system with a Wirsbo control panel (low voltage) attached to a SuperHot boiler. Seems to work okay for all zones, and always has in the past. This year, however, is different.

I have one zone that is working only intermittently (meaning it is correctly sending hot water to baseboards).

What I've observed:
  • The control board at the boiler does show heat being requested (the light for the zone is on)
  • Boiler does not fire when only this zone calls for heat
  • When other zones are also calling for heat, this is when it seems to work
Since the other zones seem to be doing fine, I'm wondering what would cause just one zone to act weird; and if there's another way to test out the logic board/control panel at the boiler? My assumption is it is not the zone valve in this instance since the boiler doesn't even fire up, and I do occasionally get heat to the zone.

Hopefully not too in the weeds. Thanks for any tips!

Comments

  • Zman
    Zman Member Posts: 6,980
    It sounds like the end switch on the zone valve is not proving.
    Honeywell valves often have an issue where the motor does not fully open the valve and the end switch does not close.
    More details and pictures would help.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    DZoro
  • HVACNUT
    HVACNUT Member Posts: 4,194
    Agree with @Zman, the valve is open, but a failed end switch prevents the burner from firing.
    When another zone is running, you'll get heat from the problem zone because the valve is open.
    Get the numbers off it and get a new ZV. Depending on the model, you might be able to replace the actuator only.
    DZoro