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nest thermostat with taco zone valves

any diagrams for wiring into taco 555 with nest... thx


  • SteveSan
    SteveSan Member Posts: 234
    Found an old diagram Paul, hope this helps.
  • Geosman
    Geosman Member Posts: 26
    I would not attempt wiring a NEST stat to the 555 the current draw on the 555 is far too great to be managed by NEST. A better choice would be to replace the 555 with a TACO Zone Sentry zone valve and save yourself the aggravation. However....if you have no other choice, the only way I see is to use an isolation relay and operate the 24 volt coil of the isolation relay from your Nest Thermostat. The contacts for the relay would then wire to the 555 just as though they were an old style dry contact thermostat that can handle the .9 + amps this heat motor valve will pull when it is opening and cycling to stay in the open position. Additionally, the 555 uses walking contacts that are continually making and breaking to control the valve open position. These contacts eventually erode and fail to make contact, causing the valve to fail in its closed position. If your valve head has a manual opener lever; it may come in handy when the valve fails to open otherwise you'll be out searching for a new valve head or digging for a "C" clamp to hold the cartridge pin in so the valve will stay open. Lets just say the 555 is not one of my favorites.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,818
    edited January 2021
    The normal function of the Taco actuator breaks the #1 terminal of the valve when the hydraulic piston expands past a point so the wax does not overheat. once this circuit opens the nest thermostat reads this as an open circuit and sends an error code and will not recover without manual intervention. since this will happen many times during a call for heat the nest is not compatible with the 555.

    use an isolation relay of some type. RIB relays are popular

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
    Alan (California Radiant) ForbesGeosman
  • Geosman
    Geosman Member Posts: 26
    Ed....Thank you for posting your wiring diagram....This says it all. I did not have time to create one with such detail. The same goes for many newer boilers that are using control boards yet need a "dry contact closure" for the thermostat. All too many installers / home owners who think they can use a new electronic thermostat with their equipment have a great deal to learn. The electronic (triac / transistor) switches in electronic thermostats will always allow some residual current to pass through the switch which shows up as voltage at the boiler control board. If that control is looking for voltage it will initiate a heat call when the thermostat may not be signaling for heat. I keep a supply of RIB relays on hand for just those occasions when a call comes from a DIY customer who installed a new smart thermostat and finds that it does not work.
  • Paul_69
    Paul_69 Member Posts: 251
    Thanks all.. I ended up using a common maker relay and went by their diagram and worked fine..