Calculating Amount Of Exhaust Vents Needed - HVAC - DIY Chatroom Home Improvement Forum

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06-08-2009, 02:03 PM   #1
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## calculating amount of exhaust vents needed

I have installed a 5700 cfm wholehouse fan and would like someone to tell me how much exhaust space is needed for it to operate most efficiently. I need to know how to test for enough exhaust and how to calculate the amount needed for this size of fan. Thanks.

06-08-2009, 05:28 PM   #2
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Quote:
 Originally Posted by bitty071203 I have installed a 5700 cfm wholehouse fan and would like someone to tell me how much exhaust space is needed for it to operate most efficiently. I need to know how to test for enough exhaust and how to calculate the amount needed for this size of fan. Thanks.
An air change in ~8 minutes?
One rule of thumb is that the intake area = 4x the fan blade swept area.

Last edited by Yoyizit; 06-08-2009 at 05:32 PM.

 06-09-2009, 01:31 AM #3 Newbie   Join Date: Jun 2009 Posts: 2 Rewards Points: 10 i need to know how to calculate EXHAUST space needed. To determine if I need to add more venting for my whole house fan. Thanks

 06-09-2009, 12:07 PM #4 Member   Join Date: Jul 2008 Location: NW of D.C. Posts: 5,990 Rewards Points: 2,000 It's probably the same 4x rule; incoming air = outgoing air and since the outgoing air is under pressure it would need even less than 4x. For more precision you need the fan curve, http://www.cartage.org.lb/en/themes/...e/FanCurve.htm showing pressure vs. CFM, then you need to be operating the fan in an area of that curve recommended by the manuf. Some fan curves are "S" shaped, which makes for unstable regions in the curve. Last edited by Yoyizit; 06-09-2009 at 12:11 PM.
 06-09-2009, 08:13 PM #5 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 33,756 Rewards Points: 6,476 Roughly 1444 sq in free area. 2-16 X 32 louvered grilles(if gable vents) should give you the area you need.

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