calculating amount of exhaust vents needed - DIY Chatroom

bitty071203 · 06-08-2009, 03:03 PM

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.

Yoyizit · 06-08-2009, 06:28 PM

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.

bitty071203 · 06-09-2009, 02:31 AM

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

Yoyizit · 06-09-2009, 01:07 PM

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.

beenthere · 06-09-2009, 09:13 PM

Roughly 1444 sq in free area.

2-16 X 32 louvered grilles(if gable vents) should give you the area you need.