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  1. #1

    Trunkline opinion

    1.5 ton ch&a heat pump and ductwork being insatlled in new construction 900sf home, single story, slab on grade. Ductwork will be in attic with ceiling supply registers. Ductwork design is rectangular galvanized plenum and trunkline with round rigid metal supply lines...no flex anywhere.

    Supply branch ducts are to be as follows:
    2 of 5" rigid metal round ducts (one for bathroom and one for laundry room)
    3 of 6" rigid metal round ducts (one for livingrm, 1 for diningrm, one for bedrm)
    2 of 7" rigid metal round ducts (one for mstrbedrm, 1 for kitchen)

    Totoal CFM's needed for house is 670

    The AHU is centrally located in the house and I will be running two trunklines off the plenum, each trunkline running in opposite direction and each trunkline approx 12' long. My question is...I know that if the AHU was located on one end of the house that I would need to run a single trunkline sized to carry 670 cfm which would be a rectangular trunkline sized at 16" X 8". However, I am running two trunklines off the plenum...one trunkline will carry 385 cfm and the other will carry 285 cfm. I need some opinions here...should I make the 385 cfm trunkline at 10"x8" and the 285 cfm trunkline at 8" x 8" and have two different sized trunklines coming off the plenum? Would it be better to have both trunklines the same size...what effect would slightly oversizing the trunklines have?

  2. #2
    Join Date
    Jan 2004
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    Lancaster PA
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    I find it strange that .1 FR is what you think the duct should be sized at if you set the air handler at one end of the house. And also if you set it in the middle of the house.

    Bet if you used Manual D, it would come out to something different.
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  3. #3
    Join Date
    Aug 2002
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    The one requiring more cfms would be larger.Manual d is needed to determine the correct FR to use with your ductolator, as Beenthere stated.

    Manual D from www.acca.org

    Looks like you did Man. J,if so ,great start.

    If you are in the business,take the time to apply,after minimum posts ,for Pro membership on this site,it's worthwhile,and you'll get all the help you need with Man. D and also Man. S .
    Last edited by dash; 07-28-2008 at 11:02 AM.

  4. #4
    My house is being built in the country and there are only 3 hvac installers listed in the phone book. After having them both out to my house it is obvious they just "throw in" hvac systems. They both love flex duct...both want to design the entire return and supply systems with round flex duct...even the supply trunkline. I saw one of the companies vans in a dreiveway of a new construction house and so I stopped to look at their work. I saw lots of flex duct, not pulled tight, sagging, loops, crushed areas, etc. I personally won't allow this. I want all rigid galvanized. I called the other two companies listed in the phone book. When I expressed that I do not want flex duct used to one of the companies that came out for a consultation, the two men just turned around got in their van and left. The other company that came out...the guy was not enthused about working with rigid, but willing to work with me.

    So I talked with him some more...he suggested that I hire him on a LABOR ONLY basis and that he would design a rigid system and then I would hire a metal shop to build it and then I would pick up all the materials and bring them to the jobsite and then he would install it.

    So last week I paid him $150 for two hours of his time in design work. What he came up with was a supply plenum sized to the supply opening of the AHU with a trunkline coming off both sides of the supply plenum...each trunkline sized 12" X 16". Then 4 supply lines at 6" each coming off one trunkline and 3 supply lines at 6" each coming off the other trunkline. All supply registers sized at 10" X 6" X 6". I am questioning this design for effeciency.

    After reviewing his design I drove 2 hours to the big city library and looked at HVAC reference books. I was able to figure heating and cooling load for the house. I called him and asked how he arrived at the trunkline and ductwork sizes...the reply was "with a 1.5 ton system you don't want it any different size"..."It's a small house, this ought to be about right for balanced air flow." This kind of mentality is the reason I question his design work. His design work does not seem to be based on real calculations...just guesses. Maybe I am wrong here. That's why I am posting to this forum to confirm or dis-confirm this information.

    After 4 hours in the library reading HVAC calc books I think that I have the cfm needs of each room with correct supply line sizing. I do not feel the bathroom and laundry rooms which are each 5' X 9' rooms will require 6" supply lines as he has designed because I have calculated the bathroom to need 35 cfm and the laundry room to need 50 cfm. Also, I calculated the kitchen needs 125 cfm and the Master Bedroom to require 135 cfm...to me this means a 5" supply line to the laundry and a 5" supply to the bathroom. Then a 7" supply to Kitchen and a 7" supply to master bedroom...right? The other rooms having a 6" supply line will be fine I think. I also want dampers on each supply line...not register dampers, but in-line dampers. II now have a question as to the correct sizing for the trunklines. He says 12" by 16" and I think this is too big.

    I am just looking for more opinions on trunkline size because I do not have an abundance of professionals in my area from which to draw information from.

  5. #5
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    The 16 x 12 is large.

    The laundry takes 50CFM for cool? So what does it need for heating.
    Many areas need more CFM for cooling, but some take more for heating.

    Depending on length of run, a 7" may be enough for your mstr bedroom, or it may not.
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  6. #6

    cooling load

    The cooling requirement was larger for this house than the heating requirement so system is sized for cooling.

    Laundry room is 5' X 9' (5' wall is a north facing exterior wall). HL for this room is 357 btu and HG is 508, however the laundry is next to and open to the kitchen and so I estimated a slightly lower cooling requirement than the numbers showed.

    Mstr Bdrm is 11' X 13 ft (13; wall is a West facing exterior wall and 11' wall is a North facing exterior wall). 1354 btu HL and 827 btu HG for this room. I estimated the cooling load to be slightly larger than the numbers showed because of the West facing wall and the burning afternoon sun along with a small 36 sq. ft. walk in closet in this bedroom

  7. #7

    Master Bedroom supply run

    The run to the Mstr Bdrm far wall supply register will be 16' from the supply plenum (10' of trunkline and then 6' of 7" supply duct).

  8. #8

    loads

    On my earlier post I meant that each room was sized for the larger load...most rooms were larger cooling load

  9. #9
    Join Date
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    I would recommend as stated before. You get a manul D. Or go and look at one.
    And size your ducts from there.

    For the most part, your sizing will work.
    The question is, do youwant something that will work, or do you want it to work right.
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  10. #10
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    Keep in mind that an all metal duct system can be a little noiesy,lower duct velocities help,a Flex/canvas connecter between equipment and duct,also two 90 degree turns between the furnace fan and the return grille.

  11. #11
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    Flex connector is code required. Atleast here for metal duct.

    And you know no one would ever ignore code.
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  12. #12

    Manual D

    Well, I want something that will work right. Unfortunately I do not have any licensed professionals that are willing (or able) to give me a design that is the best for my situation. That's why I am here...trying to get some more information. It will be two weeks until I can take a day to travel to the library to read Manual D and try and size the Supply trunkline correctly. Is there a way to look online for friction loss and to find out how much friction loss I will have since I'll have two trunklines instead of one?

  13. #13
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    I don't know of any site off hand.

    But don't forget, your return has to be added in for TEL to find the FR you should use.
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