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  1. #1
    Join Date
    Jul 2003
    Posts
    23
    Hi everyone,

    Based on recommendations I have read here, I installed a VisionPRO TH8321 thermostat (with dehumidification control) this past weekend. Everything is working well.

    However, I did have a question during installation. Before I ask my installation question, the following is important: This thermostat is operating two separate systems -- a furnace in the basement and an air handler in the attic, each with its own transformer.

    My question: The thermostat I replaced was powered via a common "C" wire, which is connected to the heating transformer. The VisionPRO installation manual states that "when installing the thermostat in a two-transformer system, use the common from the cooling transformer to connect to the C screw".

    When installing the thermostat, I used batteries and did not connect the common wire as I was afraid of damaging the thermostat. Is there a reason why the thermostat has to be connected to the common from the cooling tranformer? I assume that it would not work if connected to the heating transformer?

    Thanks in advance!

    Tony

  2. #2
    Join Date
    Dec 2002
    Location
    Rochester, MN
    Posts
    5,305
    If you used the battery, you won't need the common. Common allows you to turn on the back light on the display to be on all times.

  3. #3
    Join Date
    Jul 2003
    Posts
    23
    Thanks mayguy. I would actually like the light to stay on all the time, which is why I was hoping I could use the common connection. Because my common wire is connected to the heating transformer, can I still use it?

    Thanks,
    Tony

  4. #4
    Join Date
    Dec 2002
    Location
    Rochester, MN
    Posts
    5,305
    Tony,

    I over looked your reply, and never got back to you.. Sorry...

    What did you end up hooking the Common to?

    The books says must be on the cooling's common.

  5. #5
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,915
    It needs to be the common from the cooling transformer because the thermostat pulls power for the backlight and to run itself from Rc.
    If more government is the answer, then it's a really stupid question.

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