return and feed size duct?
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  1. #1
    Join Date
    Jun 2008
    Posts
    58

    return and feed size duct?

    I have a quick question for you guys. In the process of building a new home. 2 story home rougly 2500 square feet. I have 2 2 ton lennox xp 16 Hp's going in and 2 g71 furnaces as well. Here is my question.

    The furnace located in the basement went in fine lots of room. I don't have the exact measurements and the proper vocab so please bare with me. coming from the long duct runs that run the length of the basement floor. There are to spots were the furnace gets its return air and then the feed size.

    output 20x16

    return 20x10


    Now on the second floor there is a truss that was interfeering with the install the HVAC guy pointed this out to me and because of this were the long runs in the atic hook into the furnace the

    output opening is 15x 15
    return is 11.5x14.

    Im not sure if this is going to be a problem it seems like the amount of CFM is going to be greatly reduced. LIke I said I would have never known but the HVAC guy noticed and told me that they are a few inches smaller than normal. What do you guys think in your opinion. Sorry for the lack of proper terms.

  2. #2
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    A rectangular duct duct is more restrictive then a square,when the cross sectional area is the same,due to the perimeter being greater.More perimeter,more restistance.


    For air flow;
    16X15= 20X11

    14X15= 20X10

    So they are closer in abilty to carry X amount of air then it would appear.
    Last edited by dash; 10-20-2008 at 11:40 AM.

  3. #3
    Join Date
    Jul 2005
    Posts
    717

    Talking

    Quote Originally Posted by dash View Post
    A rectangular duct duct is more restrictive then a square,when the cross sectional area is the same,due to the perimeter being greater.More perimeter,more restistance.


    For air flow;
    16X5= 20X11

    14X15= 20X10

    So they are closer in abilty to carry X amount of air then it would appear.
    ************************************************** ********
    16 x 5 = 20 x 11....huh!!! i don't think so.

  4. #4
    Join Date
    Jul 2008
    Location
    Poestenkill, NY
    Posts
    769
    Quote Originally Posted by deejoe View Post
    ************************************************** ********
    16 x 5 = 20 x 11....huh!!! i don't think so.
    I think that was supposed to be 16 x 15

  5. #5
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Quote Originally Posted by larobj63 View Post
    I think that was supposed to be 16 x 15
    Correct,and corrected.

  6. #6
    Join Date
    Jun 2008
    Posts
    58
    I actually had messed up all the measurements. I just got back and re measured.

    Return 20x10
    output 20x16 First floor


    second floor

    return 11.5x14
    output 15x15

    With the dimensions being smaller is this a problem. I am paying a fair amount of coin for this system and don't need it comprimised because of a stud placement interfearing with duct work. Your advice is appreciated

  7. #7
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    We typically see larger return ducts then supply,or the same size.

    15X15=20X11

    11.5X14=20X8,this one may be an issue,or the basement ducts are oversized.

    Plus elbows and T's in the basement ducts could require them to be larger,where the attic ducts run strighter,less elbows ,no t'S,so they can be smaller.It's all about the total resistance of the ducts to air flow,Static Pressure.

    Correct duct size could be transitioned to a smaller size to get past he truss,then back to full size,if that's the case.

    If both were designed by using Manual D ,from ACCA ,then you should be fine.

    With your concens ,I have a chat with your contractor.

  8. #8
    Join Date
    Mar 2008
    Posts
    10

    Insulation?

    Is the duct lined inside, if so your return air will be too small. If it is not lined it will be adequate.

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