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  1. #1
    Join Date
    Oct 2008
    Location
    Indiana
    Posts
    78

    Why does furnace have an O terminal?

    I have a 2-stage variable-speed Goodman high efficiency furnace. There is an 'O' (reversing valve) terminal on the terminal block with the other thermostat connections. None of the sample wiring diagrams show it being used.

    I am trying to figure out what purpose this (an O terminal) serves on a furnace. The only thing I could come up with is maybe O on and G off tells the furnace that in a heat pump setup the heat pump is operating in heat mode, so the furnace fan should switch to heat speed instead of cooling speed? But that doesn't really make sense either since if it wasn't a heat pump there would be no O connection.

    Which raises the question, when used with a heat pump, can the furnace tell when the heat pump is operating in heat mode and switch the fan to heat speed?

    (I did find a wiring diagram for a Goodman fossil fuel kit (which I don't have, I have an IAQ stat with built-in fossil fuel capability)), and it shows O on the furnace wired to O on the 'furnace' section of the fossil fuel board, and that in turn connects to O on the heat pump and O on the stat.

    "I gots ta know"

    -jim-

  2. #2
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,785
    Yes.
    The O terminal lets the furnace know if its blower should be in HP speed or cooling speed.
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  3. #3
    Join Date
    Oct 2008
    Location
    Indianapolis
    Posts
    172
    I have seen applications where the 0 terminal on the furnace circuit board is a dummy terminal. All it does is gives you a convenient spade terminal to hook up the O from the stat to the O on outside unit which in turns energizes the reversing valve.

  4. #4
    Join Date
    Jun 2001
    Location
    Louisville, KY
    Posts
    12,189
    In a variable speed application (as with the OP), the O signals cooling or heating to the variable speed module.

    This will have an affect on the ramp profile of most OEM.

    It can also help drive the humidifier signal.
    Perhaps you should have read the instructions before calling.

  5. #5
    Join Date
    Oct 2008
    Location
    Indianapolis
    Posts
    172
    Thanks for setting me straight...I didn't read the post close enough to realize that it was variable speed.

  6. #6
    Join Date
    Oct 2008
    Location
    Indiana
    Posts
    78
    Quote Originally Posted by beenthere View Post
    Yes.
    The O terminal lets the furnace know if its blower should be in HP speed or cooling speed.
    But, what if I have straight AC instead of heat pump? O would be left unconnected (because there is no O), and the furnace would interpret this as heat mode.

    Also the sample wiring diagrams in the furnace manual don't show O connected to anything.

  7. #7
    Join Date
    Apr 2006
    Location
    Chicagoland
    Posts
    690
    It cause your furnace has an

    Sorry - yesterday was the deadline for all complaints.

  8. #8
    Join Date
    Nov 2008
    Posts
    1,673
    If there is a wire on that terminal and that wire never carries current in any operating mode then it must not do anything. . .?

  9. #9
    Join Date
    Oct 2002
    Location
    Yo.... Here!, I'm right here..
    Posts
    6,236
    Just have it serviced, and ask the tech those questions jim you will feel better about how the system was set up and operates ..........AND..........make sure you're good for the season, also establish a good relationship with a service contractor...........buy a Service Contract.....maybe.......

  10. #10
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,785
    Quote Originally Posted by WhoIsThat? View Post
    If there is a wire on that terminal and that wire never carries current in any operating mode then it must not do anything. . .?
    If you have A/C, the O terminal on some units still gets wired, or jumped.
    Varies with brand.
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  11. #11
    Join Date
    Aug 2008
    Location
    Booneville, Mississippi, United States
    Posts
    263
    Dual fuel setup maybe........

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