Help with attic fan efficiency

13 years ago

I'm trying to determine if my attic has enough vent openings to efficiently run an already installed attic fan. The fan I have is the following.

http://www.grainger.com/Grainger/DAYTON-Whole-House-Fan-3C325

Looking at the specs, it appears on high speed the fan is capable of exhausting 8480 cfm?

Now, upon my research, I've found it's recommended to have 1 square foot of attic vent opening per 1,000 cfm fan capacity. And if your vents have screens on them, you need more like 2 square feet of vent opening per 1,000 cfm fan capacity.

So, in my attic, I have two louvered and screened square vents on each end measuring 14.5"x10.75". There's two screened circular vents in the roof with 9" diameters. And that's it. No gable vent or soffit vents.

According to my calculations, I should have close to 17 square feet of screened vents for an 8480 cfm fan, however, in reality, I only have almost 3 square feet.

Am I on the right track here with figuring this out? Would the builder not put in enough attic vents? The house was built in '76 and I'm pretty sure the fan is original with the house.

I started checking into this when I heard a radio handyman talk host mention it should only take 10-15 minutes of running your attic fan to cool the house. For my house, I can run the fan for a couple of hours and still not get the house cooled.

Can anyone help me figure this out?

Thanks.

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Comments (9)

User
13 years ago
"I started checking into this when I heard a radio handyman talk host mention it should only take 10-15 minutes of running your attic fan to cool the house. For my house, I can run the fan for a couple of hours and still not get the house cooled."
Are you sure the radio guy wasn't talking about a whole house exhaust fan?
turtlegreen
Original Author
13 years ago
Well, I guess I'm getting my terminology mixed up. A whole house exhaust fan is what I'm talking about. I erroneously said attic fan.

My whole house fan, referenced above, is located in the hallway ceiling on the second floor. There's louvers in the ceiling which open when the fan is turned on.

Sorry about that.
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User
13 years ago
Attic fans can only "cool" a house to the ambient temperature outside. And they can only do that when there is enough openings from the house as well as exhaust openings in the attic. You don't appear to have enough openings for either your inlet or exhaust.
User
13 years ago
jaegermeister,

I should have realized what ytype of fan you were talking about from it rated capacity.

My bad.

Sorry.
heatseeker
13 years ago
how do you get 17 sq ft of exhaust from 8000 cfm? If it is 1 sq foot per 1000 cfm would that not be 8 square feet of exhaust"? Attic fans will move air to make it seem cooler but you have to open doors and windows first then it will only cool to ambient.
turtlegreen
Original Author
13 years ago
The recommendation is 2 sq ft of exhaust per 1,000 cfm if the exhausts are screened, which mine are.

It takes hours of running the fan to get the interior temp from 80 down into the low 70s when the outdoor temp is in the low 60s.

Anyways, I've been looking around the neighborhood and there's some houses identical to mine that have at least doubled the size of their attic vents (they're huge!). So they may have had the same thoughts that I'm having.
brickeyee
13 years ago
"It takes hours of running the fan to get the interior temp from 80 down into the low 70s when the outdoor temp is in the low 60s. "

You need to open all the first floor windows when the fan is running.

Relying on infiltration is not going to be very successful.
turtlegreen
Original Author
13 years ago
Yep, that's with the exterior doors and windows open.
heatseeker
13 years ago
Is the fan running backwards?