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  1. #1

    NEED HELP How do I increase airflow???

    I have a line from under the house off of the trunk line with 6" flex duct through the floor and straight up into attic to add heating/cooling to an addition. It has a minimal amount of airflow right now. How can I cheaply increase the airflow? This is a fairly long run, probably 50 feet. Should I change it out to rigid metal duct? Should I make the duct wider than 6" since it is such a long run? Add an in-line booster fan? I have 4 registers attached to this run. The first one has the most air flow and the last has very little. I can't run larger than 6" due to space limitations in some areas. Is it ok to go back and forth from a larger size duct to the 6" and back to larger?

  2. #2
    Join Date
    Sep 2003
    Location
    Portland.OR
    Posts
    313

    duct science..

    One six inch duct is good for about 110 cubic feet per minute. one per bedroom

    one 9" is close to 2 6 inch ones, so add a 9 and divide it between the last two room. and do the same for the other two room..
    JUST as important is the returns from those locations.. so make sure
    there is good airflow in that direction too.

    You dont get heat in the room, you REMOVE the cold air.......[and its filled
    with WARM air...

  3. #3
    Join Date
    Jun 2007
    Location
    Round Rock
    Posts
    3,550
    You probably just need a larger duct. A 6" duct at 50 ft isn't much for an average size room as you can tell. Small, big, small or whatever you are suggesting isn't going to work, neither will those stupid duct booster fans, I wish they would take those off the market. Call somebody out to evaluate what you have and make suggestions to remedy it.
    I like DIY'ers. They pay better to fix.

  4. #4
    Join Date
    Jan 2010
    Location
    Athens GA
    Posts
    1,234
    OK while this question should be very simple it is starting to hurt my head.Maybe I'm just tired.
    Number one; if you replace the 6"flex with hard pipe you should pick up 25-30% more cfm.
    Two you never increase,reduce,increase,etc a duct run.
    Then you say you have 4 registers off this one 6" run.Good luck with that,it will never
    provide any useable comfort.
    As already mentioned,where ever there is a supply register there has to be cold air return ,even if its leaving a door open to a real return air grille.Otherwise you are just blowing into an empty bottle.ie.no warm air can enter where cold air exists until there is someplace for the cold air to go.

  5. #5
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,977
    You say you have 4 registers off this one 6" run.
    That will never work properly!

    First, figure the BTUH/CFM requirements of all those areas served by the 4 or 5 supply registers. Also, whether each boot & register/diffuser is sized right to properly serve that area's CFM/BTUH requirements.

    The type of main-take-offs also affect pressure drops & airflow, & there's more...

    Then, the initial main duct has to be sized to properly to serve all the the required BTUH/CFM for all those 4 or 5 register served areas.

    Then, "the main-run needs to be reduced after each supply outlet," or velocities will usually drop too low down the line.

    'If' the air handler will support the required airflow it could be made to work, - providing there are adequate Returns from each area served - according to its CFM requirements.

    The problem is we don't know what the air handler will support (you need a Manual D performed on the duct system).

    Simply adding a Return is not likely to properly resolve the apparent airflow delivery problems to 'all' those areas. - Darrell
    Last edited by udarrell; 03-03-2010 at 03:11 PM. Reason: Clarifications...

  6. #6
    Join Date
    Dec 2006
    Posts
    82
    Concur that axial duct booster fans are stupid, ineffective and should be taken off the market. Centrifugal booster fans such as the FKD series from fantech are a whole different story though. However, like any HVAC component, they should be considered as part of an overall system, can be tricky to size/set up correctly, and may introduce unacceptable noise if not professionally installed.

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