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  1. #1

    Trane CGAC Controller W7900 Question

    I am looking at a Trane CGAC chiller that uses a W7900 controller. The chiller is set up to use only one of the two circuits (100 of the 200 nominal tons), which is sufficient for the application. So it has traditionally run only Compressor 1. Compressor 1 recently failed, which shouldn't be a problem - we just switched compressor #1 "off" and compressor #2 "on". However, there is no lead/lag switch on this chiller. Previously, the W7900 was set to three stages (using the bottom dial), which made it work correctly; it loaded and unloaded compressor 1 in accordance with the setpoint and delta T dials (top and middle).

    But it's not that easy anymore. Changing the dial to six stages (the default) makes the controller go through the first three stages on the non-operating circuit one before engaging the operating compressor at all, and then compressor 2 starts unloading WAY too early.

    Is there a straightforward way to swap the compressor sequence so that stages 1-3 would operate the compressor that is in position 2? I considered getting out the wiring diagram and starting to go point-by-point through the control wiring, and will do that if needed. But I hope that there is a "swap these connections" type of fix. I can't be the first person that has dealt with this before, right?

  2. #2
    Join Date
    May 2010
    If I remember right that controller breakes down the controller LED's as comp 1 and 2.

    Comp 1 using LED's 1,2,3 and Comp 2 using LED's 4,5,6. So first LED for each compressor would be operating unloaded and 2nd LED would load one head and 3rd LED would load second head. So you should be able to move your three output leads for comp #2 on W7900 to output teminals 1,2,3 on W7900 for comp#1 Just trying to remember back...

  3. #3
    Good point, I think you may check if comp2 is with the same capacity with the comp1, before you decide to alter the control to start the comp2 only.

  4. #4
    Join Date
    Aug 2006
    Mixing oil and fire with a big spoon.
    you can NOT just switch the boards outputs from 4, 5, 6 to 1, 2, 3. the boards inputs are looking at other safeties in the circuit as well as for normal operation.

    disconnect the suction pressure switch (electrically) from left side of the board. this will make the board think that the switch is below its cut out. at power up, the board will (eventually) lock out circuit #1 on low pressure cutout and run only compressor #2.
    "By Grapthars Hammer....what a savings." Galaxy Quest

  5. #5
    Thanks for all of your replies.

    PortaChill: yes, your recollection of how the LED's operate is correct. That was my original thought, but I am not sure (without spending some times with the print) how the safeties are checked, so I was hesitant just to swap the wires.

    Michael: Yes, the compressors are the same size.

    Jayguy: That sounds like the kind of approach I am looking for. Once the controller locks out circuit #1, do you know how it "does the math" regarding operation of circuit #2? I'd expect it to use the setpoint as the circuit #2 operation point, and to take half of the "delta T" value and "number of stages" values (from the lower two dials), as this would be the best approach if no one was changing those values.

    Does anyone know where I can get information on the controller? I have the chiller operation and installation manual, which gives only about 2 pages of information. And I have the controller checkout manual, which is solely focused on making sure the controller is functioning properly. A search of this board turned up a reference to a document AM-CON18, but I can't find any information about that anyplace.

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