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  1. #14
    Join Date
    Mar 2018
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    Thread Starter
    it's a VRF DX AHU

  2. #15
    Join Date
    May 2009
    Location
    SC
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    Quote Originally Posted by lucaspenalva View Post
    My goal is to maintain constat total flow rate through ahu (bypass + vav's).

    My thought is I must to maintain constant delta P across fan.
    Therefore my guess is that I should measure right before and after fan. Any thoughts?
    You might measure OA flow but the amount of bypass flow nobody cares about.

    The VAV is a variable volume and it needs a fairly stable air pressure to be able to have time to react and control the flow it needs to deliver. That's what the 2/3rds sensor is measuring and trying to maintain by driving the bypass damper.
    Scott Jalbert, WebCTRL ninja, Naiagara AX and EcoStruxure Building Expert newb, SiteScan Retired

    The master has failed more times than the beginner has even tried

  3. #16
    Join Date
    Jan 2003
    Location
    USA
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    Quote Originally Posted by MaxBurn View Post
    The VAV is a variable volume and it needs a fairly stable air pressure to be able to have time to react and control the flow it needs to deliver. That's what the 2/3rds sensor is measuring and trying to maintain by driving the bypass damper.
    Indeed. What the VAVs don't need will flow through the bypass. So airflow on the unit will be roughly the same. Under low loads, if cooling is enabled it most of it may go from supply directly back to the return which you need to account for.
    Propagating the formula. http://www.noagendashow.com/

  4. #17
    Join Date
    Mar 2018
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    166
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    Quote Originally Posted by orion242 View Post
    Indeed. What the VAVs don't need will flow through the bypass. So airflow on the unit will be roughly the same. Under low loads, if cooling is enabled it most of it may go from supply directly back to the return which you need to account for.
    Agreed.
    FYI where I came up with the sequence, it was from an AHU with actual returns ducted in, and the bypass was installed near the end of the supply which bypassed right into the return. This was done as there were no VFDs in the fans.
    I think the sequence should work for what you got but may not be as effective without the duct work being setup like above above.


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