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  1. #1
    Join Date
    May 2008
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    15

    Confused Building Pressure Dilemma

    I need some suggestions for controlling building pressure for the system below:

    1. Four packaged VAV Roof Top units (one for each floor).
    2. One dedicated VAV Outdoor Air unit with energy recovery wheel that serves all four floors.
    3. Combination of parallel and series fan powered boxes, and some vav boxes.

    The RTU systems are straightforward with economizer and have they capability to perform building static pressure control.

    The dedicated OA unit serves selected areas throughout the building based on CO2 in some cases or occupancy in others. This unit also handles the bathroom exhaust for the building via exhaust boxes, (vav), at each bathroom. There is an exhaust box on each floor connect to this unit for ‘volume’ control.

    The OA unit exhausts more air on each floor than it supplies, so there is an overlap between the systems, (the RTU serving the floor and the OA unit).

    Air flow stations are called for on the supplies and returns of all units. Engineer’s intent was to total the supply cfm, subtract the return cfm (with an offset), and modulate the exhaust ‘volume’ box to maintain that value, thus controlling building pressure.

    The shafts are the only location for air flow stations, and they’re not the best. In addition the owner isn’t happy with the added cost of them.

    Any thoughts or comments would be greatly appreciated.

    Thanks – IDJIT

  2. #2
    So you have a negative building..right?

    I really depends on the configuration/layout of your building on how to control it best.

    Find a good spot for a building static transmitter controlling the ERU relief (Damper/VFD's/Both?) would be the place to start.
    But I am sure you dont run the ECU in all conditions...so Locking out the rtu static control when the ERU is on and, when its off ... enable the RTU relief capability on each unit via FMS and use the same building static sensor input used for the ECU relief (if possible on that particular manufacturers equipment with what FMS system you have)

    Some FMS systems will make this or something simular easy to do or it could turn into a Kluster ..making you wish ..that the engineer was dead along with his lousy spec.

  3. #3
    Join Date
    Mar 2005
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    Pacific Time Zone
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    4,228
    Quote Originally Posted by freddy-b View Post
    Some FMS systems will make this or something simular easy to do or it could turn into a Kluster ..making you wish ..that the engineer was dead along with his lousy spec.
    That's almost good enough to have as another signature!

    I serviced a building spec'd with airflow stations - kept having pressure problems and they were high dollar stations. Anyway I came in about 7 yrs later on a controls retrofit and added building static pressure transducer and had that factor in more than the airflow stations and it really helped. Anyway I second Freddy's advice.
    "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/


  4. #4
    Join Date
    May 2008
    Posts
    15

    Thanks

    Thank you freddy-b and crab master for your feedback.

    Freddy-b – Can’t say if the building is positive or negative at this point, it’s just coming out of the ground. Which gives me some time, (very little), to work this issue out. The other good thing is that I have a pretty good relationship with engineer, and he is working hard on his end for a solution.

    I hear what you’re saying about locking out the RTU static control, but unfortunately both the RTUs and ERU/DOAU are specified run on the same schedule. In this case, the primary function for the ERU/DOAU is to provide additional outside air for high occupancy areas, (conference rooms, auditoriums, etc.), and bathroom exhaust. Additionally, the engineer’s intent is to control building pressure with this unit.

    I’m checking with the RTU rep, (AAON), to see exactly how these units are configured. If they’re configured with a true return fan, in lieu of a power exhaust setup, I’m thinking I can set the RTU return fan to track the supply fan. Then let the ERU/DOAU and associated exhaust boxes control space static pressure. This will take some tweaking with the balancer.

    I’d like to get away from air flow stations if possible.

    Thoughts?

    Thanks - IDJIT

  5. #5
    Join Date
    May 2002
    Posts
    9,564
    Is the DOAS unit all variable speed for both supply and exhaust?

  6. #6
    Join Date
    May 2008
    Posts
    15
    sysint -

    Yes. All fans on RTU and DOAU are variable.

  7. #7
    Join Date
    May 2002
    Posts
    9,564
    You say: The OA unit exhausts more air on each floor than it supplies, so there is an overlap between the systems

    Always? If so this isn't your controlling medium. The rooftops make up the difference for OA. Also, if it's always under-aired it's probably running all the time during occupied and off at night. So, during the day the net effect of the DOAS is negative, and by night it's neutral.

