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  1. #1
    Join Date
    Oct 2009
    Location
    Mukwonago, WI
    Posts
    250

    How to size attic fans

    Just got a call from a customer, he was looking for some info on sizing attic fans. I really haven't sized an attic fan before as we stick to mostly commercial stuff.

    he has two areas hes looking at, they are both connected in the attic.

    Area #1. 30x63' room, 4pitch roof, the room has a pool in it, so moisture in the attic is a large problem.

    Area #2. 24x36' room, 8 pitch roof.

    he was thinking of two fans in area #1 and one fan in area #2. How should I go about sizing this? how many air changes per hour?

  2. #2
    Join Date
    Jul 2011
    Location
    East coast USA
    Posts
    1,002
    Quote Originally Posted by Miller257 View Post
    Just got a call from a customer, he was looking for some info on sizing attic fans. I really haven't sized an attic fan before as we stick to mostly commercial stuff.

    he has two areas hes looking at, they are both connected in the attic.

    Area #1. 30x63' room, 4pitch roof, the room has a pool in it, so moisture in the attic is a large problem.

    Area #2. 24x36' room, 8 pitch roof.

    he was thinking of two fans in area #1 and one fan in area #2. How should I go about sizing this? how many air changes per hour?
    exhaust is basically how many air changes per hour you want....I assume he has ridge vents of some kind. But i can tell you this will not help with the moisture, maybe a little but, because of the pool he needs to contact somebody who understands this kind of environment. I worked in YMCA's in the past and regardless of how much exhaust we had, we still had to deal with moisture. One thing i learned its all starts in how the room was constructed from the start. use caution and explain it may help but its not the solution

  3. #3
    Join Date
    Oct 2009
    Location
    Mukwonago, WI
    Posts
    250
    House is 45 year old, so i'm sure there is no vapor barriers in the ceiling. He has ridge vents and soffit vents. Currently he has an HRV installed to exhaust the room air, which he said works well as long as the pool cover is on. I recommended some kind of Desert Aire system, and he didn't want to go that route.

  4. #4
    Join Date
    Aug 2012
    Location
    Gainesville, FL
    Posts
    325
    One rule of thumb is 1 CFM per 1 square foot of attic. A house with a 2500 SF attic footprint would need a 2500 CFM of fan and enough ventilated soffit for makeup air.

    If you have a reflective roof or a low slope roof you can deduct 20% each

    Another one is 10 CPH, so you'd have to calculate the attic volume. On a gabled house it would be roughly half of the volume of the square footage of the attic, multiplied by the height of the roof above the top plate, then divided by 2. Vaulted ceilings and complex roofs make for fun calcs.


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  5. #5
    Join Date
    Dec 2002
    Location
    Arizona
    Posts
    4,423
    Quote Originally Posted by Miller257 View Post
    Just got a call from a customer, he was looking for some info on sizing attic fans. I really haven't sized an attic fan before as we stick to mostly commercial stuff.

    he has two areas hes looking at, they are both connected in the attic.

    Area #1. 30x63' room, 4pitch roof, the room has a pool in it, so moisture in the attic is a large problem.

    Area #2. 24x36' room, 8 pitch roof.

    he was thinking of two fans in area #1 and one fan in area #2. How should I go about sizing this? how many air changes per hour?
    If moisture in the attic is a problem now, wait until he fires up those PAV's.
    Last edited by jimj; 08-30-2012 at 09:36 PM.
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  6. #6
    Join Date
    Dec 2002
    Location
    Arizona
    Posts
    4,423
    You want the pool room to be in a negative pressure compared to the rest of the home. When the PAV's are running it will be at a higher pressure than the attic! Not a good idea.
    Last edited by jimj; 08-30-2012 at 09:37 PM.
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  7. #7
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,915
    If he insists on PAV's, there will most likely need to be a rather extensive amount of work done creating an air tight barrier between the conditioned space, the pool area, and the attic, or the most likely outcome is that the fans will make the existing problem worse, and cause additional issues.
    If the house has combustion appliances, the additional issues are potentially deadly.
    If more government is the answer, then it's a really stupid question.

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