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  1. #1
    Join Date
    Jul 2008
    Posts
    433

    McQuay 80 ton unit (intermittent problem)

    McQuay 80 ton air cooled cond unit (mod# ALP080C ser#55L8132201 2 35HP copelands 1 per stage) serves AHU feeding VAVs. For as long as ive been serving the particualr building the 2nd stage of above said unit. Has a history of intermiitent High pressure tripping and it has to be reset. Now im going to get the obvious out of the way first off, coils are clean enough to eat off, all fans work and all controls to stage fans on ( first runs all the time on a call, 2nd on pressure switch, last 2 on OAT) as they should. Even in these low OAT (northern MN) the mech clg is enabled at about 57* and intermmitngly even in this weather has to be reset- even though the high side pressure isnt even 200 psi- there is NEVER a high pressure condition while checking. The pressure control is a NO switch that closes on rise that makes the holding relay coil until reset. Ive replaced both the relay (R2 on diagram) and the pressure switch (HP2) to no avail. The only other thing would be intermmittent shorting of wires 203 and 204 due to vibration making the holding circuit somewhere and ive inspected every inch of wiring on said circuit and found nothing. My next course of action is to install an outside manual reset HP switch so it is protected and put it in series with the LPC or wherever to see if it still happens over time to see if there is in fact an intermittent HP problem or if its with the circuitry. Thoughts or similar experiances?

  2. #2
    Join Date
    Dec 2009
    Location
    Southern US
    Posts
    504
    Have all the fans been checked for proper amp draw, or have any been replaced possibly with motors rated less than name plate amps. Are the bearings on all fans good. May have a fan that runs for a period of time and overheats and drops out on overloads and resets when it cools down.
    I'm good at making things cold...You can ask my first two wives!

  3. #3
    Join Date
    Jul 2008
    Posts
    433
    All fans good are good. Even if the last 2 fans arent calling due to low OAT and if 1 of the 2 was locking up intermittengly, I would think at 55* 1 fan should be adequete.

  4. #4
    Join Date
    Sep 2006
    Location
    Independence mo
    Posts
    34
    Do you have fan speed controll? I have had problems with the P66 controllers causing the fans to run on overload an several McQuay units at my contracts. The controller calles for a motor with a 1.25 SF and the motors in unit are 1.15. I talked with the factory and they told me thats what overloads are for hugh. I replaced with a cheep Total Line head pressure controller and had no more problems.

  5. #5
    Join Date
    Jan 2008
    Location
    FL
    Posts
    28
    I have seen HP trips in Higher tonnage machines with small(ish) footprints, condenser area seems marginal at times. On cool mornings just before making the extra fans on temperature if the wind blows the right way to recirculate hot condenser air. Just a thought.

  6. #6
    Join Date
    Feb 2007
    Location
    michigan
    Posts
    71
    Are the condensor fans spinning the right way? Also if a fan is not running and is pinwheeling backwards, it will make the head pressure go up. Just food for thought.

  7. #7
    Join Date
    Mar 2010
    Location
    California, USA
    Posts
    1
    dave1234,
    If you have a Copeland compressor, it looks like your problem is in the compressor control protection circuit. You might want to replace the Klixon Electronic Motor Protector located inside a box on top of the compressor. I hope this helps.

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