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What size flexible duct to use?

We did have an HVAC company come in review the system and do a load analysis. They said that the current unit should be sufficient to handle the additional room. They simply took measurements of the exterior house, windows and doors, and took inside measurements of the 21x23 room and looked at my current system. They did caution that on extremely hot days since that large room has a west facing bay window it might not keep that room as cool as the rest of the house. The R value of the 12" duct work is 4.2.


  • mh92
    mh92 Member Posts: 2
    What size flexible air conditioning duct work to use.

    We purchased a house where a living room was added 21x23. There is no air conditioning duct ran to this room. The rest of the house has air condition via a central unit and 8" flexible hose ran to each room from the central attic unit. There is one room that is 17x15 and they ran two 8" ducts to it. My question is should I run a 12" duct (about 50') to this living because of the large size? I can only easly put one duct to this room because of the ceiling cable electric heat.
  • Empire_2
    Empire_2 Member Posts: 2,343
    Be careful

    Flex duct has a greatly reduced CFM capability due to the wavering surface inside the duct. 12" flex duct will deliver aprox 500cfm as apposed to the 700cfm from galv. pipe. Also return duct is just as important as the supply duct. You have to give the room air changes. Ceiling height is also needed to figure out the cooling needed, along with outer exposure and orientation to the sun, N-S-E-W.....and don't forget insulation in the walls.

    Mike T.
  • Eugene Silberstein 3
    Eugene Silberstein 3 Member Posts: 1,380
    A number of Concerns/Issues

    First of all, it is impossible to determine, with any degree of accuracy, the required cfm for the space without all pertinent construction and fenestration information. By this I mean that we would need to to know the construction materials used, the R-values of the insulation, what is above and below the occupied space. In addition, the types of windows, the direction they are facing, the size of the windows, the type of glass used all play a role in determining the amount of air required for the space.

    Second, you cannot simply add an additional supply run without affecting the rest of the system. You mention that the run is going to be approximately 50 feet. This will have an effect of the static pressures in the duct system, especially if flex duct is to be used.

    Third, by adding a large supply takeoff to the air handler, the amount of air delivered to the other areas will be greatly reduced. A 10" round duct will carry enough air more than 1 ton of a/c, and a 12" round will carry considerably more.

    There is likely a very good reason as to why there was no air conditioning brought to this area: The system is too small to handle the additional load.

    What you need to do is have a qualified technician come in and peform a heat gain on the structure and evaluate your air distribution system.

    You can almost guarantee yourself a completly uncomfortable home should you proceed and just add the additional run as you proposed.

    Good luck and please keep us abreast as to your decision and the results.

    "The Professor"
    CHARLES_4 Member Posts: 61
    Flex duct

    What they said...All of them. You're looking to add nearly a ton (10,000-12,000 Btu) load to your existing system, depending on various factors mentioned. Is your current system THAT much oversized?? So it's only half that much too big; it's still going to be that much too small. And the techs already warned you that on a warm day, it might not cut it. So the first time it doesn't work, they say "we told you so". (How warm is warm?) Why not go with a small, ductless mini-split for that area and zone it?
  • clammy
    clammy Member Posts: 3,113
    hitting the nail on the head

    Charles has it right ,instead of compromising your existing system you should look at a small ductless split ,then your existing a/c system would not be over loaded on hot days and your less likely to have to do any sheet rocking or spackling plus 50 feet for a flex run for me is out of the question plus a supply only may not cool the room enough unless your pulling some return also go for the ductless split they are now arraiable in a 14 seer 410 a system a much better set up then pulling it off the existing systenm peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
  • jim lockard
    jim lockard Member Posts: 1,059
    50 feet

    did not know that code would let you run 50 feet of supply flex. J.Lockard
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