To gauge your cooling needs, multiply your home's square footage by
three to calculate the cubic feet of air moved per minute [CFM]. To
calculate the required vent space, divide your CFM by 750 - this gives
you the number of square feet of attic vent space needed. If your vents
are screened or louvered, double that amount. For example, a 3000 cfm
fan would require an open hole of 4 square feet to the outside. If the
hole is covered by an insect screen, the hole would need to be twice
that size

Use alone or with an air conditioner. Outside temperatures and
humidity levels can affect how well a whole-house fan works. A good
rule of thumb is to use the whole house fan when outside temperatures
are below 85 degrees. When the temperature climbs, or if the humidity
level is uncomfortable, it's more effective to use your air
conditioner.

Make sure you leave a window or door open to keep the air moving. And
do insulate the fan opening during the winter - the louvers don't seal
tightly, making it a prime place for heat loss.

http://www.powerhousetv.com/stellent...bu_000599.hcsp

A Good Guide from Iowa State Univeristy:
http://www.powerhousetv.com/stellent...htv_000449.pdf

Why not use a huge attic fan?

Noise. Any sound rating of a fan in "sones" must be performed in a
certified laboratory. At this time, there is no laboratory or
standardized test for sound level of whole house fans. Any sound
rating in "sones" in the specifications of a whole house fan is
only a manufacturer's guess.

Energy Costs vs. CFM. Running a big fan typicaly saves you no money
over running your A/C. So , look at how many Watts the fan draws on its
full setting. A Typical attic fan draws more than 500 Watts.

For example a Tamarack HV 1000 uses about the same amount of
electricity as two sixty watt light bulbs(116 Watts). If you have air
conditioning, you can reduce the costs of A/C by as much as 30%.
Motorized, gasketed and insulated doors close to form an air tight
seal, preventing heat loss in colder weather.

You can see them he
http://www.houseneeds.com/shop/vent/...sefansmain.asp

Should you get one? It depends on where you live and your home.
Here is a good guide on if a Whole House/Attic Fan makes sense for you
from our local Efficiency Org:

http://www.efficiencyvermont.com/pag...chael/Cooling/

Lacustral wrote:
I'd like to use an exhaust fan (fan from a ceiling grille to outside) to
run overnight in the summer, with the windows open, so that my house is cool
in the morning. I'm not sure how much CFM is needed. I don't want a big
powerful whole house fan because I'd like it to be quiet. Just a small
fan that keeps running overnight.

If you're using a fan for that purpose, can you tell me how much CFM gets
your house down to the temperature of the outside air, and how many cubic
feet of airspace you're ventilating, and how long does it take to cool
the house down to the outside temperature?

Just trying to get a ballpark idea.

(I could calculate the cubic feet of airspace in my house, divide by CFM
of a fan, and come up with a guess, but I'm sure it's not that simple -
the hot stuff in the house is heating up the air, fans aren't completely
efficient about clearing out the inside air, etc.)

Thanks
Laura