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what are the o & b terminals on a tstat.....and how are they wired for a heat pump???
ASM Mechanical Company
Located in Staten Island NY
Servicing all 5 boroughs of NYC.
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  • JStar
    JStar Member Posts: 2,752
    edited March 2010

    ...on the exact system, and the way the t-stat is set up, but typically O/B will energize the reversing valve for cooling. You'd wire this terminal on the t-stat to the air handler and condenser.
  • sasman
    sasman Member Posts: 32

    it does depend on what t-stat ypu are using

    if O?B it's for reversing valve, engage in  heat or cool

    if the O  and B are in different locations, you really have to be careful what you hook to

     those terminals..........
  • Empire_2
    Empire_2 Member Posts: 2,343
    edited April 2010
    Look at the manual.

    In the North East where we use "Fail safe heat",  where we energise the RV on a call for cooling "O" is generally energised on a call for cooling.  "B" is actually energised as soon as you flip the heat/cool switch to heat, where as the "O" will energise when the call is made.  "B" in this case can be used to energise a Isolation relay to separate the cooling circuit,...Y so that is will not back feed and energise thru the stat thus energising Y(Cool) G fan and "O".  In a nut shell "B" allows the separation of the 2 Modes at the condenser.

    I know that doesn't make sense, let me look for the Wiring diagram to clear it up.  Don't forget it can do anything you want it to do, as long as we all think it through.  Think about it in HP mode and in heat, W1 calls for hp condenser....What is to stop that w1 circuit from back feeding right back through Y1 at the stat, which is also interconnected to G......and O??? Now were calling for cooling again..  What's really weird once you find and understand a wiring diagram you like,................Lennox, Heil, Carrier...etc.  every manufacturer is different.  Go figure.........I scratch my head sometimes.

    Feed back please


    Mike T.
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