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  1. #1

    Building Pressure

    i AM TRYING TO USE VFDS TO MAINTAIN MY BUILDING PRESSURE SO MY DOORS TO NOT PUSH OPEN. WHAT BUILDING PRESSURE SHOULD I USE AND WHAT WOULD BE THE BEST VFD SET UP FOR THIS?

  2. #2
    Join Date
    Mar 2005
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    VFD's on what? It all depends on your overall setup. You want to use a slight positive pressure like 0.03" w.c or so. Again location of the the sensors and proper installation is important. You need a good outside reference. Anyway explain you situation/building setup - supply and return fans/power exhaust, supply fans only with power exhaust, other? Do you have airflow measuring stations?
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  3. #3
    i have a supply and return fan at the air handling unit. it goes into economizer mode depending on enthalpy comparison of outdoor and return air. no air flow measuring stations

  4. #4
    Join Date
    Dec 2006
    Location
    Hartford, Connecticut
    Posts
    442
    I have used a calculation on the RA Fan, so that it will track a certain % behind the SA Fan. Would not be my preference, but if that’s all you have it might be the easiest w/o spending some $$.

    How difficult would it be to add a pressure sensor to read building space pressure?

    Do you have problems frequently, or only when you are in full economizer mode?

    Do you have an end switch on the OSA Damper to know position?

    You said the setup is S-FAN/R-FAN, how does the air get exhausted or relieved from the space. Is there an exhaust air damper on the AHU or do you have a separate exhaust fan for the space?

    Depending what you have for exhaust maybe you can enable the exhaust when you are in economizer mode. I am still not real clear on the exact setup. Are you sure you have a return fan, not an exhaust fan?

  5. #5
    Join Date
    Jan 2007
    Posts
    735
    Okay,

    I see this problem time and time again. Here's what we do and it works all of the time.

    The purpose of a return fan as opposed to an exhaust fan is two-fold. First it is there to help control buolding pressure, but second it is also there to overcome the resistence of the air trying to get back to the unit.

    Normally what we do is we track the return fan with the supply fan. Typically they are design ed in such a way that the return fan always lags behind the supply in terms of CFM. Of course the ideal way to do this is to use airflow stations, but due to cost, this may be a restriction. So the alternative is to connect the supply fan VFD signal to the return fan signal.

    Now what you want to do is to set up your economizer dampers to work in unison with each other from one common output signal. Once this is done, you really shouldn't have any problems.

    Let me know how this works out for you.

    And yes, like twisted pair said, this is a return fan and not exhaust, correct?

  6. #6
    Join Date
    Mar 2005
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    Virginia
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    I run my returns based on supply fan inlet pressure.

  7. #7
    Join Date
    Jan 2007
    Posts
    735
    Codewriter,

    How do you do that?

  8. #8
    this is a supply and return setup with no seperate exhaust fan

  9. #9
    Join Date
    Mar 2005
    Location
    Virginia
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    I ramp the return air fan to maintain proper inlet pressure on the supply fan, since a return air fans only purpose is to overcome duct loss in ducted return systems.

  10. #10
    with a supply and return fan along with a building static sensor, how would i use the sensor to control fans?

  11. #11
    Join Date
    Mar 2005
    Location
    Virginia
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    1,144
    What type of building relief do you have? Where is it located? Get specific, every detail is important.

  12. #12
    Join Date
    Jan 2007
    Posts
    735
    Codewriter,

    Can you explain in more detail please?

    Low side of pressure transmitter into the suction side of the fan? What are the setup details?

  13. #13
    Join Date
    Mar 2005
    Location
    Virginia
    Posts
    1,144
    On a unit with straight return and supply fans, I believe the return air fan should modulate to maintain a slight negative at the supply fans inlet, typically the exact same you would expect for the exact same unit that did not require a return air fan to overcome duct loss.

    Now if you throw building pressure into the mix, it depends on where the relief damper resides, if the return air fan doubles as an exhaust fan via dampers, etc... Lots of different options out there it could be.

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