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Thread: 3M-50 and Variable Speed HVAC

  1. #1
    Junior Member

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    3M-50 and Variable Speed HVAC

    Hello All,
    Does anyone know if the 3M-50 will work with a Variable Speed system?

    I am hoping to be able to use this thermostat on a twenty year old Trane XL system that is a variable speed system. As far as I can tell, I have the required set of wires at the air handler unit. However, the existing thermostat is another story. It is a Trane thermostat and the wires connect to a circuit board. I don't see the expected R, W, Y, etc. connection points.

    I am a bit weary to dive in (taking apart the thermostat, etc.) without knowing for sure that the 3M-50 does indeed work with VS systems.

    Any thoughts or insights appreciated,
    BeeBee



  2. #2
    HVAC Thermostat Specialist

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    It should yes, but typically a VS system is 2 stage. Please provide the model number of your furnace. Also, what terminals do the thermostat wires terminate to on the furnace control board?

  3. #3
    HVAC Thermostat Specialist

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    They had variable speed 20 years ago?
    How about photos? They will help us a lot.

  4. #4
    HVAC Thermostat Specialist

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    Quote Originally Posted by Servicetech View Post
    They had variable speed 20 years ago?
    How about photos? They will help us a lot.
    Yes, they did........

  5. #5
    Junior Member

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    Quote Originally Posted by hvactechfw View Post
    It should yes, but typically a VS system is 2 stage. Please provide the model number of your furnace. Also, what terminals do the thermostat wires terminate to on the furnace control board?
    Thanks for your reply. I will get you the info and some pcitures the next time I am on-site which should be in a few days. Stay tuned. Thanks!

  6. #6
    Junior Member

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    Quote Originally Posted by Servicetech View Post
    They had variable speed 20 years ago?
    How about photos? They will help us a lot.
    Yep, they did. According to the guy that serviced it lately, it was state of the art back then and very efficient albeit a bit complex to troubleshoot, etc. I will have photos and wiring info shortly.

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