Page 2 of 2 FirstFirst 12
Results 14 to 25 of 25
  1. #14
    Join Date
    Dec 2014
    Posts
    2,952
    Post Likes
    Thread Starter
    Quote Originally Posted by WAYNE3298 View Post
    Sounds like you are getting close. Are the areas sub-cooling? Most VAV min set points are 10 to 15% of the max. It could very well be the mins are set too low and when enough of them satisfy the problems start. Your minimums need to be set at about 50% of the max which in most buildings would sub-cool the space.
    Yes they subcool. 72 cool and 68 heat. If a box satisfy, the damper drives to an unknown minimum, and continues to dump 53-60f air in the space. Controlled from return air and number of calls, determines the mode. So basically its heat,cool,heat,cool...........
    Maybe someone has adjusted the minimums in an attempt to ease up on the subcooling, overcooling.
    People I need info from are on vacation right now but possibly back on Thursday.

    Sent from my SM-G950U using Tapatalk

  2. #15
    Join Date
    Dec 2014
    Posts
    2,952
    Post Likes
    Thread Starter
    Quote Originally Posted by bradluke0 View Post
    Good going . I would take that new duct pressure sensor and install it closer to the unit instead of way at the end . Like Wayne said if the minimums are 10-15% they need to be raised . If there is electric heat on the boxes you can set the minimums at 90-100 cfm/kw . If the 1st stage of cooling has unloaders do you know how far it will unload? That and if you have electric heat will determine how low you can set the minimums . If the supply temperature is set at 51 or 52 you can raise that a bit to help with sub cooling and keep the boxes open a bit more . I would not go above 56 . ( you did verify the sensors' calibration didn't you ? )
    Can unload to 16%, (out of 2 pumps)

    The VAVs have hydronic reheat coils.

    You know, by fixing the SP sensor, (only 20ft from the unit)
    I'll create another problem. Theres a 10" box feeding 5 rooms. Was a 12 ft wide 12ft high 100 ft hallway but now remodeled. They starve for air there at 3" static. Now it'll be 2".
    Ah well.

    Sent from my SM-G950U using Tapatalk

  3. #16
    Join Date
    Feb 2016
    Location
    Louisburg Kansas
    Posts
    3,869
    Post Likes
    I'm sure you know this is a bogus system. I think you intuitive were on the right track to start with. From here to the finish line as you stated access to the control program is a must. I would drive all the boxes to min and remeasure the airflow. I have a hunch you will find that flooding back due to low airflow is possible. If that's the case the owner is going to spend money one way or the other.
    No man can be both ignorant and free.
    Thomas Jefferson

  4. Likes icy78 liked this post.
  5. #17
    Join Date
    Aug 2019
    Location
    Central Florida
    Posts
    106
    Post Likes
    Why is it taking 3" of static to get the correct air through the boxes? Are there manual dampers in front of the boxes ? Are the down stream dampers at the air distribution partially closed ? Typically 1.5" of static should get you plenty of air . You really need access to controls and pretty much calibrate the boxes and then , as Wayne said , measure your airflow at minimum . I almost wish you were close , I would love to get my hands on this sucker .

  6. #18
    Join Date
    Feb 2016
    Location
    Louisburg Kansas
    Posts
    3,869
    Post Likes
    Brad I agree with you concerning the SP set point but I think his need for 3" SP is the location of the takeoff from the main duct to the box. I have had boxes that when the box dampers were fully opened the diffusers sucked air in. I recently balanced a school that had one FPB that dictated the SP set point. The problem was made worse with the room being on the southwest corner of the building.
    To solve this problem an extractor could be installed at this takeoff if the takeoff is accessible.
    No man can be both ignorant and free.
    Thomas Jefferson

  7. #19
    Join Date
    Dec 2014
    Posts
    2,952
    Post Likes
    Thread Starter
    Quote Originally Posted by bradluke0 View Post
    Why is it taking 3" of static to get the correct air through the boxes? Are there manual dampers in front of the boxes ? Are the down stream dampers at the air distribution partially closed ? Typically 1.5" of static should get you plenty of air . You really need access to controls and pretty much calibrate the boxes and then , as Wayne said , measure your airflow at minimum . I almost wish you were close , I would love to get my hands on this sucker .
    I guess I didn't explain it right. I was checking the calibration of the static pressure sensor and I found that it was reading anywhere from 1.6 to 2 in of static . Whereas my probe with two separate manometers at that same location read 3 inches of static.
    So my comment about the one area that was starving as it is now at 3 in SP, will be worse off when its running at 2 in SP. However that is not something that I'm concerned about at the moment .
    I simply want to know that total flow is correct, know that my compressors are protected, and know that when other changes need to be done that the SP is now accurate, and the minimum cooling cfm is correct.

    Sent from my SM-G950U using Tapatalk

  8. #20
    Join Date
    Aug 2019
    Location
    Central Florida
    Posts
    106
    Post Likes
    Has anything been determined here yet ?

  9. #21
    Join Date
    Dec 2014
    Posts
    2,952
    Post Likes
    Thread Starter
    Yes, I calibrated the static sensor but ended up with a new one. It at least worked, but still off by 0.4".
    Found in config that someone selected 0-3 " sensor range. But these are 0-5". Changed that works great.
    Got email yesterday about the cool mins. They add to 4300 instead of the 6500 I'd like. So that will be changed soon too.

    Starting on the next one soon.

    Sent from my SM-G950U using Tapatalk

  10. #22
    Join Date
    Aug 2019
    Location
    Central Florida
    Posts
    106
    Post Likes
    Looks like you are getting somewhere . If you can unload all the way to 16% then your theoretical minimum would be right around 2000 cfm . 4300 cfm for a minimum has plenty of safety built in . 6500 for a minimum is 57% of maximum cfm . I would say that is a bit high . 4300 minimum sounds like it might even be enough for the flow sensors to read correctly when the boxes are at minimum . Continued good luck .

  11. #23
    Join Date
    Feb 2016
    Location
    Louisburg Kansas
    Posts
    3,869
    Post Likes
    Good points brad.
    No man can be both ignorant and free.
    Thomas Jefferson

  12. #24
    Join Date
    Dec 2014
    Posts
    2,952
    Post Likes
    Thread Starter
    Quote Originally Posted by bradluke0 View Post
    Looks like you are getting somewhere . If you can unload all the way to 16% then your theoretical minimum would be right around 2000 cfm . 4300 cfm for a minimum has plenty of safety built in . 6500 for a minimum is 57% of maximum cfm . I would say that is a bit high . 4300 minimum sounds like it might even be enough for the flow sensors to read correctly when the boxes are at minimum . Continued good luck .
    Hmmm. Good point.

    But if the unit stages the compressors/unloaded to meet supply air STPT too aggressively...itll still be too low cfm.

    Dont know if its done in Carrier algorithm or BAS .

    Unit as installed states 6000 minimum. But you make sense.



    Sent from my SM-G950U using Tapatalk

  13. #25
    Join Date
    Aug 2019
    Location
    Central Florida
    Posts
    106
    Post Likes
    If the submittal or other documentation says 6000 cfm minimum then there may be a good reason for that . It would be nice to have a sequence of operation that spells out what is supposed to happen . There is always more than one way to skin a cat and my way may not be the best .

  14. Likes icy78 liked this post.
Page 2 of 2 FirstFirst 12

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •