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  1. #1
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    Nov 2019
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    HVAC design, duct sizing

    Hello everyone

    I had a contractor do an estimate fro redoing the entire hvac system on my 2000 sq.ft single story house. They did load calculation and manual D. When first asked, I told them I wanted to use round metal duct throughout the house. In their estimate and on the manual D, they specified flex duct. So I reminded them about the metal duct which they corrected on the estimate but they are still using the same sizes than they specified with the flex duct. I was reading in another post that it is not necessarily the case and round metal duct should be smaller. Should I complain or are they correct ?
    I also ask them to add a supply register in the master bathroom because the room is kind of split in half and all they did was add another 10"x6" register with 6" duct on that other side, which is same size as original design with only one register for the whole room.
    Also I have a 10"x6" register with 6"duct in the guest bathroom that is 10 feet by 4.5 feet in size, and same size 10"x6" register with 6" duct in guest bedroom that is obviously fairly bigger at 11 feet by 12.5 feet.
    Now, I'm obviously no expert, but I don't want to complain unless I have a good reason to do so.
    Thanks for your help, if you can

  2. #2
    Join Date
    Aug 2009
    Location
    Beatrice, NE
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    Generally Flex will move 80% less air than metal, they may have sized the flex the same as the metal, can't say for sure. 6x10 registers seem large unless they are ceiling registers.

    If they did a load calc and man D then they should be able to tell you hard numbers for the amount of air needed for each room. Just don't be surprised if they don't want to share them. It takes a lot of time to go through those 2 worksheets even on computer and they may feel if they give them to you you may pass them on to another dealer saving him the trouble. However for the few rooms in question they may give you their numbers and reasoning.

    Duct sizing is about getting the air to the register. Register sizing is about supplying that air to the room at the proper volume and velocity, too much or little of either "V" can cause comfort issues. The location of the register/grill can have an affect on this also.

  3. #3
    Join Date
    Aug 2001
    Location
    Pavilion, NY
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    Quote Originally Posted by BNME8EZ View Post
    Generally Flex will move 80% less air than metal, they may have sized the flex the same as the metal, can't say for sure. 6x10 registers seem large unless they are ceiling registers.

    If they did a load calc and man D then they should be able to tell you hard numbers for the amount of air needed for each room. Just don't be surprised if they don't want to share them. It takes a lot of time to go through those 2 worksheets even on computer and they may feel if they give them to you you may pass them on to another dealer saving him the trouble. However for the few rooms in question they may give you their numbers and reasoning.

    Duct sizing is about getting the air to the register. Register sizing is about supplying that air to the room at the proper volume and velocity, too much or little of either "V" can cause comfort issues. The location of the register/grill can have an affect on this also.
    80% less or did you mean 80% of?

    Sent from my SM-N960U using Tapatalk
    ...

  4. #4
    Join Date
    Aug 2009
    Location
    Beatrice, NE
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    20% less, 80% of

  5. #5
    Join Date
    Nov 2004
    Location
    SW FL
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    9,725
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    Quote Originally Posted by filipi View Post
    Hello everyone

    I had a contractor do an estimate for
    redoing the entire hvac system
    on my 2000 sq.ft single story house.

    They did did load calculation and manual D.

    When first asked, I told them I wanted to use round metal duct throughout the house.
    In their estimate and on the manual D, they specified flex duct.

    So I reminded them about the metal duct
    which they corrected on the estimate but
    they are still using the same sizes
    that they specified with the flex duct.

    I was reading in another post that it is not necessarily the case and
    round metal duct should be smaller. Should I complain or are they correct ?

    I also ask them to add a supply register in the master bathroom
    because the room is kind of split in half and
    all they did was add another 10"x6" register with 6" duct on that other side,
    which is same size as original design with only one register for the whole room.

    Also I have a 10"x6" register with 6"duct in the guest bathroom
    that is 10 feet by 4.5 feet in size, __ probably need less than 25 CFM; 8" X 4" diffuser may be typical
    and same size 10"x6" register with 6" duct in guest bedroom
    that is obviously fairly bigger at 11 feet by 12.5 feet. __ probably need 80 CFM +/- 20%

    Now, I'm obviously no expert, but I don't want to complain unless I have a good reason to do so.
    Thanks for your help, if you can
    ZIP _ _ _ _ _

    Age of Residence __ Built in 2002 or _ _ _ _ ?

    ACCA Manual J Room-by-Room calc is the method
    to pursue to determine required air flows to each area.

    Duct size: Provide Internal Area for ~ 500 FPM Velocity.
    Specific duct size would be determined in a Manual D calc.


    I am not certain what hemisphere you may reside in __ ?
    ______________________________________________ Portugal ?
    Last edited by dan sw fl; 11-24-2019 at 06:28 PM.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

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