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  1. #1
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    VAV Static Reset Control using AX

    Saw one of the last threads on VAV Static and thought I'd throw this out. I have a job where the engineer is asking for Static Stpt Reset based on VAV box position. We are using HW XL10's that have an "Airflow Alarm" or starved box indication. What is a good sequence using program ojects available in AX?
    Would you use the Airflow Alarm to raise the stpt, or should the target be to get at least one box up to 80%
    Thanks
    b-man

  2. #2
    Join Date
    Jan 2011
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    Are you using Jace 6?
    I couldnt find Airflow Alarm in the XL10's configuration. Havent got a chance to program a pressure reset, I would use the latter method you mentioned above. When Maximum opened box is below the low limit say 70% for a certain period, setpoint-x,but not less than the limit. When any box opens>90% for a certain period, setpoint+x, but not more than the upper limit
    For Jace programming, I am thinking of using maximum and compare, and limit?

  3. #3
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    Dec 2007
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    I am not familiar with the controls you are using. I work for a ALC dealer. The way we do it is in our VAV program will send a "box 100% open" request to the parent AHU whenever the damper is at 100% open. The AHU program will increase the static pressure setpoint until it starts seeing damper open 100%. The result is the AHU only providing the amount of air the system requires. No more and no less. We have had customers tell us they saw up to a 30% decrease in power consumption when we changed program to do this from the old way of just setting a setpoint and leaving it alone. I have seen AHU's operating at just .35 iwc with no problems.

  4. #4
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    Quote Originally Posted by The_USC_2001 View Post
    The way we do it is in our VAV program will send a "box 100% open" request to the parent AHU whenever the damper is at 100% open. The AHU program will increase the static pressure setpoint until it starts seeing damper open 100%.
    I think you mean until it starts seeing the damper go less than 100%. This is fine for zones that are sized correctly, but all too often I've found rouge zones (poor ductwork/other issues couldn't reach max flow) and had to simply throw them out of the count. I've found terminal load to be very effective in such zones.

    I've operated a reset range from 0.5" w.c up to 1.25" w.c. The 0.5" w.c was on parallel fan powered boxes when OAT < 35 deg F.


    Also check these out
    http://hvac-talk.com/vbb/showthread.php?t=500702
    http://hvac-talk.com/vbb/showthread.php?t=177999
    "How it can be considered "Open" is beyond me. Calling it "voyeur-ed" would be more accurate." pka LeroyMac, SkyIsBlue, fka Freddy-B, Mongo, IndyBlue
    BIG Government = More Dependents
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  5. #5
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    Quote Originally Posted by crab master View Post
    I think you mean until it starts seeing the damper go less than 100%. This is fine for zones that are sized correctly, but all too often I've found rouge zones (poor ductwork/other issues couldn't reach max flow) and had to simply throw them out of the count. I've found terminal load to be very effective in such zones.

    I've operated a reset range from 0.5" w.c up to 1.25" w.c. The 0.5" w.c was on parallel fan powered boxes when OAT < 35 deg F.


    Also check these out
    http://hvac-talk.com/vbb/showthread.php?t=500702
    http://hvac-talk.com/vbb/showthread.php?t=177999
    Yes I said it backwards.

    Our setpoint will gradually decrease until AHU starts receiving commands from VAV's indicating the VAV damper is open 100%. Once it starts receiving those commands it will then start increasing setpoint until the commands stop and then it starts over.

    I can see when a building that has been neglected badly and ducts modified and such having issues but we have yet to have a building that we have not been able to implement this control strategy. And we have taken over some bad systems.

    Now on new construction most all mechanical engineers require this sequence now.

