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Old 09-29-2008, 09:17 PM   #1
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Whole House fan Again!


How much roof venting do I need if I am pulling 6,000 cfm from a 14" x 20" room vent into the attic. I can create more soffit vents but I believe the 8 x 4" pot vents about 4' down from the ridge are insufficient. I cannot install ridge vents so what is my alternative? I thought of a cupola but I cannot get to the ridge from the attic. How much lower down can I get from the ridge and still be effective with vents?

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Old 09-30-2008, 08:28 AM   #2
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Whole House fan Again!


What is the fan rated at? In CFM?
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Old 09-30-2008, 09:37 AM   #3
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Whole House fan Again!


2 x 3,000 cfm
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Old 09-30-2008, 11:34 AM   #4
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Whole House fan Again!


Pulling 6000 cfm through a 14x20 hole with obstructions is going to be a real challenge. More than likely it will make a lot of noise and more than half of your cfm will come thorough holes in the structure. I you have bird blocks with mesh and any blow in insulation it will soon be all over the neighborhood.

To get 6000 cfm out of your attic you would need to have an unobstucted opening of at least 20x20 (400 sq in). If you have a screen or any kind of vents then add 30% to 50% on that opening (520 to 600 sq in). Screens alone cut air flow about 30%. Trying to push too much air through a small hole will cause lots of noise and make the fan work that much harder causeing wear and heat.

Fans cause tunnels of pressure and is especially true of exhaust fans. If your goal is to get the heat out of the house make sure you have open windows in every room. It is better to open several window a little than to open one window all the way. The air will flow from the opening to the exit by the shortest means possible and won't make much difference in the surrounding air.

If you have ceiling fans then by all means have them on as this will help mix the air and disrupt the tunnels.

But at first glance you will need a much bigger hole going into the attic. I can almost see your roof caving in as your fan causes so much vacuum in the attic (I watched to many cartoons as a kid).
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Old 09-30-2008, 12:27 PM   #5
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Whole House fan Again!


To remove 6000cfm@1000fpm you will need a 30"x30" hole. your 8"X4" vent is good for about 220cfm@1000fpm.
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