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  1. #1
    Join Date
    Jul 2009
    Posts
    24

    McQuay building static

    I've been working on a building that I'm incountering some high building static pressure problems. I've been told that this has been going on for about 6 years now. This is a three story building and has two McQuay RDT roof top units one controls the west side of the building all 3 floors and the other the east side all 3 floors. The building doors will hang open when the units are in Economising mode only, when mechanical cooling is on or in the heat mode this seems to not be an issue. What I have checked is all fire and smoke dampers in the building. all exhaust fans for the building are running and none are interlocked with the roof top units. Inspected return and ODA dampers and linkages. Reilief dampers on the RTU's are none mechanical and will swing freely open. I've inspected RA fan for proprer rotation. The east unit seem's to have a lower speed and less air comming from the relief damper and has had my main focus. One thing I did find on the east unit was that a new VFD has been installed for the supply fan and was not communicating with the Microtech controller. I've installed a 10VDC to 10VDC to boost the voltage signal to the RTU and now the supply fan is working properly with the roof top and can be controlled from the microtech. I guess in short im wondering if anyone had some ideas that are common with these units where im getting my excess ODA from. Any help would be great.

  2. #2
    Join Date
    Mar 2011
    Location
    Edna Bay, Alaska Highest concentration of black bears in the US
    Posts
    623
    No relief air damper anywhere?

  3. #3
    Join Date
    Jul 2009
    Posts
    24
    None that I know of I'll go through the blue prints of the building and look.

  4. #4
    Join Date
    Oct 2001
    Posts
    6,954
    your balloning the building...ODA and the RA dampers are 100% opposite each other within that unit?where are you setting the staic from and what is its setpoint now..where is it being read on the duct(s)....are you monitoring space pressure taking that his building is VAV not CV.one thing for sure the return back up to the unit isn't being exhausted as quick as it is sucking in that OA.Trane rooftop exhaust fan comes on at a 1/4 open on the OAD as they swing into econo mode to full open and the exhaust damper stay closed with the fan air blasting thru them the space is sensed with a pressure run back into the econo section and that set point is made by those exhaust damper slowly opening and modulating...do you see anything on prints of the unit or as builts saying space pressure controlling
    "when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY

  5. #5
    Join Date
    Jul 2009
    Posts
    24
    Found no building relief damper. Yes the ODA and return dampers are one set and 100% opposed. Static pressure readings on the unit are from a duct static pressure sensor, and are currently set at 1.50" W.C. the unit seems to also be able to hold this duct setpoint. This roof top unit is not equipt with a building static pressure sensor and is a VAV system. The exhaust system on this RTU is a return fan only with static activated dampers on the end of the unit no seperate exhaust. The RTU is using a return fan tracking method and varies with the supply fan as the supply maintains duct static.

  6. #6
    Join Date
    Oct 2001
    Posts
    6,954
    is the return fan in the return duct or in the return section of the rooftop trane has a dedicated exhaust fan in the return section that doesn't run when the unit isn't economizing and only starts when the dampers swing to do econo.those staic dampers on the relief are depending on the space balloning to open up and in turn the doors are taking the pressure also.if you could run the return fan 100% speed or CV and discharge it 100% the pressure would drop in the spaces.when a rooftop has static dampers on the return section they are waiting on space pressure to open them the install wasn't designed to exhaust its doing what it was installed to do http://www.google.com/imgres?imgurl=...2&tx=123&ty=86 as shown on this trane without exhaust options
    "when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY

  7. #7
    Join Date
    Jul 2009
    Posts
    24
    The return fan is located in the return section of the RTU no other fan in the duct or return section. I'll try bumping up the return fan speed to match supply fan now I have the return fan set to 90% of supply fan speed.

  8. #8
    Join Date
    Oct 2001
    Posts
    6,954
    then that RF is facing the return dampers and dead heads its supply when your doing econo? so the return air has to go out the staic exhuast dampers..i would CV that return anytime the unit supply is running.did you see the supply VFD drop in Hz when you swung into econo with the unit pulling right off the roof that freah air... if that was an exhaust fan design the building return would be going over the fan shroud and facing the exuast dampers with its cycle tied into the damper swing during econo
    "when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY

  9. #9
    Join Date
    Jun 2006
    Location
    New Jersey
    Posts
    4,333
    I don't know what exact unit you have. Since you said you don't have building pressure controls. Normally, the economizer dampers and return dampers should be tracking together. Are they? Please make sure before following next advise.

    I will assume your unit is using the fan tracking control for the return fan vfd. It's was probably never set up right. Here's a link how to set it up. Jump to page 54.

    I'm assuming on what you have. A model number would help.
    Attached Images Attached Images

  10. #10
    Join Date
    Oct 2001
    Posts
    6,954
    i don't think there is the exhaust option with his units..just static dampers in the return section with 100% opposite dampers for RA and OA
    "when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY

  11. #11
    Join Date
    Jun 2006
    Location
    New Jersey
    Posts
    4,333
    Quote Originally Posted by maxster View Post
    i don't think there is the exhaust option with his units..just static dampers in the return section with 100% opposite dampers for RA and OA
    I agree......but the setup is still in the link I provided(go to setup without exhaust fan) I have seen countless times, that this wasn't balanced during startup. The default setting are 90% wrong for most buildings.

  12. #12
    Join Date
    Jul 2009
    Posts
    24
    ASCJ That looks like the information I need I'll give that a try if I can get over there tommorrow and post back thank you.

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