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  1. #1
    Join Date
    Aug 2009
    Posts
    28

    Confused Supply plenum size vs Opening size in air handler

    Here's a question:

    I have a 5 ton air handler with an opening in the top of it obviously.
    That 5 ton unit was obviously designed so the opening can actually push 5 tons of a/c through it.
    If my supply plenum is the same size as this opening then it should be adequate I would think since the hole in the air handler is all the air handler can put out anyhow.

    BUT, I had a guy come out with a ductulator and measure the supply plenum and said it was too small for a 5 ton unit ! How can this be? It is the same size as the opening in the top of the unit.

  2. #2
    Join Date
    Aug 2009
    Location
    Kansas City
    Posts
    258
    The opening on the air handler is adequate because there is no back pressure(static pressure) to resist the amount of airflow that is required. Once you start adding ductwork with bends and elbows to the equation, the size of the air handler opening is a mute point.

  3. #3
    Join Date
    Nov 2008
    Location
    Chapel Hill, NC
    Posts
    1,070
    ????? But once you connect all of that ductwork (with its inherent back pressure) to the furnace, isn't the blower is pushing against it through that opening, just the same?????

  4. #4
    Join Date
    Sep 2002
    Location
    Northeast Ohio
    Posts
    4,842
    The exit opening of the blower itself is what is dictating the dimensions of the flanged connection on the airhandler. From that point, the duct system must be sized accordingly for the cfm capacity of the unit, the duct system and all of it's branches follow suit. You will notice that the return side is considerably larger.
    A good HVAC tech knows how, an educated HVAC tech knows why!

    DEM


  5. #5
    Join Date
    Aug 2009
    Location
    Kansas City
    Posts
    258
    Quote Originally Posted by sktn77a View Post
    ????? But once you connect all of that ductwork (with its inherent back pressure) to the furnace, isn't the blower is pushing against it through that opening, just the same?????
    Typically the velocity coming off the top of the AH is considerably higher than what the supply duct is designed at.

  6. #6
    Join Date
    Aug 2009
    Posts
    28
    so, the supply duct that is coming directly out of the air handler should not be used to measure cfm on a ductulator because in my case that shows the opening will only flow 1300, when it needs to flow 2000 for a 5 ton.

    which then asks the question: if a 5 ton needs to flow 2000 cfm, where should that measurement be taken on a ductulator ?

  7. #7
    Join Date
    Aug 2008
    Posts
    416
    Its probably what comes off the plenum that is the problem.
    How many branches come directly off the supply plenum??
    What are their sizes??

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,761
    Need to know the total equivalent length of the duct.
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