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  1. #1
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    Why do we control AHU fans to duct static pressure

    I know this is a basic HVAC concept but it is something I have never understood as I have had little experience with the control side of an HVAC system. I just aid in the equipment selections and layout. Why do we need to control AHU fans or VFD's to static pressure? In normal operation (in my mind) If a VAV or fan powered VAV box is calling for more primary air the ahu will ramp up to meet demand or ramp down as needed especially if you have demand control ventilation. How does this relate if the fans are also controlled to maintain a duct static pressure of lets say 1.5" WC. for instance?

  2. #2
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    Basically when that damper at the end of the line opens and starts dumping air, static drops. That's how the ahu knows to ramp up... Now it's gotta maintain the same pressure as before but with another eight inch hole in it.


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  4. #3
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    So your saying that the Fan powered box does not communicate with the AHU directly or thru BMS to say I need 300 more CFM, instead the AHU is just always monitoring the static pressure and when more air is discharged somewhere, pressure in the ductwork reduces and the AHU picks up on this loss and ramps up as needed to maintain static pressure setpoint?

  5. #4
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    Quote Originally Posted by dglenn9000 View Post
    So your saying that the Fan powered box does not communicate with the AHU directly or thru BMS to say I need 300 more CFM, instead the AHU is just always monitoring the static pressure and when more air is discharged somewhere, pressure in the ductwork reduces and the AHU picks up on this loss and ramps up as needed to maintain static pressure setpoint?
    Correct


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  7. #5
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    That way it can maintain a constant pressure at ALL the VAVs. Helps with flow, balancing, etc.
    -----Stop, step back, relax and have another go at it.-----

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  9. #6
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    There are control systems that use satisfied and unsatisfied VAV'S flow input to alter the static pressure set point as needed to prevent starving the VAV'S. The principal is as others have stated here with a slight twist.

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  11. #7
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    Why do we control AHU fans to duct static pressure

    Quote Originally Posted by WAYNE3298 View Post
    There are control systems that use satisfied and unsatisfied VAV'S flow input to alter the static pressure set point as needed to prevent starving the VAV'S. The principal is as others have stated here with a slight twist.
    Yeah in fact I have a carrier system that appears to be ignoring the apparent static SP because either

    A) it's simply designed to ramp based on box demand (which I can see it is monitoring but cannot pinpoint the direct mapping)

    B) box demands are applying a reset to the SP, Creating an "effective" setpoint that I cannot see.

    Since that's not my actual problem I didn't dig deeper but be advised not all things are created equal by any stretch. Also I'm not a carrier guy....



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    Last edited by B_roche; 09-11-2018 at 08:01 PM.
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  12. #8
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    Static pressures are based on duct sizing and unit CFM design.

    I think the “standard” units have evolved over time to reflect “tolerable” sound levels to achieve actual unit design requirements.

    I wonder if it’s the age old “chicken or egg argument”?.....did duct sizes evolve to achieve static pressure requirements? OR...did duct static and noise levels dictate duct sizing requirements??

  13. #9
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    Most applicating I see are to maintain a minimum duct pressure. This keeps the vavs well fed and allows lower energy usage that running the fan constantly. I have also see it set up to "open" on maximum static pressure. Usually on smaller package units with a few zones. Think of it as a limit in case the zone weights weren't set up right, it keeps the motor form burning up.

  14. #10
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    Quote Originally Posted by WAYNE3298 View Post
    There are control systems that use satisfied and unsatisfied VAV'S flow input to alter the static pressure set point as needed to prevent starving the VAV'S. The principal is as others have stated here with a slight twist.
    Finally! A topic that I can answer. For what it's worth, I am a system engineer with a large company that makes both VAV systems and their controls.

    All of the answers above are correct, but I will try to tie them together here.

    When VAV systems were first invented in the late 70's, there really were not any good ways to control fan speed. VFD's, if they existed at all for HVAC were very expensive and unreliable. In those days, the fan size and speed was set for maximum system airflow, and as VAV boxes closed the static pressure in the system would go up and the fan would "ride up the curve." Very often a bypass would be placed at the end of the system to allow some air to flow if all of the boxes closed. Systems like this were available until recently. The Carrier VVT is an example. Also, residential zoning systems work this way.

    As volume control (e.g. variable inlet vanes) and VFD's became available, designers were able to control based on static pressure in the duct. Typically, a static probe was set about 2/3 of way down the system from the fan to the farthest box. The fan speed would be reset to maintain a fixed pressure, typically 1.2" to 1.5". Though the holy grail was to control to the widest open valve, it was not feasible to have a signal from every box in a pneumatic system or even the first generations of DDC. This prevailed for many years and was a requirement in ASHRAE 90.1. In fact, many building codes today are still based on those earlier versions of 90.1 and it is not unusual for systems to still be designed this way. If you are a service tech, I'd think that most systems you see in existing buildings work this way.

    Today, the expectations are higher. These are the requirements in ASHRAE 90.1-2016:

    6.5.3.2.3 VAV Set-Point Reset
    For multiple-zone VAV systems having a total fan system motor nameplate horsepower exceeding
    5 hp with DDC of individual zones reporting to the central control panel, static pressure set
    point shall be reset based on the zone requiring the most pressure; i.e., the set point is reset
    lower until one zone damper is nearly wide open. Controls shall provide the following:

    a. Monitor zone damper positions or other indicator of need for static pressure.

    b. Automatically detect those zones that may be excessively driving the reset logic and
    generate an alarm to the system operator.

    c. Readily allow operator removal of zones from the reset algorithm.


    In addition, there are requirements to reset the leaving air temperature from the AHU or rooftop, so besides changing airflow the system may send colder or warmer air.

    If you want more information, ASHRAE recently released Guideline 36 - Sequences for High-Performing Air Systems. This document provides more in-depth explanation and recommends specific sequences for the use of designers and controls programmers.

  15. #11
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    Congrats cool,
    The first system I was involved with that used variable speed drives was a primary/secondary chilled water system. We had to decide whether to use variable frequency or DC drives. Both were expensive but we used VFD. That thing was huge and one of the problems reported with them was voltage leak to ground. I don't know anything about that just heard the complaint.
    You should also include in your explanation above how and why duct static pressures evolved.

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