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  1. #1
    Join Date
    Jun 2013
    Location
    DeSoto Parish, Louisiana
    Posts
    31

    Is this return too close to the supplies?

    I am in the process of getting my HVAC system designed. If you care to read more information please check out my thread here:

    http://hvac-talk.com/vbb/showthread....would-you-make

    One of my returns is sized to deliver 1200 CFM, and it will be a 16" rigid pipe set behind a 24" by 24" return grille.
    The supplies in the photo are 10" (14" by 14" by 10" Speedi-Boot) and are sized to each deliver 325 CFM at 0.06" WC/100ft.

    Are these supplies too close to the return grille?

    Please let me know if I can provide more info to help you make a determination.

    Thank you for your assistance.

    jonblack


  2. #2
    Join Date
    Jun 2013
    Location
    DeSoto Parish, Louisiana
    Posts
    31
    Just to clarify there will be more than one return. There will actually be four returns. One return will be in the hallway and there will be a return in each of the two offices. The return in question cannot go any higher due to small limitations as there is a 16" stuctural I-beam behind the partition wall as shown in the photo below.



    Thank you
    jonblack

  3. #3
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
    Posts
    6,051
    Looks ok to me. The supply air should throw across the room and will fall since cold air is less dense.

  4. #4
    Join Date
    Aug 2001
    Location
    Mechanicsville, Virginia
    Posts
    1,353
    It does look fine. When finished, you could verify by measuring the air temperature of the room and compare it to what is going into your return. With your particular configuration, return air that is cooler than the room air would indicate the supply air is not mixing well before being returned.

    The goal is to supply cool dry air to the room, mix it well with the existing room air and return it to the air handler to be conditioned. If this is done without anyone feeling or hearing it, then consider it perfect.
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  5. #5
    Join Date
    Jun 2013
    Location
    DeSoto Parish, Louisiana
    Posts
    31
    I guess I am just worried that the return would suck in the air from the two 10" supplies that are closest to the return grille. I know about the blowing out a match versus sucking out a match analogy, but still looking for some peace of mind.

    jonblack

  6. #6
    Join Date
    Jun 2013
    Location
    DeSoto Parish, Louisiana
    Posts
    31
    I also thought about turning the return air grille upside down so it pulls air from above the return more readily than from below the return. Plus I think it will be more visually appealing since you won't be able to see inside the return grille if it is upside down.

    jonblack

  7. #7
    Join Date
    Dec 2008
    Location
    Buffalo NY
    Posts
    3,124
    Quote Originally Posted by jonblack View Post
    I also thought about turning the return air grille upside down so it pulls air from above the return more readily than from below the return. Plus I think it will be more visually appealing since you won't be able to see inside the return grille if it is upside down.

    jonblack
    This is what I would do. You can also balance those two grills closet to the return down and have more CFMs coming out of the others if it becomes a problem.


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  8. #8
    Join Date
    Aug 2001
    Location
    Mechanicsville, Virginia
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    1,353
    After re-reading your post, I calculate the velocity of your supplies at <300 FPM. Your may want to discuss the "throw" of the supply registers with your designer.
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  9. #9
    Join Date
    Aug 2001
    Location
    Mechanicsville, Virginia
    Posts
    1,353
    Quote Originally Posted by jonblack View Post
    I guess I am just worried that the return would suck in the air from the two 10" supplies that are closest to the return grille. I know about the blowing out a match versus sucking out a match analogy, but still looking for some peace of mind.

    jonblack
    I never heard that analogy before, but I love it. Too many people put too much emphasis on the location of returns when it's the supplies that they should be worried about.

    I see your concern, and as I mentioned, the throw (or blowing out) does need to be addressed.
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  10. #10
    Join Date
    Aug 2001
    Location
    Mechanicsville, Virginia
    Posts
    1,353
    Quote Originally Posted by jonblack View Post
    I also thought about turning the return air grille upside down so it pulls air from above the return more readily than from below the return. Plus I think it will be more visually appealing since you won't be able to see inside the return grille if it is upside down.

    jonblack
    Turn it so you can't see in it, but not for any other reason. Address the supplies to ensure good mixing and to prevent short circuiting the air.
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  11. #11
    Join Date
    Jun 2013
    Location
    DeSoto Parish, Louisiana
    Posts
    31
    Quote Originally Posted by penderway View Post
    After re-reading your post, I calculate the velocity of your supplies at <300 FPM. Your may want to discuss the "throw" of the supply registers with your designer.
    I am not an expert so I may be missing something, but my ductulator (McQuay DuctSizer) shows 325 CFM at .06" giving 585.8 FPM in a 10.1" diameter pipe.

    I also used the Velocity Calculator on the page linked in your signature and got 596 FPM for a 10" pipe at 325 CFM.

    Can you give me details at how you arrive a <300 FPM?


    jonblack

  12. #12
    Join Date
    Aug 2001
    Location
    Mechanicsville, Virginia
    Posts
    1,353
    In short, duct sizing and register sizing are different.

    I just did some approximation not knowing the engineering data on your registers, 14 x 14 = 15.3" round equivalent = <300 FPM@325CFM

    I would need the make and model of your grille to give you exact numbers, but that is what you hired your designer for. Discuss it with him, i'm sure he can explain it to you.
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  13. #13
    Join Date
    Jun 2013
    Location
    DeSoto Parish, Louisiana
    Posts
    31
    Thanks for the reply and for the explaining the difference between duct velocity and supply velocity. That is a facet of this subject that I have not considered in the few weeks I have been learning.

    I will double check what supply registers can be used with that size register box. I will also look at the Hart & Cooley engineering data and make sure to select a register that is going to give us the throw we want. I could always go with a different size supply box if that meant a more optimal register could be used.

    I appreciate you brainstorming with me. Every little bit of knowledge helps.

    jonblack

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