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Thread: Two-wire furnace/AC & programmable t'stat

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  1. #1

    Default Two-wire furnace/AC & programmable t'stat

    I have a customer in a mobile home that has a furnace/central air system with a 'heat/cool' switch on it, and the t'stat has only two wires to it.

    He wants to install a programmable stat to lower his energy bills, but I, being an electrician, haven't been able to locate a decent prog'le stat that can be installed on this system.

    Any help would be appreciated.
    Just my 2 worth.

  2. #2
    Moderator & Master Plumber hj's Avatar
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    Default t'stat

    I cannot figure out how it works with just 2 wires. You would normally need 3 wires, (a hot wire, one for the heating, and one for the air conditioner), at the very least.

  3. #3

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    There's no On/Off/Fan or Heat/Cool switches on the stat, just the temp adjustment.

    So the switch on the furnace must do something as the stat can only open and close the circuit. So my assumption (being an electrician, mind you) is that when it's on heat, it looks for the stat to close. On cool, it looks for it to open. The 'call for heat' or 'call for cool' is based on the switch on the furnace.
    Just my 2 worth.

  4. #4
    Moderator & Master Plumber hj's Avatar
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    Default t'stat

    In that case you may never find a t'stat that works. Without a heat/cool at the thermostat, it would not know whether to raise or lower the temperature to set it back, even if you could figure out how to get the electronics to work.

  5. #5
    Plumber jimbo's Avatar
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    Default

    Well, I think you need a wiring diagram of the furnace. But, if your existing thermostat is a typical 2 wire, what we would call a "heat only", then possibly that switch on the furnace is accomplishing the switching regarding activating the system on temp drop for heat, activating on temp rise for A/C. IF that is the case, you could use any T-stat, and just connect as if it was "heat only". But you really need to confirm if that is how your system is set up.

    The White-Rogers 1F80-240 is a heat-only programmable that might work for you. Check your system, and if necessary contact white rogers for assistance.

    http://www.white-rodgers.com/wrdhom/..._pg0012_13.pdf
    http://www.white-rodgers.com/wrdhom/.../0037-6325.pdf
    http://www.white-rodgers.com/wrdhom/..._06_pg0178.pdf

  6. #6

    Default

    Thanks! I'll post my results when I get them.
    Just my 2 worth.

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