# fanpower needed to cool house overnight

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
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I had a wholehousefan growing up. It was large, but it wasn't that loud.
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.
Best Regards, -- Todd H. http://www.toddh.net /
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Todd H. ( snipped-for-privacy@toddh.net) wrote:

I don't need a 1000 CFM whole-house fan - I don't need something to clear out the house air in a few minutes. It would require larger ducts and be more of a hassle to install.
Laura
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Lacustral wrote:

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.
Mark
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wrote:

clear
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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 attic.
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.
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M&S ( snipped-for-privacy@no.com) wrote:

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 airspace?
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 my attic.
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 hours.
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%?
Laura
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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 space.
--
snipped-for-privacy@phred.org is Joshua Putnam
<http://www.phred.org/~josh/
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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.
Jo Ann
Joshua Putnam wrote:

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Joshua Putnam ( snipped-for-privacy@phred.org) wrote:

Yes. The upper story, the one that gets hot, is 600 sq feet.
Laura
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Joshua Putnam ( snipped-for-privacy@phred.org) wrote:

Actually it's 640 sq feet (just measured it). x 8 feet high, = 5120 cubic feet.
Laura
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Lacustral 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 replaced it.
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.
cheers Bob
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wrote:

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.
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On 05 Jun 2006 17:08:16 -0500, snipped-for-privacy@toddh.net (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 hell.
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 guessing.
Since I'm here, I'll express my preference for roof fans.

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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 years.
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????? What's the matter with an old fashioned attic fan? Fast, clean and very efficient. Any hardware, lumber store can tell you how big. I recommend 36" for everything.

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Glenn wrote:

Yep. There are reasons people use them rather than what the OP seems to want.
--
Joseph Meehan

Dia duit
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Lacustral wrote:

Get a good LARGE whole house fan that offers more than one speed. The larger size fan will move more air with less noise.
--
Joseph Meehan

Dia duit
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Lacustral wrote:

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.
Bill Gill
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Bill Gill wrote:

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 actual data.
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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/stellent2/groups/public/documents/pub/phtv_se_he_bu_000599.hcsp
A Good Guide from Iowa State Univeristy: http://www.powerhousetv.com/stellent2/groups/public/documents/pub/phtv_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 here: http://www.houseneeds.com/shop/vent/tamarack/tamarackhvhousefansmain.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: