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Thread: duct question

  1. #1
    Join Date
    Jun 2006
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    i just built an addition on to the back of our home, the addition is real tight, r30, r19 in walls, double insulated doors and windows, low e. all under a 6' porch.

    main house simple rectangle 1200 sq
    addition simple 650 under air
    manual j was done said 3.5 went with a 4 ton trane 15i vari speed air handler. florida east coast

    the house is not cold enough and the addition will freeze you out. i dont think the conmpany did the duct calculations. from all the stuff i have researched i think the 2 ten inch flex ducts that feed the main house are undersized and should have been 12 inch.
    the plenum coming out of the air handler is about 22" square 1.5 inch duct board then it goes to 16" flex for 5 feet then into a dist box about 2x2 ductboard, 2 10 inch ducts about 25 feet long go to the main house and the other 10" branches off to addition. the air handler is in the addition. now the stuff i think i know.

    cfm 1600-1800 dont know for sure for a 4 ton trane
    10 flex duct cfm 340 -15%for flex
    so if i have 3 x 300 and i have a 7" from the plenum box going to the addition also(dont know why) which is i think 140 cfm so i think i have a 1000cfm system.

    so my question is if i go to 12 inch flex to the main house which should be 600cfm will i get more cfm to the hot rooms, will i loose air volectiy? i have to reduce the addition air flow, i plan to add a damper to the addition 10 to help balance the system but with the vari speed i really dont know what will help...if you need picts or i just confused you please email <<<removed email>>>
    i really apprecaite any info on this. thanks tony


    <<<Edit: No emails in posts due to site rules. Place your email in your profile biography. Thanks, Benny!>>>

    [Edited by jrbenny on 06-23-2006 at 04:57 PM]

  2. #2
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    Total of 1850 sq ft,with a well insulated 650 sq ft addition,3.5 sounds large ,4 ton sounds oversized.Lots of glass??

    The cfms you mention for certain size ducts are "rules of thumb",not anything actual or meaningful.That said it does sound like the 10" ducts going to the original house need to be larger,could be that one needs to be a 14" instead of having two 12" runs,depends on the load of the rooms that are served by each one.

    Hint the rooms of the original house that ajoin the new ,air conditioned addition,will not need the air flow the once did,before the addition.


    Manual J should have been done "room by room" to determine the cfms needed for each room,then a Manual D to size the entire duct system.It would be best to 'drop back" and have those done,instead of guessing at a solution.

  3. #3
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    Jun 2006
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    thanks for the reply, the house is all single pane,leaky structure, about average glass coverage. im trying to understand the science behind air flow. if the ducts going to the main house were bigger, that would reduce the air flow to addition and increase the cooling to the main house? does more air flow equal more cooling? also the vari speed runs on low then ramps up for several minutes then ramps down. when it is on low does the duct size really matter? thanks again tony

  4. #4
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    Larger ducts to the main hose will reduce the air flow to the addition,works kinda like water flow,follows the path of least resistance.

    More air flow to each room the room will be cooler than others,their is a science to getting it right to all the rooms.


    Even on lower air flow,it basically porportions the air to each room ,based on the ducts sizes(all the sizes it went thru to get to the room,not just the final one going to the room) feeding the room.


    Each room also needs a return or return path(like a large enough space under the door),for air to get back to the fan,when the doors are closed.Try feeling the air flow from grilles with the doors opened versus closed.If air flow drops to a room with the door closed,that's part of the problem.You can't keep blowing air into a closed "paper bag',nor will it go into a room without a path back to the return.


  5. #5
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    Jun 2006
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    dash, thanks for thaking the time to answer my questions, i appreciate it...tony

  6. #6
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    You're welcome,no problem.

  7. #7
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    Jun 2004
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    4H: Hot, Humid Houston H.O.
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    Additional insight asked

    >>Even on lower air flow,it basically porportions the air to each room,
    >>based on the ducts sizes (all the sizes it went thru to get to the room,
    >>not just the final one going to the room) feeding the room.


    Dash (or anyone), I appreciate your sharing your knowledge on this subject! I would like to try a possible brain-teaser on you, if you don't mind:

    Suppose your central AC duct system is built to handle 1600 cfm, and you install a whole house dehumidifier which has a fan which can blow 200 cfm. Like the Thermastor old design, which has that capacity at about 0.1 inch w.g. (going from memory). The dehu blows air into the supply plenum. The main AC air handler is not running.

    Will there be a tendency to distribute air throughout the house, as the duct system was designed to do at an ESP of maybe 0.5? Or will there be a tendency for air to mainly go out the nearest duct because the ESP is much lower than the duct system was designed for?

    I have heard statements in both directions, wonder which is more true.


    Thanks and best wishes -- Pstu


    P.S. Even if the airflow is unequal among the rooms, I am not thinking of slamming the Thermastor type recommendations because of it. I am entirely thinking it is not as necessary to distribute dried air as precisely as it is necessary to distribute cooled air. We all know from experience how a house heats up in minutes, I am thinking that humidity changes much more slowly throughout a house. So even a one-inlet, one-outlet dehu design would be fairly effective in whole house dehumidification. That is my thought at this time.

  8. #8
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    Just a consumer opinion, but my next house won't have an inch of flex duct without a real good reason.

  9. #9
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    Originally posted by danglerb
    Just a consumer opinion, but my next house won't have an inch of flex duct without a real good reason.
    Flex ,designed and installed properly .performs the same as metal or ductboard.

    But,the choice is yours,of course.

  10. #10
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    pstu,

    Air flow as low ,cfm wise,as a dehumidifier,will likey "fall out" the nearest grilles(really don't know for sure),however the dehumidified air will quickly "mix" throughout the home.

    It's the very large cfm difference, not the the static difference, that I think will cause this .

    Many think that reducing the cfms by 50% will cause distributio problems,however that's what top of the line two speds do,with no problems at all.


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