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  1. #1
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    Question Mc QUAY LOW WATER C.O.

    Have a 55 ton aircooled McQuay chiller. Two 15 and two 12.5 scrolls in parallel. R407c.

    System has 13 fancoils with a auto bypass valve with differential controller since coil valves are two way.

    We are constantly getting Low Water Temp trip on system Our water balancer says there is more than enough flow through barrel? Even at low load conditions!
    Set for 36 degrees trip.Seems we mostly trip at low load conditions at nite when most of the coils are not being used.
    I was wandering if this micro 2 controller has a min on time per compressor?? If so is there a chance that compressor will stay on even if its be low 45F and it's in it's min on time????? Shouldn't the micro 2 shut it down at 45F before it goes lower????
    Last edited by mrhvacmechanic; 07-27-2008 at 03:46 PM. Reason: title low water temp c.o.
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  2. #2
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    Out of calibration of the leaving water sensor? Explain how the bypass works automatically?

  3. #3
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    Water bypass valve works automatically on water pressure differential.
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  4. #4
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    Don't trust what the water balancer says you have to confirm it. I had a job once with a trane rtac can't remember tonnage doesn't matter. Had a balancing report that stated the flow is virtually identical if the water is being bypassed or if it is going through the coil. Here is my readings unit at 40% load on one comp doing 14delta T across chiller while in bypass mode. Then when water flowing through coil I could only do about 7 degrees at 80% with both comps runniing. So whattay know the balancing was lying. In bypass I had poor flow causing low temp probs. Never trust anyone except yourself

  5. #5
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    Wink

    Good advice Mustard!
    I have been saying this to my office quite a few times. I feel we don't have enough flow at times. I will need to spend some time on job and see exactly what's happening.
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  6. #6

    McQuay low water CO

    I suspect a couple of possibilities here. You didn't mention flow trips, so, the flow protect device is not indicating major flow drop.( or its not functioning ) Is the chilled water loop fairly small with fan coils installed close to the chiller? I am guessing that the automatic bypass valve you are referring to is a single valve controlled by differential pressure. Particularly if this is installed nearer the chiller, that is the 1st suspect I would have causing rapid load loss by dumping supply water back into return. Probable cause would be fan coil controls hunting, or, the differential controller. The chiller controller will shut down the compressors based on a shutdown delta T below setpoint depending on the delta T setpoint in the controller. Another possible cause for trips is due to rapid load loss such as time clock shutdown of fan coils prior to disabling the chiller. If this is the case allow the chiller to shut down say 5 minutes before the fan coils to allow pumpdown. It is possible that the flow is varying due to load, but, that change would have to be pretty rapid for the chiller to hit 36 deg leaving. I don't think this is your problem, but, I would check the water pressure drop across the chiller when loaded, and, unloaded to determine if flow is varying. The drops are quite low, so, you need a single gauge reading, or, foot gauge. The following drops are close. 8 deg delta t, 165 gpm, 3.4 psid. 10 deg delta t, 132 gpm, 1.7 psid. 12 deg delta t, 110 gpm, 1.5 psid. It appears that the design is constant volume, so, you should not see greatly differing flow rates. Good luck.....

  7. #7
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    Chiller is in basement feeds 13 coils from bmt to third floor. Bypass valve is on the third floor above drop ceiling .

    Sequence is any of the 13 fan coils will start chill water pump, and chiller will enable from flow switch. If all coils have no call for cooling, pump will run for five more minutes, and chiller will shut down.
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  8. #8
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    Have a look at the chiller set point and where the machine will cycle off.

    What controller on the chiller?
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  9. #9
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    Micro II setpoint 47F.
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  10. #10
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    On the 280 panel it was start and stop differential settings, and on the Micro II, it's something real similar. With a set point of 47, you should be shutting down before you get any faults from low temp, providing, of course, that the alarm settings are reasonable for the application.

    I'd also be looking into what actually shuts down the chiller when all the coils are satisfied. Like maybe a time delay after all the valves are closed, or a valve opening percentage for a cooling call.
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  11. #11
    It seems that one possibility for trip is using the flow switch to enable / disable the chiller. I would think that you would be getting low evap flow trips, but, there is about a 3 second delay on that trip. If the load is low at the time of pump shutdown, you could hit the leaving water freeze trip first. If this is the problem, I would install an off delay timer ( of 5 minutes ) on the pump, and, you need to be sure the chiller is shut down by wiring into the remote enable digital input to turn the chiller off.

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