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  1. #14
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,273
    From an alternate perspective, you need to know What the Sensible Load requirments are for day/night based on a selected Inside Air set-point.

    CFM = Sensible Load / ( 1.08 * approx. 18)
    ie. Room is 12 * 15 = 180 Sq. Feet which requires 2,000 BTU/Hr Daytime .:. 2,000 / 20 = approx. 100 CFM.

    If you are targeting a night time set-point, the required load is going to be much less based on both decreased outdoor temperature
    and NO load from glass.
    Designer Dan
    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

  2. #15
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,273
    From an alternate perspective, you need to know What the Sensible Load requirments are for day/night based on a selected Inside Air set-point.

    x

    http://www.engineeringtoolbox.com/la...oad-d_245.html

    x

    CFM = Sensible Load / ( 1.08 * approx. 18)
    ie. Room is 12 * 15 = 180 Sq. Feet which requires 2,000 BTU/Hr Daytime .:. 2,000 / 20 = approx. 100 CFM.
    Designer Dan
    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

  3. #16
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,190
    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.

    So you are trying to "balance the system by "sucking" the air back. Sounds kinda logical BUT it is always better to Balance the supply air side of the equation. to do otherwise invites low airflow and frozen coils in the summer
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  4. #17
    Join Date
    Jul 2009
    Location
    South Carolina
    Posts
    3,191
    I think we're making this way too complicated.
    I would just block off about 10" of the return grill so it acts like a 20x20 grille. A 20x20 is about what I would use for a 3 ton if I already had the 2 - 12x12's. If the rest of the duct system is not balanced already there's nothing blocking off any returns are going to do any ways. So I say give it a shot & if it doesn't work to your expectations call in the marines(pros).
    Gary
    -----------
    http://www.oceanhvac.com
    An engineer designs what he would never work on.
    A technician works on what he would never design.

  5. #18
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,910
    Quote Originally Posted by garyed View Post
    I think we're making this way too complicated.
    I would just block off about 10" of the return grill so it acts like a 20x20 grille. A 20x20 is about what I would use for a 3 ton if I already had the 2 - 12x12's. If the rest of the duct system is not balanced already there's nothing blocking off any returns are going to do any ways. So I say give it a shot & if it doesn't work to your expectations call in the marines(pros).
    Be nice to know what size the supplies are to those rooms.
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