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  1. #1

    Freeze Stat trips, when OA temp drops to 30F, but zone is need of cooling.

    Hi, I looked all over this forum and didnt find my answer, so here is my question.

    My equipment (AHU) trips on freeze stat and shuts down the unit, I do have a few AHU's that back each other up, but here's what going on. I have a zone that is always hot and needs cooling, OA temp is in the low 30's, and OA damper is modulated and is open 100% to try to satisfy the zone after a few minutes or so, the freeze stat trips and unit is shutdown. Freeze stat is located after the HW coil, but before the CHW Coil. Freeze stat is set to trip at 33F

    What can be done to prevent the trips in the future? I have full admin rights to the BMS, and can change the coding as I please, would it be logical to override the OA dampers to go to its minimum position, if the OA temp dropps below 35F?

    But then thinking about this, wouldnt it start to open the CHW valve, and in doing so, it sends a start signal to the Chiller plant and start up the chillers when OA temp is 30F? BMS has chillers to be locked out if the OA temp drops below 45F. So chillers wouldnt start anyway. I guess what im asking is, what is the right thing to do, I know the Freeze stat is to protect the coil, but if the space is in need of cooling, then what do i do?

    Thanks in advance.

  2. #2
    Join Date
    Aug 2009
    Location
    Prattville, Alabama
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    Check the control drawing's "sequence of operation". Your answer should be there. If my buildings had air side economizing I would tell you what mine said, but I have water side economizing.

  3. #3
    Join Date
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    Mount Airy, MD
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    Control your dampers via the mixed air temp when economizing.

  4. #4
    Join Date
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    This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

    You can find the rules for posting and qualifications here.

    Additional infractions may result in loss of posting privileges.
    Last edited by jpsmith1cm; 01-18-2013 at 06:40 PM. Reason: non AOP member

  5. #5
    Join Date
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    Southold.calm
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    Quote Originally Posted by access_max View Post
    Hi, I looked all over this forum and didnt find my answer, so here is my question.
    I guess what im asking is, what is the right thing to do, I know the Freeze stat is to protect the coil, but if the space is in need of cooling, then what do i do?

    Thanks in advance.
    My guess is the unit was never designed for this use.

  6. #6
    Join Date
    Jul 2007
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    Quote Originally Posted by pecmsg View Post
    My guess is the unit was never designed for this use.
    Agreed and one would like to hope that there is glycol in these loops as well, my guess notta.

  7. #7
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    Aug 2009
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    Please realize that just because you CAN access the BMS and make changes, that doesn't mean you SHOULD.

  8. #8
    Join Date
    Sep 2008
    Location
    Western PA
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    24,970
    Trane Tech NYC

    This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

    You can find the rules for posting and qualifications here.

    Additional infractions may result in loss of posting privileges.

  9. #9
    Join Date
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    Quote Originally Posted by Chris_Worthington View Post
    Control your dampers via the mixed air temp when economizing.

  10. #10
    Join Date
    Feb 2006
    Location
    Atlanta,GA.
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    811
    can you set the discharge air temp? if you set it at 45 that should cool the space and the heating coil should open to bring the temp up to keep from tripping. check to make sure your hw valve is opening.

  11. #11
    Join Date
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    Orange County N.Y.
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    Quote Originally Posted by chillerout1 View Post
    can you set the discharge air temp? if you set it at 45 that should cool the space and the heating coil should open to bring the temp up to keep from tripping. check to make sure your hw valve is opening.
    This is where I'd be looking to go with it.

    If the freeze-stat is tripping then the air is obviously too cold and will freeze the heating coil if OA is allowed to continue to be used for any length of time. I see it all of the time.

    No one wants to be breathing air that is that cold anyway, makes for an uncomfortable time.

    ...Ron
    Roof Rat

  12. #12
    Join Date
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    Quote Originally Posted by Trane Tech NYC View Post
    This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

    You can find the rules for posting and qualifications here.

    Additional infractions may result in loss of posting privileges.

    No problem.

    What do us non AOP member know anyway right, wont happen again.

  13. #13
    Join Date
    Jul 2003
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    Quote Originally Posted by Trane Tech NYC View Post
    No problem.

    What do us non AOP member know anyway right, wont happen again.
    You could always become a professional member with the star and help these people out. Not many posters from NYC here. I bet you have tons of equipment to talk about.

    ...Ron
    Roof Rat

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