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.)
I had a wholehousefan growing up. It was large, but it wasn't that
The thing to know about fans is that you can have a large one with a
relatively low rpm, with a solid well constructed motor that moves the
same volume of air as a smaller one wizzing at deafing volume and rpm
with a cheaper motor.
My gut--without researching anything for you--is that a variable speed
whole house fan is probably what you actually want.
Most whole house fans dont use ducts. They simply vent air from the
living space to the attic where it is forced out through gable, ridge,
soffit, roof, vents.
Your question can simply be answered by how quickly you want to change
the air in your home. If you want to cool your home in 1 hour then a fan
that exchanges your homes volume 2-3 times an hour would likely be
adequate. If you are going to leave the fan running longer and cool the
home slower/continuously exchanging the homes volume once every 2-3
hours would be fine.
It will really depend on how you intend to run the fan and how quickly
you will want to cool the home. I agree with Todd that a variable speed
fan will likely be best for any application as it will allow you to run
it for long periods on low to maintain airflow but also allow you to
kick it up to high for a quick purge/cooldown should you need it. Of
course controlling it thermostatically would be the best.
I have used "attic fans" with limited results for years.
The temp difference, size and materials of the space are what is important.
In my humble attempts if there is less than 10 degrees difference the fan
does squat to very little difference. My home is masonry and has R-40+ in
The fan I use is 6000 cfm. Roars like a banshee. I pull the air from one end
of the house to the other. It takes at least 3 hours of > 10 degrees
differential to lower my home 1 degree F. Running all night sometimes I can
achieve a 10-15 degree drop. If the differential is greater than 10F then
the results are better. Still takes over night with the banshee running. My
home is ~1100 sqft with 8.5' ceilings.
Your probably wondering if I have tried a smaller fan, yup done that and
bought the T shirt. The banshee works.
Your results will vary
I guess that is reasonable, as long as it isn't damp air.
Is that other people's experience? That taking cubic feet of airspace to
been cooled, multiplying by the fan's CFM, times a fudge factor of 2-3,
would give you about how long it takes to exchange out the air in the
I just need to exchange the air on about 4800 cubic feet. Not
to have the fan strong enough to set up a breeze you can feel.
If I were exchanging the air 2-3 times/hour as you suggest, that would
be a 240 CFM fan. If I were only exchanging air every 2-3 hours that
would be a 40 CFM fan.
One person said, they need a 6000 cfm fan. But they have a masonry
house which must have a giant heat capacity. I have a wood frame house.
I have a total of about 1000 sq in of exhaust and intake ventilation in
Running a fan for just a short time wouldn't work, because on really hot
days in the summer, the low of 70 F isn't reached until the early morning
Ideally what would be great would be a variable speed control AND a
thermostat, so the fan would shut off if the inside temp gets down to
65 F ...
Fantech's variable speed controls go from 0-100%. Does anyone know if a
speed control will work fine at, say 10-20%?
Thanks for all the answers!
Just double-checking, do you have a very small home? 4800 cubic feet
would be a 600 square foot house with 8-foot ceilings. A 240 cfm fan
*would* be enough to create a breeze you could feel in that small a
email@example.com is Joshua Putnam
I don't know how many cubic feet I have, but my house is about 1600 sq.
ft. and I have 10-foot ceilings downstairs, 8 feet upstairs. When I
put my box fan in an upstairs window and open windows downstairs, I can
literally feel the breeze coming up the stairs. The method that works
best for me is to open windows at night and put the box fan in the
window blowing out, which exhausts the hot, stale air in the house and
brings in the cool night air. During the day, I keep the windows
closed and curtains pulled where the sun is coming in. I don't open up
again until it cools off in the evening. My house is well insulated
and has good windows but, at 100+ years old, is far from airtight.
This method works really well except during periods of hot, humid
weather when it stays hot at night. At that point I resort to a couple
small (5K) air conditioners and put my box fan on the floor to
circulate air. It's not high tech, but it works great.
