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What digital thermostat will work 2 black wires

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Dwheat
Dwheat Member Posts: 1
in Thermostats and Controls

Moved into an apartment.

Boiler. Hot water baseboard. Zone valve fails open (the moment I disconnect the wires on the old system it goes full heat). Two black wires.

I tried Rthb20c Honeywell. Two calls to Honeywell and it was still full heat.

Here is the old one. I put the painters tape on the one wire when I was troubleshooting.

20251130_080214.jpg

Comments

  • Ironman
    Ironman Member Posts: 7,838

    It sounds like you have “normally open” zone valve which means they have to be powered shut. That’s the reverse of most.

    Any thermostat that has cooling terminals available (R and Y) should work.

    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 19,381

    You can figure this out by disconnecting the wires from the thermostat. Leave them apart. If the valve goes closed you need a heating thermostat. The wires would go to r or RH and W

    If the valve goes open you need a cooling thermostat and would hook the wires to R or Rc and Y

  • Jamie Hall
    Jamie Hall Member Posts: 26,742

    And in either case, a digital thermostat will need to be battery powered.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England

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