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Updating Old 3 wire T92A Tstat.. HELP

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PlumBrad
PlumBrad Member Posts: 2
in Thermostats and Controls
I recently took over a very old church property, I have the boilers running smoothly, but now customer wants to update the existing 3 wire stats ( T92A21X1 ) I believe they are old Honeywell, with Honeywell Pump relays.
What would be a good replacement Tstat for this application, Hot water heat, pumped zones, baseboard and radiators for heat, Plus one hydronic fan coil..

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  • bob_46
    bob_46 Member Posts: 813
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    Those are series 90 modulating 135 ohm stats, they are probably controlling a Honeywell modulating motor on a valve or a damper. Unless you replace them with a honeywell equivalent you will have to replace the whole control system. I've been retired for 16 years so there may be some kind of new space age solid state replacement but I doubt it.

    There was an error rendering this rich post.

    PlumBrad
  • Jamie Hall
    Jamie Hall Member Posts: 23,327
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    Remember that a thermostat is nothing more -- nor less -- than a switch which is sensitive to temperature. It is in one state -- certain contacts closed and others open -- above a certain temperature, and the reverse below that temperature.

    Therefore... any thermostat will do. Just make sure that you know what the three wires are supposed to do, and hook them up properly.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    PlumBrad
  • bob_46
    bob_46 Member Posts: 813
    edited January 2017
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    Not a switch Jamie,135 ohm potentiometer that controls a balancing relay in the controlled device. A bellows in the stat moves the wiper on the potentiometer the motor you are controlling has a balancing relay that will go out of balance as soon as the wiper moves causing the motor to stroke. Another 135 ohm pot connected to the shaft of the motor moves until it puts the relay back in balance. Continuously variable modulating control. The replacement stat is T921A1191/u

    There was an error rendering this rich post.

    PlumBrad
  • Jamie Hall
    Jamie Hall Member Posts: 23,327
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    Oh cute. Thank you. Interesting gadget, then! And that explains the three wires...

    And so simple... would probably take two solid state boards and all kinds of LEDs today! I like gadgets like that.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    PlumBrad
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,545
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    @Bob is correct. Series 90 Honeywell is 135 ohm R W B 3 wire connections.

    I agree it is most likely running a zone valve. The valve end switch (2 wire contacts) is probably pulling in the relay @PlumBrad mentions.

    Wheatstone bridge
    PlumBrad
  • PlumBrad
    PlumBrad Member Posts: 2
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    GREAT INFO!!! FROM ONE WET HEAD TO ANOTHER, YOU GUYS ROCK!!!

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