Joshua Putnam wrote:
Stick a 3 speed 20" box fan in one of the upper room
windows.......blowing outward. Place a carboard shield on the sides as
needed to create a reasonable seal.
Open windows in the rooms you want to cool (open more windowns or open
them further in the "most impotant" rooms to cool)
The rooms with the open windows will cool FIRST & the room with the
fan will cool LAST.
Experiment with fan speed & window opening, adjust window openings &
fan placement to get the desired result.
fwiw I have a 2300 sq ft home in OC SoCal. It's a 1.5 story design
built in 1930 We typically get good nightime cooling (into the low
70's or less with low humidity) we can get temps in the mid / high 90's
& even low 100's.
I used to have a Nat Gas AC unit but it bit the dust & I haven't
A single 20" box fan funning on high speed will cool the house to
outside temp by morning & will provide a comfortable (& adjustable)
breeze in the bedrooms.
Don't forget that the walls and floor are warm not just the air. In other
words if you just change the air once and then stop the walls will warm the
air up again. You may need to change the air numerous times before the
temperature of the whole structure comes down.
On 05 Jun 2006 17:08:16 -0500, firstname.lastname@example.org (Todd H.) wrote:
We had one too, but it was before they invented speed controls. And
it was only 4 feet from my head (and then go up to the ceiling). So I
hated the noise.
My mother had also given me an alarm clock that ticked. Life was
But I revolted and demanded an electric alarm clock, so she got one
out of the closet, and we actually turned off the fan before bedtime.
Life was pretty good, come to think of it.
But now they have speed controls which makes all the difference.
I must say that the whole house fan must have moved 100 times as much
air as my little bathroom fan. Maybe 500 times, although Im just
Since I'm here, I'll express my preference for roof fans.
I once used an old oil heating furnace fan ... it's 110ac, just put a
heavy extension cord with length as required and plug on it and plug it
into any receptacle ... with nothing else turned on on that circuit :-)
Wait till an hour or so after dark or till it cools down outside, open
up the windows in the downstairs, turn on the fan, and in about no time
at all, all the air in the house is exchanged.
I currently have a newer smaller "surplus" gas furnace fan with
variable speed wiring waiting for installation ... just haven't had the
need for it. We get a nice cool evening breeze off the river most
I have the same message as a lot of other responders.
Get a whole house fan. I have a 40+ year old fan that I
use in the spring and fall. It works great if the
temperature over night gets down into the 60s. If it
doesn't get that cool then it doesnt' work very good. I
wait in the evening until the temperature outside drops
to about the temperature inside, then open some windows
and turn on the fan. When I get up in the morning I
turn off the fan and close the windows. By late
afternoon it is starting to warm up in the house.
When the temperature doesn't cool off enough I just
close the insulated fan cover and turn on the AC.
I live in Tulsa, Oklahoma. The temperature here in the
summer normally runs in the upper 90s to low 100s.
I too, have a "whole house fan," however
it it probably smaller than most.
I have had it for 35 years. It is a
unit that fits in the attic access hole. It
is completely self contained; louvers,
fan, pull chain switch. To access the
attic, you unplug the power cord by
accessing it between the louvers,
remove 4 safety screws and push in 4
tabs, while holding the fan. It's not
real easy, but not that bad. The fan is
probably about 22" and has 2 speed.
It does a pretty good job on a 2400 sq
ft tri-level. It required very little
installation, only mounting a strip on 2
sides of the hole and bringing in power.
BTW, I'm in the Chicago area where
summers can get pretty hot. We use
it, as others have said, when outside
temps are below 70..
We used to have, in addition, attic vent
fans, however, when we put in the
new roof, we put in continuous ridge
venting. I don't think the countinuous
ridge vent keeps the attic as cool as
the powered fan vents, but I have no
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
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
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.
A Good Guide from Iowa State Univeristy:
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 here:
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:
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