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Thread: Solar Attic Fan (Replace Wired One)

  1. #1
    DIY Member gmrules's Avatar
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    Default Solar Attic Fan (Replace Wired One)

    I currently have wired power vent in my Attic, it Quit working. Not sure
    what size it is, but I wanted to replace it with Solar version. I am not sure how
    to calculate what size fan to replace it with or if one fan is even enough,

    The 20 watt solar fan I found so far moves 1300 CFM, How do I calculate
    what size fan I need,

    My attic does have ridge vents as well and get feed from the sofits

    Is it hard to calculate the volume of the attic? What do you use to determine
    the volume of air you need to exchange to keep attic cooler.

    I read on the web if you have ridge you really do not need power fan

    I can tell hugh difference in the temp of the attic w.o the power fan. I so not even know the size fan the builder used. I am sure its the lowest CFM

    Thanks George

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Divide the attic into two right triangles (i.e.,measure from the peak to the rafters, then from the point right under the peak at the rafters to the outside edge of the attic), find the area of each, and then multiply by the length. So, if it was 8' from the rafters to the peak and 10' to the outside edge, it would be 8x10x.5 or 40 sq ft. Then say it was 20' long, take 40x20 or 800 cubic feet. Do this for each side. You want probably at least a few air turnovers an hour. See what the manufacturers recommend for air changes. My guess is 5-8, but that's a guess.

    If you have eave vents, you'll get some natural convection...aiding it can help with a fan.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Computer Programmer Bill Arden's Avatar
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    I've looked at solar fans and they just don't make sense to me.

    20W = .026 hp

    Also watch out since there is a big difference between CFM at "zero pressure" and the CFM you will actually get.

    What temperature is your attic running at?

    Mine was running around 120F last week with my fan running.
    Note: I only have three small roof vents along with eve vents on one side.
    Important note Ė I donít know man made laws, just laws of physics
    Disclaimer: I'm a big fan of Darwin awards.

  4. #4
    Plumber jimbo's Avatar
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    I think you might have some decimals crossed. 1300 CFM is a LOT of air, and not consistent with a 20 watt load on a fan. A typical bath fan, which might draw something less than 100 watts, moves 50 to 100 CFM.

  5. #5

    Default Solar Attic Fan

    Would this work better with a 25 or 30 watt solar attic fan?

  6. #6
    Computer Programmer Bill Arden's Avatar
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    Ask this question... Is direct solar voltaic a low priced way to generate electricity?

    The answer is no... This means the only reason to use a more expensive solar voltaic product is to avoid the cost of running a wire.

    It's also better to use shingles that reflect IR heat than to try and keep the attic cool using a fan.

    Edit:
    http://roofknowledge.org/main/energy...roofingoption1
    Last edited by Bill Arden; 10-14-2009 at 03:40 PM.
    Important note Ė I donít know man made laws, just laws of physics
    Disclaimer: I'm a big fan of Darwin awards.

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