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Thread: 7" Duct runs?

  1. #1
    Join Date
    Mar 2006
    Posts
    11
    This is my first time posting here, so please be very kind! I am getting an addition put onto my house that will add apporxiametly 450sq ft of living space. My current system (heat pump)is 17 years old, and I am looking to repalcement the system and upgrade it to the size needed to supply the addition as well. I ahve had a few different HVAC people come out to look at how the new feed lines could get ran to the new rooms. One wants to go up through my main floor into the attic and over to the new room, which would be ceiling mounted ducts. I really don't want to do this because it would be going through my sons room (which is already tiny to start). I had someone come out last night that said he could branch 2 7" feeds from the current trunk line (which is 10" x 18") into the new addition (16'x20' in the main room size). My question is the distance from the main trunk line to where the new feed line would be is about 26'. Id this to long of a run through a 7" pipe? The room faces south and I live in Maryland. I wuld go up through the attic if that would yeild me the best option (the other contractor was going to add a new trunk line into the attic and then branch off of it). Please let me know if more info is needed. Thanks!!

  2. #2
    Join Date
    Sep 2004
    Posts
    654
    Assuming the contractor is reputable and will stand behind his work, let him do what he feels will work. Personally, a 26' run on 7" pipe is too much, at least start off bigger and then neck down to 7" to maintain air velocity.

  3. #3
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    There's a way to know the proper size system and the correct amount of air for every room,Manual J.


    There's a way to selct the corect size equipment,Manual S.


    There's a way to size the ducts to deliver the amount of air that was determined in Manual J,to each room,Manual D.

    Read a little about them ,search function on this site and http://www.acca.org

    Call around until you find a contractor that says they can do all three,don't waste you time and theirs,if they can't/won't.If they use another method it's likely just a guess,get it done right the first time,and save.

    [Edited by dash on 03-08-2006 at 06:12 PM]

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,119
    Originally posted by dash
    There's a way to know the proper size system and the correct amount of air for every room,Manual S


    There's a way to selct the corect size equipment,Manual S.


    There's a way to size the ducts to deliver the amount of air that was determined in Manual J,to each room,Manual D.

    Read a little about them ,search function on this site and http://www.acca.org

    Call around until you find a contractor that says they can do all three,don't waste you time and theirs,if they can't/won't.If they use another method it's likely just a guess,get it done right the first time,and save.

    Shouldn't that be a "J" , in the first line.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  5. #5
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Yes it should,thanks.

  6. #6
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,295
    Originally posted by the4lams
    ... I am getting an addition put onto my house that will add approximately 450 sq. ft. of living space.

    .. upgrade it to the size needed to supply the addition as well.

    I really don't want to do this because it would be going through my sons room (which is already tiny to start).

    I had someone come out last night that said he could branch two 7" feeds from the current trunk line
    (which is 10" x 18") into the new addition
    (16'x20' in the main room size).

    My question is the distance from the main trunk line to where the new feed line would be is about 26'.

    Is this too long of a run through a 7" pipe?
    The room faces south and I live in Maryland.
    16'x 20'= 320 Sq. Feet ...
    Can two 6" diameter ducts be run smoothly to this room
    so that ~ 200 CFM is delivered?

    450 S.F.- 320 S.F. = 130 Sq. Feet
    Run another 6" diameter delvioerign ~100 CFM to this area.

    Use METAL Duct where possible!
    http://www.ductexpress.com/textDocs/inStock_PG.htm

    New Variable Speed air handler can deliver the required air flow with a somewhat more restrictive flow path.

    Limit the amount of west facing glass in the addition.

    The current 10"x18" header would generally handle
    ~1,000 CFM ... How does one "add" 300 CFM?

    IT APPEARS that you have a 1,400 S.F. house
    that you are expanding to 1,850 S.F. Investigate 2-stage, 4-ton heat pump with H.S.P.F. > 8.5.

    Is any more specific guidance needed?

    What are the current and the foreseaable future electirc rates?
    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

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