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  1. #1
    Join Date
    Jun 2008
    Location
    Jacksonville, Florida area
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    58

    Trane Summit DX VAV setup Question

    I am involved with a Trane Summit system and three of the AC systems on the network are DX VAV systems. The VAV AHU systems are set up to control to a supply air temperature and the VAV's control to their zone temperatures. The problem is overcooling of spaces. As the VAV boxes satisfy and close to their minimums the AHU continues to pump out its 55 degree or 60 degree air (depending on outside air temperature) and overcools the spaces. I thought the VAS settings were to prevent this by shutting off the air handler but this doesn't seem to work as I thought it should. Anyone with experience in this? Thank you.

  2. #2
    Join Date
    Aug 2012
    Posts
    2,663
    What happened to the reheat?Doyou have hot water reheat or do you have electric heat strips.

  3. #3
    Join Date
    Jun 2008
    Location
    Jacksonville, Florida area
    Posts
    58
    All the zones are satisfied but the AHU continues to cool. Really don't want to have the reheat come on when its 90 degrees outside, so it is locked out.

  4. #4
    Join Date
    Aug 2012
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    2,663
    Your system is designed for reheat. You could always overide the damper and close it 100%. Or you could change the supply temp. setpoint. Set it a couple of degrees higher seee if that helps tempur the space better.

  5. #5
    Join Date
    Jun 2008
    Location
    Jacksonville, Florida area
    Posts
    58
    When you say override the damper, I assume you mean the VAV box dampers. Since it is a DX system I can only go so low on the minimum air flows without worrying about low airflow/flood back to the condensing unit. The discharge air temperature from the AHU is setup with a reset program that keeps the SAT setpoint at 55 until it gets down to 80 degree outside, then it resets up to 60 degree.

  6. #6
    Join Date
    Aug 2012
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    2,663
    Time to fire the boilers

  7. #7
    Join Date
    Oct 2001
    Posts
    6,966
    what is the static set point there whe IGV/VFD should be backing down as the boxes start to move from max to minimum air then the DX will react to the lower air an make set point and unload or cycle off.
    "when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY

  8. #8
    Join Date
    Jun 2008
    Location
    Jacksonville, Florida area
    Posts
    58
    I have dropped the static from 1.5" to 1.3" and it is controlling fine. Just leary of dropping the minimum airflows on the boxes too low since it is a DX system. You are right that the lower air flow will cause the condensing unit to cycle on it's low pressure switch. The problem with the spaces overcooling may only be remedied by lowering VAV box minimums even more.

  9. #9
    Join Date
    Oct 2001
    Posts
    6,966
    is the fan VFD or IGVs might try 1" if the air handlers are on the floor being conditioned 1" might help that minimum over cooling.two location questions...where is the discharge air sensor for th DX,and then where is the static sensor.outdoor air reset is to high keeping the DX banging away not letting it react to the drop in static when you go to minimum.if the reset wasn't in the picture the static air would control the cooling with the air changes going thru the AHU.
    "when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY

  10. #10
    Join Date
    Aug 2012
    Posts
    7
    Lowering the static shouldn't help. The vav will only compensate to maintain cfm min/max. Sounds like you have too much airflow/cooling for the heat load. Is the space occupied or vacant? Was an engineer involved and a test and balance done? I always prefer to use space temp reset rather than outside air temp...seems to work better to me.

  11. #11
    Join Date
    Jun 2008
    Location
    Jacksonville, Florida area
    Posts
    58
    I agree that it seems like too much capacity for the load. It is an occupied space, in operation for 10 years but not very efficiently. Just recently got involved with the account and trying to save energy and make the occupants comfortable. Space temp reset may work better than OA as you say. Trane used an archaic programming method to do the reset control, almost looks like DOS. I'm not sure how easy it would be to change the reference from OA to space but I could look into that.

  12. #12
    Join Date
    Jun 2008
    Location
    Jacksonville, Florida area
    Posts
    58
    Maxster,
    Presently IGV, soon to be converted to VFD. AHU's on the same level as the boxes and only 6-8 boxes per system. Not sure what minimum inlet static pressure the Trane boxes need to operate correctly. I am not exactly sure where the discharge and static sensors are located but I don't think they are the cause of any issues either. I don't follow what you mean by "outdoor air reset is to high", if anything it being down at 55 most of the time is causing the overcooling.

  13. #13
    Join Date
    Feb 2012
    Location
    New York City
    Posts
    106
    Running VAV w/o reheat is doable but it requires return air reset, average or critical space reset (easy with DDC boxes). For return air reset make sure that sensor doesn't see mixed air or you under cool in moderate condition and overcool in hot conditions. Return air reset works well when the boxes aren't already networked with the ahu. Most VAV ahu controllers should have some programming for resetting rthe supply air temp based on....

    The lead comp on a DX VAV system needs hot gas bypass, full stop. Check if you've got this. It may be worth retrofitting and setting the bypass to maintain a minimum suction temp.


    Another cool thing is static reset. The fan runs a starve box program that gets boxes to 95% open but maintaining their CFM set point. Huge fan savings. On your system you might be able to get away with as low as .3-.5 entering a box (you need to work out what that means at the ahu).

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