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  1. #1
    Join Date
    Nov 2007
    Location
    Livermore, Ca
    Posts
    163

    It looks good on paper...

    First I must say that getting a few Manual J's and D's gives peace of mind.

    I've been going over the ducting, comparing the design(s) of each. I see that the ducting has a column for TEL. Also each branch is listed as a separate measurement. It's great that this is part of the equation and I can grasp the concept as for why.

    So my question is, Due to limited space in the attic am I able to streamline or cut down the length(s) of the ducting as designed? I have no problem with resubmitting a fairly detailed print or drawing of the workable routing and lengths that would result.

    I understand that rules of thumb are bad and the Manual D was drawn with the TEL in mind. I even tried to self educate by crunching the room requirements as compared to the branch line vs the trunk line vs the cfm vs fpm and so on.

    I am totally pleased with the engineered results but unfortunately it will be a PITA to make this work in my situation. It looks good on paper, but...

  2. #2
    Join Date
    Jul 2008
    Location
    Poestenkill, NY
    Posts
    769
    I'm not sure what you're asking - but duct length is much less important than cross sectional area. Maybe that helps?

  3. #3
    Join Date
    Nov 2007
    Location
    Livermore, Ca
    Posts
    163
    The TEL is set from the Manual D. I need to chop that way down to make it fit in my small arse attic. I would have ducting criss-crossing, blocking egress, etc.

    Is this doable or should I resubmit a drawing with what will fit keeping all as drawn trunk and branch dimensions and still have the same end result.

  4. #4
    Join Date
    Mar 2003
    Posts
    7,326
    use the dimensions of the ducts as your guide as to what size trunk, takeoffs etc, but cut down the lengths to fit your needs.

  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 regis101 View Post
    The TEL is set from the Manual D. I need to chop that way down to make it fit in my small arse attic. I would have ducting criss-crossing, blocking egress, etc.

    Is this doable or should I resubmit a drawing with what will fit keeping all as drawn trunk and branch dimensions and still have the same end result.
    Are just using shorter runs,or reducing the duct size??

  6. #6
    Join Date
    Jul 2008
    Location
    Poestenkill, NY
    Posts
    769
    Quote Originally Posted by dash View Post
    Are just using shorter runs,or reducing the duct size??
    I can't figure out what he's asking either.

  7. #7
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,167
    Apparently he has a drawing. That we can't see. And wants an answer based on us not knowing what he see's.
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  8. #8
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Okay,read the OP again,think I understand.

    Yes you can reduce the lengths,may need balancing dampers,as the shorter they are the more air they will get,but this is also the case with the original design.

    You should not;
    Change supply or return grille sizes or location
    Add more elbows or turns
    Use fittings(transitions,etc.) other then shown on the print.
    Last edited by dash; 08-13-2008 at 08:17 PM.

  9. #9
    Join Date
    Nov 2007
    Location
    Livermore, Ca
    Posts
    163
    The operative word in the opening post is, length. Duct diameter is not going to be reduced. Defeats the purpose. I need that diameter to get the air into the room (s).

    Can I cut down on the length(s). Will that defeat the purpose.

    I think some of you read too fast
    Go easy on me, I'm new

  10. #10
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Uh did you read my post?

    I used the plural of the operative word!!

  11. #11
    Join Date
    Jul 2008
    Location
    Poestenkill, NY
    Posts
    769
    Quote Originally Posted by regis101 View Post
    The operative word in the opening post is, length. Duct diameter is not going to be reduced. Defeats the purpose. I need that diameter to get the air into the room (s).

    Can I cut down on the length(s). Will that defeat the purpose.

    I think some of you read too fast
    Go easy on me, I'm new
    We're going easy on you because I think many of us suspect English is not your first language, not because you're new. Which is fine, of course. But if it is your first language - you type to fast - we don't read too fast. No-one in here could figure out what you were asking - and I said the cross section was more important than the length in the first reply!

  12. #12
    Join Date
    Nov 2007
    Location
    Livermore, Ca
    Posts
    163
    Thankx dash.

    I am under the assumption that the length(s) of the ducting is part of, not just the reason, of the Man D to help get the airflow down to the rooms requirement.

    As dash mentioned, a shorter run will get more air. So this means a longer run will slow it down? If so, will having to take a branch line from 20ft as drawn down to 8ft affect the airflow as described for that room.

  13. #13
    Join Date
    Jul 2008
    Location
    Poestenkill, NY
    Posts
    769
    Quote Originally Posted by regis101 View Post
    Thankx dash.

    I am under the assumption that the length(s) of the ducting is part of, not just the reason, of the Man D to help get the airflow down to the rooms requirement.

    As dash mentioned, a shorter run will get more air. So this means a longer run will slow it down? If so, will having to take a branch line from 20ft as drawn down to 8ft affect the airflow as described for that room.
    The velocity of the air is determined by the cross-sectional area of the duct.

    Longer runs do impose more static pressure, but the difference between 20 feet and 8 feet is almost negligible. For the third or forth time, I lose count, shorten the runs and maintain the other dimensions the same as designed.

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