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  1. #1
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    VAV system too cold with min. air.

    New here, and this is my first post. I came from a building with a duel duct system. In that system, the cold deck minimum air was set low enough on all the boxes that this was not an issue I ran into in summer. I recently got promoted to take over a larger building as the only engineer, with little training on its system. My new building has a nice new EMS system and VAV boxes with reheats. As we have shut down the boiler for summer, I am getting cold complaints in the early morning. This is because of the minimum air setting normally required for heating. I am hoping the solution is not to set minimum to 0 for the summer on every box, and back in the winter. I have been messing with the supply settings, but I am hoping for some insight. All the units have previously run at a 68-73 degree return input span to adjust a 55-60 degree supply reset. What is the most effective way to keep supply temp warm enough in the morning when boxes are still putting out a minimum air, while maintaining a low enough supply as it gets closer to afternoon? I am assuming it would be to increase the hi supply reset temp, but am wondering if the return input should also be adjusted. I am afraid to set the supply reset at too large a curve that it will cause big fluctuations. Thanks in advance for the help.

  2. #2
    Join Date
    Aug 2002
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    Southold, NY
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    VAV's need a heat source all year long

  3. #3
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    Jun 2015
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    Quote Originally Posted by pecmsg View Post
    VAV's need a heat source all year long
    Tell that to the gal who does our budget! lol. The guys who did the install were surprised when they were told we shut off the boilers for summer. But apparently they have operated this building like this for at least the last 10 years.

  4. #4
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    Aug 2013
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    Visalia California 93291
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    Re Heat needed year round or set minimums to 0 or deal with the cold calls.............or add computers and or staff to generate the needed load
    "If you think it is expensive to hire a professional to do the job, wait until you have hired an amateur"

    Red Adair

  5. #5
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    Mar 2015
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    Most ideal situation would to enable the re-heat, running a building incorrectly for 10 years is really no excuse, just because "you" have done something wrong for so long, doesn't mean it has to continue.

    I would find a way to reset the water discharge (120F-180F) on the Boiler according to outside air temp, this may relieve some of the cost in operating the boiler.

    What will be the associated cost in you having to send techs to walk around the building every time they call about being cold, you could adjust supply temp trying to find the sweet spot, but i'll tell you that will be a constant battle.

  6. Likes lkapigian liked this post.
  7. #6
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    Jan 2003
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    Quote Originally Posted by Oldmanmike View Post
    All the units have previously run at a 68-73 degree return input span to adjust a 55-60 degree supply reset. What is the most effective way to keep supply temp warm enough in the morning when boxes are still putting out a minimum air, while maintaining a low enough supply as it gets closer to afternoon? I am assuming it would be to increase the hi supply reset temp, but am wondering if the return input should also be adjusted. I am afraid to set the supply reset at too large a curve that it will cause big fluctuations. Thanks in advance for the help.
    Ditch the return air reset, you don't give a crap what the return is...unless you have occupants in the return duct... Reset based on the space deviance from setpoint. No call for cooling in the space = warm supply air.

    If the VAVs have a heating CFM that is higher than the minimum CFM, that should get locked to minimum when the boilers are off.

    Between both of these we typically don't have too many issues with shutting down the boilers as long as the cooling loads on all the boxes are approximately equal.
    Propagating the formula. http://www.noagendashow.com/

  8. #7
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    Jul 2008
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    Quote Originally Posted by orion242 View Post
    Reset based on the space deviance from setpoint.
    bang on baby! ... for vavs the only real indication of load that we have is deviation from SP

    RA temp and damper position have very little to do with what is required in the space.

    for tridium people:
    I wrote an DevFromSp object that when dropped in a wiresheet, it works out where the SP and ZT objects are and automatically connects itself to them.
    Using the program robot you can add a connected DevFromSp to every VAV very very quickly.
    Then for demand calculations, run a BQL to work out what the space conditions are.
    1 + 1 = 3 ( *** for very large values of 1)

    ...everybody wants a box of chocolates and long stemmed rose

    Be brave. You cannot get eaten by an imaginary tiger.

  9. #8
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    Quote Originally Posted by MatrixTransform View Post
    for tridium people:
    I wrote an DevFromSp object that when dropped in a wiresheet, it works out where the SP and ZT objects are and automatically connects itself to them.
    Using the program robot you can add a connected DevFromSp to every VAV very very quickly.
    Then for demand calculations, run a BQL to work out what the space conditions are.
    I calc the deviance in the controller and BQL to the reset logic, same thing no robots.
    Propagating the formula. http://www.noagendashow.com/

  10. #9
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    Quote Originally Posted by orion242 View Post
    I calc the deviance in the controller and BQL to the reset logic, same thing no robots.
    Ha, we specialise in fixing bad buildings and integrating foreign systems...basically my plan is to have robots take over the world.
    1 + 1 = 3 ( *** for very large values of 1)

    ...everybody wants a box of chocolates and long stemmed rose

    Be brave. You cannot get eaten by an imaginary tiger.

  11. #10
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    Its the borg. Resistance is futile...
    Propagating the formula. http://www.noagendashow.com/

  12. #11
    Join Date
    Apr 2014
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    The problem with deviation from setpoint is that you are resetting the discharge AFTER you've deviated. I like to reset to keep the boxes at around 90 % cooling demand. Basically 90 percent of max cooling setpoint. That way you keep everything right on setpoint. Of course that's what Johnson Controls does best. We sh1t gold and p1ss perfection. just ask and I will tell you all about it for hours and hours.
    To the OP...just turn the boilers back on with as low as a setpoint as the boiler design allows for. It's how the system was designed, and will make sure all the fresh air required will be supplied and the spaces will be properly dehumidified.

  13. #12
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    Quote Originally Posted by JCIman View Post
    The problem with deviation from setpoint is that you are resetting the discharge AFTER you've deviated.
    If you have a 0F deadband maybe... Otherwise, you can easily see the trend. What exactly is your "cooling demand"?? Output of a PID that's looking at error from setpoint maybe?? You could be 1F off setpoint for hours because its undersized and outputting 100%, nothing meaningful in the whole building scheme.

    Quote Originally Posted by JCIman View Post
    We sh1t gold and p1ss perfection.
    Yea that's the same thing the hookers behind ghetto liquor store say around here. Well maybe its suck gold and piss extra, either way I can see where JCI may have got it.
    Propagating the formula. http://www.noagendashow.com/

  14. #13
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    Apr 2014
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    Quote Originally Posted by orion242 View Post
    If you have a 0F deadband maybe... Otherwise, you can easily see the trend. Cooling demand doesn't mean squat on a undersized box. What exactly is your "cooling demand"?? Output of a PID that's looking at error from setpoint maybe??



    Yea that's the same thing the hookers behind ghetto liquor store say around here.
    Our cooling demand is the span between min cooling air flow setpoint and max cooling air flow setpoint. So when a box is at min flow, the cooling demand is 0% and when the box is at max flow, the cooling demand is at 100%. We do frequently program 0 deg deadbands.
    Cooling demand means just as much as a deviation from setpoint in an undersized box.
    With cooling demand you know how hard the box is working BEFORE it ever deviates from setpoint.

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