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Thread: duct work

  1. #1
    Join Date
    Nov 2005
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    3 ton rheem air handler with 8x18 duct. this goes strait into a 20 by 20 plenum. my question is is this duct the right size for a 3 ton heat pump. I have a 2200 sq. ft home and it doesn't seem to heat very well, and i'm wondering if this is a airflow problem?

  2. #2
    Join Date
    Jun 2000
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    Some older Rheem airhandlers required a smaller duct,2'-3' to be attached on unit before the plenum to build up static pressure. We installed some of them in the ealy 90's.
    Check the installation instructions if you have em.
    Home not heating well could be one of 100 things wrong.
    never say never

  3. #3
    Join Date
    Jun 2004
    4H: Hot, Humid Houston H.O.
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    Seems clearly small to me

    Your 8*18 inch duct has 144 sqin, that is 1.00 sqft. When your 3 ton air handler is blowing at its normal speed, that is 1200 CFM. Therefore your duct is flowing 1200 ft/min if indeed the air handler can push that much air through such a small duct. That is for air conditioning mode, you should check whether design airflow is reduced for heating mode.

    There's tons that I don't know, but I do know Manual D specifies air speed should be no higher than 900 feet/min max, and that is for rigid supply ducts. Flex and returns have lower speed limits. I did not see whether your duct is in the return or supply. At most, Manual D would support 900 CFM through that duct, and 700 CFM if that is the return side.

    So you are probably in violation of that airspeed limit, which happens when your ducts are too small or airflow too high. I would guess (I'm not a pro) that your system will have excessively high ESP (External Static Pressure). It may be because of the ESP, your air handler is not actually delivering the rated CFM. It is a common error in duct design and installation where I live.

    There are other parts of Manual D for good duct design, but that is one facet of it. I will learn from what the pros tell you about your system.

    Best wishes -- Pstu

    P.S. What Contactor said, I never knew that! Thanks. I kind of agree that a tech on site, could answer this far better. With this much (little) info on the board, it is a riddle.

  4. #4
    Join Date
    Oct 2003
    Post Likes
    You should ask your local HVAC contractor all your questions. We couldn't possibly know from here.

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