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  1. #1
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    Pressurized Return Air

    I was ask to air balance a school where the HVAC system has been redesigned reportedly because the HVAC has never heated or cooled the building properly. The building has fan powered boxes virtually everywhere and a new RTU. The boxes calibrated and balanced very nice but I noticed on the controls program one of the labs was 66 F with a set point of 72 F. I had to check the lab static pressure relative to the hall and when I cranked up the RTU the room sub-cooled with the airflow at minimum. The return which is a wild return ceiling plenum was pushing 410 CFM of supply air into the room.
    My guess is at one time duct static pressure was controlled by a dump damper and the damper is open. Do any of you have any other ideas of the cause. I was there after hours and this RTU was the only one running.
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  2. #2
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    Not surprising about no response as yet. Venting here a little because the reasons for pressurizing the return are limited to duct leaks no matter the reason. When a building starts out being a problem it seems like nothing wants to go right until you give it personnel attention.
    One RTU with a return fan had over an inch of negative static at the outside air damper. No way to control outside air with that much static. The start-up guy came out and found that with the supply fan running at 60 Hz the return fan was running at 35 Hz even though the drive indicated the fan was running at 60 Hz. Never saw that before.
    Another wouldn't run continuously. There are some VVT'S and for some reason some of them wouldn't read airflow and the beat goes on.
    No man can be both ignorant and free.
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  3. #3
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    Quote Originally Posted by WAYNE3298 View Post
    I was ask to air balance a school where the HVAC system has been redesigned reportedly because the HVAC has never heated or cooled the building properly. The building has fan powered boxes virtually everywhere and a new RTU. The boxes calibrated and balanced very nice but I noticed on the controls program one of the labs was 66 F with a set point of 72 F. I had to check the lab static pressure relative to the hall and when I cranked up the RTU the room sub-cooled with the airflow at minimum. The return which is a wild return ceiling plenum was pushing 410 CFM of supply air into the room.
    My guess is at one time duct static pressure was controlled by a dump damper and the damper is open. Do any of you have any other ideas of the cause. I was there after hours and this RTU was the only one running.
    I guess that means that the lab was definitely positive to the hall! LOL
    Sounds like lots of issues and I will offer that I think they hired the right guy!

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  5. #4
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    Thanks for the compliment icy. This job is taking forever because of so many problems. I just hope the owner doesn't get too impatient. So far there are pleased and so is the engineer.
    I have another school that is also a pain for them. The new MAU's have exhaust fans with heat recovery wheels and I can't get the contractor to understand the exhaust fans have to run full speed to exhaust the locker rooms and recover waste heat. The locker room temperatures are 80 F plus and the football coach is getting pissed. Just sent the third email on this one to help the engineer explain how they have to set these units up.
    No man can be both ignorant and free.
    Thomas Jefferson

  6. #5
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    Quote Originally Posted by icy78 View Post
    I guess that means that the lab was definitely positive to the hall! LOL
    Sounds like lots of issues and I will offer that I think they hired the right guy!

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  7. #6
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    You mentioned VVT. That means there is a bypass damper somewhere. If the new rtu is on a vav then that damper needs to be closed.
    IV IV IX

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  8. #7
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    Wayne are you saying you have 410 cfm air coming from the supply, or from infiltration?


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  9. #8
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    Quote Originally Posted by viceman View Post
    You mentioned VVT. That means there is a bypass damper somewhere. If the new rtu is on a vav then that damper needs to be closed.
    Ill second that.


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  10. #9
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    Wayne, also possible the “return” is not communicating with the true return of the rtu.

    Ive see that a few times when remodeling is done, fire walls get installed all the way to the deck, blocking off return pathways.

    Or just too small of pass throughs in fire walls.


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  11. #10
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    The return grilles had 410 CFM of supply air. The problem is resolved. The inlet duct to the FPB was off. I didn't have time to search for the problem last night.
    The labs were barely positive with respect to the hall.
    These are old labs and not designed in accordance with present day standards. For one thing they are all interconnected by the prep rooms that have doors but they are left open. I checked their fume hood and it is useless.
    Will be doing the final check on pressurization this evening. They will not be very negative but as balanced should work OK.
    There is no bypass damper. It was removed several years back.
    No man can be both ignorant and free.
    Thomas Jefferson

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  13. #11
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    Something else amusing on this job. My brother and I went to start finishing the air balance this afternoon. On the way in the building I told him I had gone through all the paper work and we should be able to breeze through the work that is left.
    We got in the building and couldn't get on the WI-FI to manipulate the controls. The teacher said it had been down all day and they didn't know when it would be working.
    I ask him if the building was built over an ancient burial ground with only criminals buried there.
    We will back up and try again tomorrow.
    Thought you guys would enjoy this new development.
    No man can be both ignorant and free.
    Thomas Jefferson

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