Duct sizing?

Discussion in 'HVAC Heating & Cooling' started by Nate R, Mar 12, 2008.

  1. Nate R

    Nate R New Member

    Messages:
    472
    Location:
    Milwaukee, WI
    Looking at doing something as an alternative to air conditioning whenever possible. My house does not have central air, so we use window units when necessary.

    The downstairs of the house isn't too bad in Summer. Tolerable enough for the most part. (LR and Kitchen)

    I'm more concerned about keeping the 2 bedrooms upstairs more comfortable for less money.

    I looked at whole house fans, but they require a larger opening than I would like as we have no place large enough to hide the grille out of sight. My wife does not want it in plain sight. They also seem to be somewhat loud.

    I'm only looking at 1800 cubic feet of space that I'm looking to move air through. (The 2 bedrooms)

    What I've come up with is using the Broan L900L in the attic, w/ 2 inlets. One in each closet in each bedroom. (Closets have no doors and are open to the room)
    900 CFM and only 3 sones! Some of the small inline units I was looking at were more expensive and were 14 sones to move 1000 CFM. 14 Sones 8 feet above where I sleep may be an issue.

    The inlet/outlet duct size on this unit is 12X8. The instructions seem to indicate they intend you to use 12" diameter ducts intake and exhaust or 12X8 ducting. Not a problem for the most part. I'm assuming they need to be this big to keep the noise down. They sell wall outlets for 12" duct that are just giant dryer vent type outlets.
    I only wonder on how to size the intakes before they come together since I'd have 2.

    12X8 is 96 square inches. If I have 2 inlets feeding this duct, how do I determine what size they should be? 1/2 the area would be 48 inches. An 8 inch round duct would be ~50 square inches. If I use two 8" ceiling inlets, will that be enough? Or do I need something more? Or is this more than enough?

    I'm open to other less expensive ideas as well!
  2. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    If ventilation will be adeqate for your needs, then I suggest an exhaust fan mounted in the roof or gable. You can put vents in the ceilings of the closets.

    The fan will exhaust the attic which will help reduce the heat transfer to the upstairs rooms, and will draw outside air through open windows.

    The fan in the roof will be barely heard in the house. There is usually a temperature switch that causes the fan to operate when the attic is warm. You could have a circuit with controls that would permit you to operate it with the temperature switch or manually.
  3. Bill Arden

    Bill Arden Computer Programmer

    Messages:
    584
    Location:
    MN, USA
    An attic based A/C unit would be expensive to run as the attic heat heats up the unit and heats the room air as it travels threw the ducts.

    I'm putting the final pieces together for a well-water based cooling system.

    The ground temp in WI should also be cold enough.

    -
    Option 1. Water "A" coil in used furnace.

    I used a car radiator and a fan last summer. I used a drain pan under it to collect the moisture that was condensed.

    I now have a "A" coil that fits my furnace.

    -
    Option 2. "celling beam cooling".
    The only problem is that you have to have a dehumidifier to remove the moisture so that the beams do not start to drip.

    I used draw cold air in at night to cool the house off. brrr
    Last edited: Mar 13, 2008
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