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Thread: Trying To Understand Honeywell RTH 5100B 1009 Therm Functionality, Please

  1. #1
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    Trying To Understand Honeywell RTH 5100B 1009 Therm Functionality, Please


    Have a typical forced hot water home system; gas fired.
    Trying to understand how it all plays together, and was wondering about the Honeywell Thermostat I now have on it.
    It's a RTH 5100B 1009

    Pretty basic, and probably dumb, questions, but if anyone could help
    me out in understanding this would be most appreciative.

    Questions, please:

    I guess that the sole output of the unit when calling for heat is to simply close a single pair of contacts.
    True ?

    If so, does it have a "small" mechanical relay inside ?
    Or does it use a solid-state closure ?

    And, that the 24 V dc is provided, probably, by a transformer in the Honeywell control box mounted
    on the side of the furnace, and not by the Thermostat ?
    True ?

    Thanks for help,

  2. #2

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    Correct. With a hot water heating most systems only need to connect two wires from the thermostat. That can trigger pumps or valves as well as the boiler (there are various configurations). Mechanical thermostats use a coil that bends when heated. Most battery operated thermostats have a mechanical relay. WiFi thermostats commonly use electronics to bridge the wires.

    Most controls are 24v AC but a few are millivolt. Multi-unit housing, commercial buildings and electric heat can be line voltage. The transformer can be in control box or external, not at thermostat. Sometimes multiple transformers.

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