    So, your static is controlled really via your rooftop units since they have to make up the difference. You better find out what they have for exhaust and/or return fan.

    Do you have much time in the shoulder months and winter that require economized cooling?

  8. #8
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    Won't work properly. Think about it. The additional OA brought in by the DOA needs to be compensated by more exhaust. Airflow stations on the OA and variable exhaust w/airflow stations would be better.
    Last edited by crab master; 03-10-2009 at 12:06 PM. Reason: clarification
    "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/


  9. #9
    Join Date
    May 2002
    Posts
    9,564
    Quote Originally Posted by crab master View Post
    Won't work properly. Think about it. The additional OA brought in by the DOAS needs to be compensated by more exhaust. Airflow stations on the OA and variable exhaust w/airflow stations would be better.
    He said the DOAS exhausts MORE than it brings in and does it on a variable scale. NET effect is negative with this unit for the OA it supplies. (whether or not I believe this is a whole different story-- I can only go off what is stated)

    So, the DOAS brings in X amount of OA and exhaust X + Y% more (on a variable scale).

    I understand an engineer wanting to use this for CO2 as whatever it's bringing in is tempered air from it's exhaust thereby saving money. The engineers best hope is that the air introduced is neutral to the space...

    Anyway, I think you misread my post because your solution is closer to what I'm thinking so that's why I asked what the units are doing for exhaust and/or return fan. Better not be barometric reliefs...

  10. #10
    Join Date
    Mar 2005
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    OK I stand corrected.

    However, as mentioned with Economizers in use this is going to be a real PIA to control properly. The only way I see this done properly with airflow stations is to monitor all variable flow OA intakes and all variable flow building exhaust. Maintain a slight bit more OA intake than exhaust.
    "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/


  11. #11
    So the When the ECU is running..this does not provide all the Outside air needed for economizing/minimums to all the units while in those modes. The RTU's economizers are still in play anytime they are running? That sounds kind of counter productive from a Energy saving point of view to me.

  12. #12
    Join Date
    May 2008
    Posts
    15
    Guys, I really appreciate the feedback, and I’ve spent some time today really looking at this and here’s a little clarification. Each floor has about 9 series boxes with ducted primary air and ducted outside air. Some modulate the outside air damper based on space CO2 and some maintain a constant outside air CFM based on an occupancy sensor. So, the outside air from the DOAU is demand based. The return, (exhaust), air for this unit is through to exhaust boxes, one serves the bathrooms, constant volume, the other is supposed to modulate to maintain floor pressure.

    After reading your responses and scratching my head more then once, here’s what I’ve come up with:

    When an RTU for a given floor is commanded to run, the DOAU must also be commanded to run because it serves the bathroom exhaust. But, because the boxes with ducted OA only modulate or open when the space they serve are occupied or the CO2 setpoint is reached, I can see a scenario when there is very limited demand for additional OA to the spaces served. So during these conditions, the DOAU's return fan would be operating, but the supply fan would be off or at minimum.

    I think the best solution would be to control the DOAU supply fan from a duct static pressure sensor located towards the bottom of the shaft, say the first floor. I think the return fan should be controlled in a similar fashion. Locate a duct static sensor at the top of the shaft, say somewhere between the roofline and the first box tap to control the return fan. This would allow this system to run along in it's own little world and do it's thing.

    The RTU would be setup like a typical RTU: Supply fan modulating based on a duct static setpoint, and modulating the return fan to maintain a space static pressure setpoint. Not sure what to do with the floor Volume Regulator box at this point, it becomes usless.

    This setup makes the Energy Recovery Wheel in the DOAU less efficient, because more air is being exhausted through the RTU. But at least I think both systems will be controllable and there shouldn’t be a building pressure problem.

    Does this make sense, or am I just getting punchy?

    Thanks - IDJIT

  13. #13
    Join Date
    May 2008
    Posts
    15
    Kluster, PIA and Counter Productive - ALL GOOD DESCRIPTIONS

    I'm not even going to add the kitchen exhaust hoods to this mess yet. I'm glad it's not just me scratching my head on this one.

    - Thanks

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