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  1. #1
    Join Date
    Oct 2007
    Location
    BC
    Posts
    354

    Engineered Air MUA with mixing dampers

    Came across a set of makeup air units today, both with DJE controllers. Each unit has a direct fired burner, as well as a chilled water coil. Each unit has mixing dampers, that is a set of return air dampers, as well as fresh air dampers that modulate via a bellimo damper motor. (as one opens the other closes, like an economizer)

    I have never seen a makeup air unit with return air. Both of the units had the damper motor disconnected. The wiring diagram indicates a "mixing damper signal 4-20 mA provided by others" which is nowhere to be found. So right now you just set the damper position by hand.

    What is the purpose of providing return air to a makeup air unit? What should be signalling the dampers to move? Why would you want that? The whole thing has me scratching my head.

    This is for a new customer, and the original call was for no heat on one of the units (which has been fixed). But I would like to know more about these units.

  2. #2
    Join Date
    Jul 2011
    Location
    East coast USA
    Posts
    956
    what is it supplying a school? it would appear its designed to maintain a certain air discharge temp and or Humidity or CO2 levels. It would act as an economizer (but not really) if you had the proper controls and sensors installed. Depending on what it was originally designed for it could also have come with CO2 sensors. Its not called a return air damper but a, recirculated air damper between that and the outside air damper they work together to maintain proper mixture of air to meet space ventilation requirements. without the proper controls installed your wasting a lot of energy. whats the model #.

  3. #3
    Join Date
    Jan 2007
    Location
    Stongsville Oh
    Posts
    893
    I have seen makeup air units with RA before. Is this unit controlled by a BAS. Are there exhaust fans associated with it. During unocc mode exhaust shuts down OA may close and unit maintains nite setpoint. Controlls should control dampers even if it's two position. Look at the application. Also check for as someone already mentioned co2 sensor. If it's a newer install check the control section in the spec. Many times during costruction the mechanical is to supply unitary control of some items but the equpment comes without control. the controls contractor does not have it in his spec so it gets overloked unless there is a commissioning agent on the job. Ihate to even say commissioning agent but manyu times they find these kind of issues. The startup guy should catch this and report the dampers are not under control but many times the guy is of the it's not my job attitude and does not persue these gray areas.
    ckartson
    I didn't write the book I just read it!

  4. #4
    Join Date
    Feb 2012
    Location
    Upstate SC
    Posts
    84
    I've never seen one either but I always thought it to be a good idea. It could be used for humidity control when unoccupied and when mated with a CO2 sensor mitigate the OA intake. But set up as you described....worthless.

  5. #5
    Join Date
    Mar 2003
    Posts
    7,321
    during unocc periods in winter, it allows for space heating without using excess energy.

  6. #6
    Join Date
    Oct 2007
    Location
    BC
    Posts
    354
    It's for a magazine printing facility.

    There is no controls system. There is a heat switch and fan switch along with temp set point in the space. The sequence of operations in the mua indicates the space setpoint temp resets the discharge air setpoint on the DJE controller. There is an additional cooling thermostat in the space. A call on Y from the cooling thermostat starts the chilled water circ pump and locks out the heat switch.

  7. #7
    Join Date
    Oct 2007
    Location
    BC
    Posts
    354
    I don't think they care about humidity when unoccupied, but they definitely care about humidity when the facility is running.

  8. #8
    Join Date
    Mar 2012
    Posts
    19
    I have seen An ICE make up air, dampers control building pressure

  9. #9
    Join Date
    Jan 2006
    Posts
    140
    Usually when I get to a point of head scratching I would ask the customer if they had any engineering documents, preferably a sequence of operation for the particular unit. In most cases you can figure out the intent of the design. In this case it seems like more of an efficiency thing. Alot of HRV's in the residential market use this method too, they will pull from inside to grab the stale air and mix it with OA to "Blend" the temperatures so that the MAU doesn't have to change the air so dramatically by itself.
    The only decision in life is to decide what to do with the time given to you

  10. #10
    Join Date
    Mar 2012
    Posts
    19
    WHERE I LIVE YOU CAN ONLY USE INDIRECT- FIRED FOR THIS APPLICATION

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