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  1. #1

    How much return?

    I recently had a 3.0 ton Trane xl16i system installed in my home. The new air handler is set to 350 cfm

    The old system was a 2.5 ton and had a single air return which is a 20" x 30" filter with a 14" flex duct.

    When I had the new system put in, I added 3 new 12" x 12" air returns for each of the 3 bedrooms the unit services. The obvious purpose was to get more air circulating in those rooms and allow the hot air to escape.

    So now I have 4 air returns on the system.

    I want to restrict the flow of the 20" x 30" air return to encourage more air to be pulled from the 3 new air returns located in the bedrooms.

    My question is:

    How much would it be appropriate to restrict that 20" x 30" air return and still not starve the air handler of return air?

    Any help would be appreciated!

  2. #2
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    Need more info. The 12 x 12 's, is that the size of the filter grills? What size flex and what is the length?
    Doug

  3. #3
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    Are the new returns filter grilles or standard grilles? How do you filter the air if they are standard grilles?

    As posted, more info is needed.

  4. #4
    Quote Originally Posted by George2 View Post
    Are the new returns filter grilles or standard grilles? How do you filter the air if they are standard grilles?

    As posted, more info is needed.
    All three are filter grills

  5. #5
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    Quote Originally Posted by kengreenwood30 View Post
    All three are filter grills
    Now we need to know what size flex was run to those 12" x 12" filter grilles?

  6. #6
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    I want to restrict the flow of the 20" x 30" air return to encourage more air to be pulled from the 3 new air returns located in the bedrooms.
    Why?
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  7. #7
    Join Date
    Aug 2003
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    Fort Worth, TX
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    First ensure each bedroom is getting enough supply air. If the returns are sized properly for each room, the rooms can still be uncomfortable if they don't get enough supply air. HVAC air distribution is a balancing act.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  8. #8
    Join Date
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    No need to restrict the 20X30 grille. The 14" flex(barely ok for 2 tons) is already restricting it.
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  9. #9
    Join Date
    Nov 2004
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    __Q = CFM * dT * 1.08
    _CFM_ = Q / [ dT * 1.08)
    _CFM_ = Q / 20 using dT =~18'F of coil

    http://www.engineeringtoolbox.com/co...ons-d_747.html

    If the bedroom cooling loads are known, the targeted air flows are determined.
    for a LARGE ( > 180 sq feet) bedroom ...
    i.e. 2,000 / 20'F = 100 CFM
    Duct area = 6 ^2 * .785 / 144 = 0.2 Square feet
    Check to determine if air velocity < 600 FPM.
    ____ 100 CFM / 0.2 Sq feet for 6" Diameter = 500 FPM

    __________________________________________________ ___________
    MAXIMUM flex duct length from diffuser to supply header or register to return header: 12 feet
    __________________________________________________ __________________________



    DIAMETER
    ___ CONSTANT____ Area
    inch ______ Sq. inch Sq. Feet
    6 0.785 28.26 __ 0.196
    8 0.785 50.24 __ 0.349
    10 0.785 78.5 __ 0.545
    12 0.785 113.04 _ 0.785
    14 0.785 153.86 _ 1.068
    16 0.785 200.96 _ 1.396

    600 FPM in a 14" diameter yields 650 CFM maximum to assure a Very Low pressure drop.

    A total, integrated model of all the components in the entire system is needed to adequately address a Duct Layout design per ACCA Manual D.
    Last edited by dan sw fl; 08-26-2012 at 02:26 PM.
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    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  10. #10
    First one correction: The 20x30 filtered return grille has a 16" flex duct, not a 14" as previously stated. This 16" duct is about 12' long

    The three 12x12 filtered return grills each have a 12" flex duct.

    The three 12" flex ducts are 6', 15' and 20' feet long

    So my question is: it appropriate to restrict the larger return grille, and if so how much?

  11. #11
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    Do the three 12in ducts enter the return plenum directly? It is hard to see your layout from our screens. How about a diagram?
    Doug

  12. #12
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    Quote Originally Posted by kengreenwood30 View Post
    First one correction: The 20x30 filtered return grille has a 16" flex duct, not a 14" as previously stated. This 16" duct is about 12' long

    The three 12x12 filtered return grills each have a 12" flex duct.

    The three 12" flex ducts are 6', 15' and 20' feet long

    So my question is: it appropriate to restrict the larger return grille, and if so how much?
    Take a piece of kleenex and go around to the bedroom returns a "see" how it sucks to the grille. Then put in a more restrictive filter in your 20"x30" grille and recheck the suction of the bedroom grilles.
    Don't get to carried away. You don't want to starve the system.

  13. #13
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    What size are the supplies to those bedrooms.
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    How many times must one fix something before it is fixed?

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