  6. #6
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    No it can be implemented on basically on any system, but you lose efficiency gains of what you could have if you have a zone that is running 100% damper way more than other areas, thus the accounting for rouge zones, that's why I like terminal load plus damper position. If the zone temperature is fine but it 'thinks' it doesn't have enough airflow let the fan speed run at the same settings until the terminal load increases enough to justify the ramping up of the fan speed.
    "How it can be considered "Open" is beyond me. Calling it "voyeur-ed" would be more accurate." pka LeroyMac, SkyIsBlue, fka Freddy-B, Mongo, IndyBlue
    BIG Government = More Dependents
    "Any 'standard' would be great if it didn't get bastardised by corporate self interest." MatrixTransform
    http://threedevilskennel.com/ - not my website.
    Versatile Hunting Dog Federation - www.vhdf.org/


  7. #7
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    Dec 2007
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    Hopkins, South Carolina
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    Now we do have a base setpoint that the system start up at. Typically 1 inch to 1.5 inches. After 5 min the setpoint will either increase, decrease or stay the same depending on feedback from VAV's

  8. #8
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    Hopkins, South Carolina
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    Supply Air Duct Static Pressure Control
    When the fan is energized the Control Module shall measure duct static pressure and shall modulate the supply fan VFD speed to maintain duct static pressure setpoint in response to cooling requirements. The speed shall not fall below 30% (adjustable). The initial duct static pressure setpoint shall be 1.5 in. H2O (adjustable). This setpoint shall be reset using a trim and respond algorithm based on zone damper position. If no zone dampers are at 100% open then the setpoint shall gradually decrease over time to a to a lower value (adjustable). If any zone damper is at 100% open then the setpoint shall gradually increase over time to a higher value (adjustable). This sequence is used to reduce cooling energy use.

  9. #9
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    Mar 2008
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    Wow!
    Guess I missed all that info from previous threads. Thanks for all the input.
    b-man

  10. #10
    Join Date
    Feb 2005
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    345
    I'd suggest that rather than use a threshold of 100% for your VAV damper position to trigger your reset scheme, you use 95% instead. This way you can also add code to ignore any box that's open > 98% and to remove it from your reset equation. If a box is open more than 98%, chances are it's indicitive of it trying to respond to a load for which it was not designed, or there's a problem with the box itself (size etc.). Regardless - if the large damper postion is truly indicitive of the spot loading, as time passes other sister zones will aslo start to warm-up and they will trigger a higher duct static SP as their dampers open. Ths is turn will supply more air to the one that was previously removed form the reset equation.

  11. #11
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    Good suggestion, that would be an 'easier' method.

  12. #12
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    Quote Originally Posted by The_USC_2001 View Post
    Yes I said it backwards.

    Our setpoint will gradually decrease until AHU starts receiving commands from VAV's indicating the VAV damper is open 100%. Once it starts receiving those commands it will then start increasing setpoint until the commands stop and then it starts over.

    I can see when a building that has been neglected badly and ducts modified and such having issues but we have yet to have a building that we have not been able to implement this control strategy. And we have taken over some bad systems.

    Now on new construction most all mechanical engineers require this sequence now.
    All of the best engineers in the world cannot convince me that this strategy is in someway practically superior as compared to a traditional control strategy. Any of thirty boxes installed on a system can send the whole thing in the tank. The VAV controllers are some what fragile, we all know that, not to mention the human interface factor (close a diffuser, open a damper, override a setpoint) . Anyone that has a decant amount of controls experience can go on anybody's site and pick out 3-4 boxes that are not up to stuff, or at least one or two that are totally whacked. It's kind of a dream world strategy, it's just not practical.

  13. #13
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    Quote Originally Posted by mechdorn View Post
    All of the best engineers in the world cannot convince me that this strategy is in someway practically superior as compared to a traditional control strategy. Any of thirty boxes installed on a system can send the whole thing in the tank. The VAV controllers are some what fragile, we all know that, not to mention the human interface factor (close a diffuser, open a damper, override a setpoint) . Anyone that has a decant amount of controls experience can go on anybody's site and pick out 3-4 boxes that are not up to stuff, or at least one or two that are totally whacked. It's kind of a dream world strategy, it's just not practical.
    All I can say is we have been using this method for over 5 years now on all our jobs and have not had issues. And one of our customers that is a medium sized university changed all his VAV AHU's to this sequence after he saw it work. I was also skeptical when we first toyed with sequence but have now been converted to new way.

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