Managing static pressure
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  1. #1

    Confused Managing static pressure

    My building has 16 floors with a 300-ton York HVAC. I was told that our operations staff has been manually controlling the air handler supply air fan to increase airflow. I have been monitoring the system with an Alerton DDC, and have observed that when they set the SAF to 0.6", this results in a supply air static pressure of 1.8", and when they set the SAF to 0.8", this results in a SA static pressure of 2.4".

    I thought the objective was to keep static pressure to a minimum. What is the point to turning on the SAF (RAF is off, RA static pressure is 0), and creating supply air static pressure? Doesn't this just make the HVAC have to work harder?

    Sorry if this is a dumb question - I'm just management.

  2. #2
    Join Date
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    It depends on what the vav system was designed for. To. Little static and you cant achieve airflow requirements whichn means you short on cooling or hearting nand too few air changes. To hign and you shouild be tripping a high limit switch to prevent blowing the ductwork apart.

    Under conditionmed aces couild be a poorly tuned vav system, or too high supply temp.



    Sent from my SGPT12 using Tapatalk 2

  3. #3
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    Last edited by jpsmith1cm; 03-17-2013 at 08:09 PM. Reason: non AOP member

  4. #4
    Quote Originally Posted by motoguy128 View Post
    It depends on what the vav system was designed for. To. Little static and you cant achieve airflow requirements whichn means you short on cooling or hearting nand too few air changes. To hign and you shouild be tripping a high limit switch to prevent blowing the ductwork apart.

    Under conditionmed aces couild be a poorly tuned vav system, or too high supply temp.



    Sent from my SGPT12 using Tapatalk 2
    I thought that static pressure is independent of airflow pressure, so if there is static pressure in the supply ducts, this is the amount of resistance that the supply air fan has to push through. If that is true, then don't you always want zero static pressure? Thanks for the education.

  5. #5
    Quote Originally Posted by craftyringwiseveteran View Post
    1. Turn on the return fan
    2. Don't believe anything your operations staff is telling you (they're the ones who turned off the return fan?)
    3. Find out where the actual static pressure sensor is in the duct. (The rule of thumb is 2/3 of the way out in the duct.)
    4. Spend some money and find a good air balancer (best of luck on this one!) and have him figure out what pressure you need.

    Good luck, it sounds as if you may have one of those golf tee, duct tape, and chewing gum operation staffs.
    The operations staff is just doing what the former building engineer (who has been gone for 5 years) told them to do - they don't know why they're doing it. They do use duct tape and chewing gum - haven't seen any golf tees yet.

    Does the return air fan help reduce static pressure on the supply side? Thanks for your help.

  6. #6
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    craftyringwiseveteran

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  7. #7
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    Zero static, would mean zero airflow.

    Large ir handlers normally have and frsh air inlet damper that has a minimum position. Then you have an exhaust dampler and the return fan. In the system we have at our commerical offices spaces, the rerurn fan is always running at minmum speed and modulaates ro control vuilding pressure. It kept slightly positive. Fresh air has a minimum position and between 40 to 55f it economizes, bringing in outside air for free cooling. When doing this the exhaust opensan equal amount and a damper between the two closes to force air out of the exhaust.

    The rerurn fan is much smaller than the supply.


    There are other control schemes. But in ours suply air is kept at 50f when occupied, and 160f water rehats the air as needed. Vavs modulate airflow to each office as needed in proportion to the cooling or heating demand, but have a minimum position for adequste air changes.

    Theybused to run ours in manual before we had several transmitters repaired, and the system programmed properly. Someone used to manully controlthe fans a switch from heating and cooling.

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  8. #8
    Join Date
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    Your system has a supply fan AND A RETURN FAN THAT YOU SEEM TO HAVE TURN OFF Me thinks your system was made to have both fans ON and unless you have a good reason to know you don't need the Return Fan, you are probably creating a bunch of balancing issues. Better get the drawings that show how your system was designed in front of someone who cna guide you before you continue turning the knobs and experimenting
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

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