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Thread: MUA's

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  1. #1
    Join Date
    Oct 2007
    Location
    Lakewood, CO
    Posts
    73

    MUA's

    Shouldn't any MAU unit have preheat coils to prevent the freeze stat from tripping all the time?

    If it doesn't, does a logical solution entail programming the hotwater coil to circulate with the hot water valve opening based on a OAT reset before the OAD's slam open to 100%?

    Wondering 'cause a unit we "inherited" does trip all the time.

    - Basparky

  2. #2
    Join Date
    Oct 2003
    Location
    Minnesota
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    1,380
    Quote Originally Posted by basparky View Post
    Shouldn't any MAU unit have preheat coils to prevent the freeze stat from tripping all the time?

    If it doesn't, does a logical solution entail programming the hotwater coil to circulate with the hot water valve opening based on a OAT reset before the OAD's slam open to 100%?

    Wondering 'cause a unit we "inherited" does trip all the time.

    - Basparky
    Well, having a preheat might be nice.

    But most units I've worked with don't have em.

    Gets a bit nippy around here, from time to time. So when we've no other alternative we open the OAD REAL slow, so as to give the coil control (HW valve, elec heat, or gas burner as the case may be) a chance to keep up with the decreasing temp of the incoming air.
    A site where I stash some stuff that might be interesting to some folks.
    http://cid-0554c074ec47c396.office.l...e.aspx/.Public

  3. #3
    Join Date
    Mar 2003
    Location
    NY
    Posts
    2,152
    I've seen a lot of them with the freeze-stat installed 'before' the heating coil. These will obviously trip when the temperatures drop. Be sure the freeze-stat is located after the heating coil.
    "Controls is a lifestyle not a job" -klrogers

  4. #4
    Join Date
    Jan 2006
    Posts
    530
    put in a time delay

  5. #5
    Join Date
    Apr 2007
    Location
    Amarillo by mornin'
    Posts
    853
    "If it doesn't, does a logical solution entail programming the hotwater coil to circulate with the hot water valve opening based on a OAT reset before the OAD's slam open to 100%?"

    That might work, or reset your discharge air temp set point to something really high shortly before you start the unit and then slowly reset the DAT so that your PID stays under control. If you do as you stated above if OAT reset doesn't have enough water moving through the coil and your OAD goes from 15% to let's say 85% fairly quickly, your stat could trip before your valve has enough time to react.

    You might even think about moving the freeze stat. Several of the OAU we have done lately we've had to put the freeze stat after the CHW coil even with preheat. {OAD-->Elec. Heat Strips-->CHW Coil-->Freeze Stat--> Elec. heat strips}. Reason for this is the units have multiple stages of electric heat. If it's cold enough as soon as one of the stages goes off then it will trip the stat. I've had to put in the program that if the OA is less than the CHW supply temp then flip the CHWV 100% open. This will ensure that there is (or should be anyways) CHW flow. Also, you are potentially making some free chill water.
    "It's not that I'm smart, it's that I stay with the problem longer”
    Albert Einstein

  6. #6
    Join Date
    Jan 2012
    Posts
    147
    I was thinking that instead of opening the OAD real slow, would it be better to have the heating valve fully open when OAD is opening, and to stay open a few minutes after the MUA has been running? This way, the heating valve can then modulate to maintain temperature thereafter.

  7. #7
    Join Date
    Oct 2003
    Location
    Minnesota
    Posts
    1,380
    Quote Originally Posted by Questionz View Post
    I was thinking that instead of opening the OAD real slow, would it be better to have the heating valve fully open when OAD is opening, and to stay open a few minutes after the MUA has been running? This way, the heating valve can then modulate to maintain temperature thereafter.
    <Shrug> Whatever yah want to try. It's your call.

    I just stated what we do, and it works for us.

    The way you seem to want to do it may well work for you. As long as the folks on the discharge end don't complain about a burst of heat for longer than they're willing to live with it.
    A site where I stash some stuff that might be interesting to some folks.
    http://cid-0554c074ec47c396.office.l...e.aspx/.Public

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