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  1. #1
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    Constant flow AHU with VAV - Best way to control bypass air

    Hi there,

    I have an AHU that must have constant flow and is equipped with VAVs.

    What is the best way to maintain and control constant flow, using motorized bypass damper?

    Where would be best to place pressure sensor to feed bypass damper?

    Name:  Face-and-Bypass-Dampers-Positioning.png
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  2. #2
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    You don’t need the face damper. The bypass damper should go between the supply and return duct. The pressure sensor would be located just like you would for a VFD controlled fan...usually 2/3 of the way down the longest duct run, but you can make it work almost anywhere in the supply duct.

  3. #3
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    Be cautious returning cold air directly to the unit, I would suggest a dump zone (hallway etc) like your picture shows.

    kontrol out
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  5. #4
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    Actually my picture shows returning it back to the unit. My temperature sensor is on supply air and not on return, so I don't see a problem with that.

  6. #5
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    Agreed usually 2/3s duct length, of the one with the most VAVs.
    I’m thinking you determine the pressure setpoint with all VAVs at max cooling flows, then that pressure at reading plus some would be a good starting setpoint for your bypass damper. As vavs shut down pressure will build and the damper should open relieving the air from the supply right back into the return, but if goes below pressure setpoint then the damper will close allowing more air go to the VAVs.


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  7. #6
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    I don't get the 2/3 length. Why not close to discharge and before damper ?

  8. #7
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    Quote Originally Posted by lucaspenalva View Post
    I don't get the 2/3 length. Why not close to discharge and before damper ?
    I have always been told that the area before the bypass damper is a bad place for the sensor because the static pressure fluctuates (turbulence) too much because of the bypass damper moving back and forth. You want it located 2/3 down because you are trying to maintain the static pressure at the inlets of your boxes. Since the static at the boxes is what you are concerned about you need to have the sensor pick up down stream of the bypass damper.


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  10. #8
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    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?

  11. #9
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    Quote Originally Posted by bolt950rider View Post
    Agreed usually 2/3s duct length, of the one with the most VAVs.
    I’m thinking you determine the pressure setpoint with all VAVs at max cooling flows, then that pressure at reading plus some would be a good starting setpoint for your bypass damper. As vavs shut down pressure will build and the damper should open relieving the air from the supply right back into the return, but if goes below pressure setpoint then the damper will close allowing more air go to the VAVs.


    Sent from my iPhone using Tapatalk
    The way that Bolt950rider states it above is how manufacturers like Carrier and Trane recommend that you setup a system with a bypass damper. Essentially when you drive all of your VAV boxes to their maximum CFM setpoints, you will read the static pressure at the sensor 2/3 down the duct. That static pressure that is obtained is what is necessary to maintain your design CFM through all of your boxes. Based on that, in theory when all of your VAVs are close to minimum and your bypass damper is open based on the static pressure increase down stream of it, you will be essentially bypassing the remaining amount of air that you are no longer moving through the VAV boxes. Moving air through the bypass is less resistive than moving air through the distribution system. So everything just tends to work out pretty linearly with solely looking at the static downstream of the bypass damper.


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    J. King

  12. #10
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    Is the AHU a DX unit or chilled water?

  13. #11
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    Thread Starter
    Dx

  14. #12
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    Quote Originally Posted by lucaspenalva View Post
    Actually my picture shows returning it back to the unit. My temperature sensor is on supply air and not on return, so I don't see a problem with that.
    These down and dirty systems are very common and they can work reasonable well.

    You want control the bypass as described here. The supply air temp also needs attention. You need reasonable high/low temp limits and min off times to prevent cycling the living crap out of the equipment or cooking/freezing it to death. VAV min flows or a mix of VAV and VVT control sequences can help. Plan on the unit supply feeding right back into the return and avoid in programming any hassles with the sensors you have.

    Ideal, nope. Can it work done right, yep.
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  15. #13
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    what is the must need for the constant volume through the ah?

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