Splitting a 12" trunk flexible duct...please help!!

Discussion in 'HVAC Heating & Cooling' started by Linclink, Aug 18, 2010.

  1. Linclink

    Linclink New Member

    Messages:
    5
    Location:
    Florida
    Ok...I live in a mobilhome and I'm tired of possums screwing up my ac ducts. So I'm in the process of doing a ductwork and make it come from the top of the house

    My trunk duct that comes from the package unit is a 12" diameter duck, and doing some calculations ( in terms of area) I realize that I would have to use around 9 ducts of 4" diameter to keep up with the same area of a 12" duct.

    As the pic below shows, I was thinking about 4 splits of 4" ducts. Now I'm not sure if it will be enough to release all the air coming in.

    Any help will be really appreciated!!

    Attached Files:

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  2. gator37

    gator37 Retired prof. engr.

    Messages:
    108
    Location:
    Alabama
    What manufacturer and model number is your HVAC unit or how many tons? Is it a heat pump, AC with electric heat or a furnace? How long is your mobile home? You should not rely on the unit's duct connection size to be you main trunk line. In your case you indicated 12".
    Just a note: in your case you may want to consider metal rectangular internally insulated duct and install it back under the mobile home again.
    GO GATORS
    Last edited: Aug 18, 2010
  3. Linclink

    Linclink New Member

    Messages:
    5
    Location:
    Florida
    Hey gator...thanks for dropping me a line.....

    Tempstar PA9542AKA/PAPCA2GA it is a 3.5 ton and I disconnected the the heating a while ago so it just works as a AC.

    My Mobil home is 24 by 40 and roughly 1000 square feet.....

    Yeah...GO GATORS !!!!!

    PS - Ohh man....working under the MH is a nightmare !! I realy wish I could finish this thing and have it not under there :)
    Last edited: Aug 18, 2010
  4. Linclink

    Linclink New Member

    Messages:
    5
    Location:
    Florida
    Ok...took a pic of the data sheet . The compressor was changed for another copeland a little bit bigger but the guy that installed it for me told me all was fine.

    Attached Files:

    Last edited: Aug 18, 2010
  5. gator37

    gator37 Retired prof. engr.

    Messages:
    108
    Location:
    Alabama
    I had the same problem with my mobile home and changed from a split system to a gas heating, electric cooling package unit. I ran metal round duct underneath with external insulation all the way to the center of the home and tapped into the existing distribution system about mid way. I ran a new return air underneath also and placed a new return and metal grill can in the corner of my living room. I put the minimum of 4 self tapping 3/8” screws equally spaced around the duct joint circumferences and sealed all the joints including the snap lock joint (long duct joint).

    Originally when we purchased the home 30 years ago (new) they wanted to sell me a 2 ton and I told them no I want a 2.5 ton. Having dealt with rule of thumb contractors selling and installing mobile home units in Ft Lauderdale 40 years ago I knew some of the trade tricks. I still get a possum or two in the last 7 years or so but I put out food in the back at night and shoot them when they come out. They have not torn up my insulation on the metal duct yet.

    My mobile home is about 16x75 and the 2.5 tons works great even when the outside temp was 103 a few weeks ago. When I sized the duct I oversized (liberal sizing) it knowing that the existing distribution system was undersized which seems to be the standard anymore. The chart you sent is similar to what I mentioned. Check and see if the speed tap is set on Hi for either the 230 or 208 volt as applicable. On the 230v, it appears that you can only get 1467 CFM if your static losses are no greater than .45 inches which should include duct losses, filter loss, and grill loss. So you probably would not get 1467 CFM but something less. Liberal sizing for the FURNACE trunk duct at 1400 CFM would result in an 18" dia duct.

    Note that if your new unit evaporator coil (indoor coil) is freezing (everything else being correct) than you fan is either not working, you have a blockage in the duct or just not getting enough air a crossed the coil due to increased duct etc. losses.
    Is your roof cool sealed (light reflective mopped on sealant)? Do you have an exorbitant amount of glass is the glass you have double pane insulated?
  6. Linclink

    Linclink New Member

    Messages:
    5
    Location:
    Florida
    Thanks for getting back to me Gator.

    Basically I'm not runing any ducts throught the attic because it is too tight there to work, I kind of designed a system that runs inside the house and it is barely noticeably.

    Now about my original question. Do you think that 4 tubes 4" diameter each will keep up with the 12" main trunck? I just don't want my compressor to be damaged due to "high blood pressure" on the ducts.

    Thanks a million!!!
  7. gator37

    gator37 Retired prof. engr.

    Messages:
    108
    Location:
    Alabama
    Not sure what your layout is but the following may help.

    4" duct is good for about 40 CFM
    6" duct is good for about 120 CFM
    8" duct is good for about 200 CFM
    10" duct is good for about 400 CFM
    12" duct is good for about 680 CFM
  8. Linclink

    Linclink New Member

    Messages:
    5
    Location:
    Florida
    Ok... thanks for the help gator.I'm gonna try with 4 of 4" and if my AC freezes or I dont see a lot of suction on the return I will have to add more.I will keep my fingers crossed because if I have to add more ducts it it will mess up my set up...lets see
    thanks again.
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