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Thread: Duct sizing

  1. #1

    Default Duct sizing

    I'm sketching out some ideas for a remodel that might entail replacing and moving the furnace upstairs -- basically directly above where it is now (making the duct runs be pretty straightforward, I'd think, since they are already in the vicinity).

    I'd like to know, though, if there are specific rules for ducting _size_. The current furnace is installed so poorly that I don't want to assume there are enough and properly-sized ducts. I realize that an HVAC pro would make sure the new unit was properly done, but I'm trying to get a rough idea for my sketches. Any help is appreciated.

    BTW, I found a handy tool for 2D and 3D sketching of my house here:
    http://sweethome3d.sourceforge.net/

  2. #2
    Master Plumber nhmaster's Avatar
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    To do it right and have a comfortable, quiet and efficient system you need to call in an expert in duct design and lay out. All too often forced hot air systems have gotten a bad rap due to bad ductwork. There are whole engineering volumes written about proper duct design and air flow. Without knowing the heat loss and gain figures, infiltration factors and accruate framing and room lay out it is all but impossible to do anything other than guess at things. Guessing at things generally turns out badly. If you are expecting someone online to do for free what they would certainly charge for, I'm afraid you are going to come up dry. Nice drawing program though.

  3. #3
    Computer Programmer Bill Arden's Avatar
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    When guessing it's better to err on the large size.

    The larger and shorter the ducts are, the less air resistance and the lower you can run the fan. It's also a lot easier to close a grill than to increase it in size.

    On the other hand... If you combine large ducts on some runs with small ones on other runs you can end up with whistling register vents.

    The goal is to have each run drop the same amount of pressure at the needed air flow rate for the heat loss for each room.

    Quote - "anyone can build a bridge, but only an engineer can build the bridge just strong enough"

    This is one of the areas where a person in the field can do a better job and save on materials.

    So you have 4 options
    1. Hire someone to figure it out.
    2. Do the math yourself.
    3. Do trial and error increasing the vent size latter as needed.
    4. Overbuild everything.

    Overbuilding can be useful if you plan on upgrading things to a heat pump or add a central A/C unit.
    Important note Ė I donít know man made laws, just laws of physics
    Disclaimer: I'm a big fan of Darwin awards.

  4. #4
    Master Plumber nhmaster's Avatar
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    Insufficient duct flow leads to slow and uneven heating.

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