I bought a 20" Lasco box fan because the store said it would move 2500
cfm for 80W. If it sounds too good to be true, it probably is.
According to my measurements, it moves about 1000 cfm for 91W.
Bigger fans move more air more efficiently. I don't know why there
don't seem to be inexpensive box fans bigger than 20".
The 30" Westinghouse 78108 ceiling fan is supposed to move 3900 cfm for
45W. It weighs 11 pounds and costs less than $50.
If I built a stand, could it be used to blow air out a window? Would
the motor bearings handle it? Would I need to build a wire cage?
Is there a better alternative?
Simple, there is very little call for them. Huge volume is what
allows Lasko to built and sell them so cheap. Look around and see how
many windows can accommodate a 30" fan.
Can be done, but will take some doing to get it right. You need the
balance point to hold it or a large stand to take the stress of having
all the weight out front. You need a shroud to get the air to the
Spend the money to buy a proper sized fan made for your use.
Expect to pay $250 to $300 for a 3000 CFM fan suitable for
How about this for $56
(Amazon.com product link shortened)
My opening is 24 x 27. Box-fan blades aren't as long as their nominal
size, and the tips don't move much air. Mine says 20", but the diameter
of the current is about 16", with a 6" dead space at the hub. A 28" fan
should blow through the opening.
Westinghouse has a 24" model that moves 2000 cfm for 15W. That's even
Eureka! I didn't have a shroud because no air was escaping. To the
contrary, the current was picking up air, due to venturi effect, like a
jet pump. I measured with the fan on a cart. Three feet from the fan,
the current had expanded from 16" to 27".
The box fan had been hung in the window. I hung it 3 feet from the
window and found that the current pretty well filled the opening. I can
feel the increased draft in doorways and intake windows. I may have
increased the flow 70%.
(Amazon.com product link shortened)
They don't mention watts. It's strange that High produces only 9% more
current than Low. If it really sends 3263 cfm directly off the blades,
it may use 1000 watts, compared to 15 watts for the ceiling fan. I need
only enough velocity to get the air out the window. The rest is wasted.
I put my 20" box fan in the window, with the upper sash holding it in
place against the storm. Even with the window partially NOT filled by
the fan, it works fine. Gives me the option of reversing air flow even
with a non-reversible fan (turn it around). It's going upstairs that way
as I type.
Having no upstairs, I have for 15 years hung a box fan in the upper half
of my kitchen window. I use a couple of loops of rope tied around the
top of the fan frame and hung from a couple of screws. As long as the
ropes are in good shape and the screw heads aren't too small, it's secure.
I figure the top half of the window is the place to expel the hottest
air. It's also the best place to expel smoke from a cooking mistake.
I can measure a fan's thrust by hanging it from several feet of ropes
and measuring how far it swings back. Then I can calculate how many cfm
come from the blades.
I can measure how effective a fan is by closing all but one door to the
kitchen. Then I hang a two-gram balloon at several points in the doorway
to find the mean air velocity.
With the fan in the window, I was expelling 1000 cfm. With it 3 feet
from the window, I was expelling 1600 cfm! The blades could not have
been pushing nearly that much. The fan was acting like a jet pump in a
I've got a couple of 12" table fans from the local dollar store. I
tried one of them 5 feet from the window. 1000 cfm for only 33 watts!
Two of those fans 5 feet from two windows would give me twice the
ventilation of the box fan hung in the window, for less electricity.
The dollar store has 16" stand fans. I may try one. It would be easy
to move into position.
We have a fan store here that displays their fans in the front window
that way. The guy told me they have been running fine like that for
I will let others argue whether it is a good idea but the fan will be
On 9/2/12 1:16 PM, email@example.com wrote:
That answers my question.
If you doubled the speed of a fan to double the air flow and the blades
worked equally efficiently, you would be using eight times the power.
Moving twice as fast, the air would have four times the energy, and
you'd be blowing twice as much air. If instead, you used a fan with a
diameter 1.4 times greater (twice the area), you could blow twice the
air at the same speed as before, requiring only twice the power. That's
why ceiling fans seem to move large amounts of air so efficiently.
I've discovered that the advantage of ceiling fans is inflated. ANSI
calculates cfm by diameter and thrust, and a ceiling fan has a large
dead area at the hub. In a duct, a ceiling fan would not blow as much
air as ANSI says.
Besides, in the open area, such as blowing toward an open barn door, a
small, high-velocity fan will move more than calculated because venturi
effect will increase the volume of the stream. Moving fans back from
windows has improved the ventilation of my house.
Thanks to venturi effect, a 16" stand fan might work to blow a lot of
air out a window. I think I'll buy one!
I'm sure you could find larger and/or more efficient fans if you're willing
to search and pay for them. However, on a simple cost basis, it would
probably be cheaper to just buy additional box fans for more airflow.
Also, keep in mind the fan can only blow "out" the air it can pull "in"
from somewhere else. If you don't have an equal sized window open in the
room, or elsewhere in the house if you leave a door open, the airflow will
be restricted. It will be pulled through cracks, around outlets, from hot
attics, or wherever the air can be pulled from.
If you have two windows, you could try placing one fan to blow air out one
window, then place the other fan to pull air in from outside.
Finally, consider the effects of having too much airflow in the room. Dry
eyes, papers blowing around, etc.
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I found it in a neighbor's shed: a discarded 16" table fan with a
plastic blade. It has the traditional clover-leaf shape.
ANSI calculates cfm on the basis of thrust and diameter. ANSI points
out that the results are often inflated. What you really need to know
is the cross section of the air the blade pushes. That may be a much
smaller area than the diameter would indicate.
Putting my hand in front of the box fan, I found that there was little
draft outside a 14" diameter; apparently the plastic wasn't rigid enough
to design the blade to have the outer 3" do much work. The hub is 6".
The thrust was 1.7 Newtons. That computes to only 700 cfm. My older
5-blade box fan is about the same but uses more watts.
The table fan has a thrust of 3 Newtons and operates on a larger cross
section. It computes to 1200 cfm for the same 90 watts as the box fan.
I have a 24-pound steel box fan with a clover-leaf blade. It's a full
20" and has a thrust of 7 Newtons. That computes to 2000 cfm. I wish I
knew how to replace the bearings or the motor.
I had tested with several intake windows open, but an outdoor air
movement of even 1 mph could probably have a big effect on flow. I
retested with only one intake window open, on the same side of the house
as the exhaust fan.
This way, I measured 800 cfm with the box fan 4" from the screen (a
little venturi effect) and 1400 cfm 2 feet from the screen.
The 16" table fan, 2 feet from the window, expelled 1800 cfm! On low,
using only 55 watts, it expelled 1300 cfm.
The steel box fan, using 160 watts and set two feet from the window,
expelled 2300 cfm. It was easy to see why it didn't gain more from
venturi effect. The window opened 24" high, and that wasn't big enough.
I tried the 16" fan without the wire cage. It expelled 2300 cfm. It
would be foolish to use it without a guard, but I'll bet that fan could
do a lot better than 2300 cfm if instead of a cage, it had a shroud that
funneled air to the intake side of the blade.
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