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Thread: Ductwork Flex

  1. #14
    Join Date
    Jun 2005
    SW Wisconsin
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    Quote Originally Posted by trebor1065 View Post
    My old unit was a 2.5 and I recently upgraded to a 3.5 ton unit, my contractor recommended going to a 3.5 Ton unit to sufficiently cool the second floor and it does!

    But I have noticed when the blower shuts down it sounds like the ductwork is flexing? It is possible the ductwork is slightly undersized if so what effect will this have on the system?
    Flex duct has to be sized larger than metal ducts to get the same velocity & CFM airflow, so I like the others here, I think you better find someone to do some static pressure testing to see what is actually going on.

    Airflow & heatload though the evaporator determines heat transfer, you may not be getting much more than 2.5 ton from the 3.5-ton unit.

    Slowing the velocity will further reduce the needed CFM for the larger capacity evaporator.

    Here is a linked page on the general sizing of duct systems , filter racks, etc.
    Study it so you'll have a general idea what to ask a contractor for.
    - Darrell

  2. #15
    Join Date
    Jan 2004
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    Quote Originally Posted by trebor1065 View Post
    To change the duct out will cost thousands due to it being an old house and the duct runs through the walls, My best bet is to get a service tech out to set the fan motor to run at a slower speed..
    Your best bet is to get the contractor out and pull that unit out and install the right size and correct what duct work needs corrected.

    Othre wise. you might have paid for a 3.5 ton, to only have the capacity of a 2.75 ton.
    And end up with expensive repairs.

    Your contractor should have known better.
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    How many times must one fix something before it is fixed?

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