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Thread: Attic Fan Size

  1. #1
    Join Date
    Jul 2008

    Default Attic Fan Size

    I own a raised rancher. With the garage, the house dimensions are 80 ft. by 30 ft. (The first level is 80x30 and the lower level is 60x30.) With our first attic fan, the upper level was always cool during the summer. Five years ago the attic fan needed to be replaced. I saw that the fan was made to cool a 1400 sq. ft. area and told the electrician at the time that it was too small. Of course he persuaded me otherwise. Needless to say, the upper level of the house has not been cool since.

    I am now ready to replace the fan. This time the electrician has suggested going with the largest fan, one that cools 3200 sq. ft. Will that fan be too large and too powerful and less energy efficient? I think we should find one that is closer to 2400 sq. ft.

    What size fan shoud we get?

    Thanks so much.
    Last edited by jsusan; 07-22-2008 at 11:32 AM. Reason: wrong dimensions

  2. #2
    Join Date
    Aug 2007

    Default Re: Attic Fan Size


    Exhaust fans and Whole House Fans are sized according to how many cubic feet of air they move per minute (CFM).

    Thus you would have to add the ceiling height to your dimensions.

    A standard sizing calc is to take the house length X width X ceiling height (assuming 8' ceilings) and multiply by .5 to get the cfm/min--the lower, quieter speeds are used to cool the house during overnite, mornings & early evenings; high speed to quickly exhaust hot air when first entering house.

    Thus for the upper level, 80 X 30 = 2400 sq.ft. X 8 = 19,200 cu.ft. X .5 = 9,600 cu.ft./minute as the whole house fan you would need.

    Google "sizing a whole house fan" for more sites & info.

    3-speed, or at least 2-speed units are highly recommended (1 speed =3k cfm, 2 speed = 6k cfm, 3 speed =9k cfm), as well as a belt-driven fan (quieter) rather than a direct drive fan; at least 3 blades, preferably 4 (more blades quieter); thermal overload switch on motor to automatically shut down fan if it overheats to prevent fire; 3-way control switch in living quarters.
    Last edited by JacktheShack; 07-22-2008 at 08:03 PM.

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