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 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.
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.
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 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.
????? 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.
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.
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.
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.
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.
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A Good Guide from Iowa State Univeristy:
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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:
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I'd like to use an exhaust fan (fan from a ceiling grille to outside) to
A bedroom AC won't do anything to cool down the entire house overnight, which is what he's trying to do, by moving cooler night air into the house. I agree with the advice to get a whole house fan. That's exactly what they are designed to do, pull air through the whole house. Of course, a lot depends on the climate.
That depends on where you live and the house thermal mass and insulation and how cool you want it. Whole house fans can work almost as well as AC for all but a few days per year near Philadelphia. NREL says June has average daily min and max temps of 61.8 and 81.7 F. July is the warmest month, with 67.2 and 86.1 and humidity ratio w = 0.0133 pounds of water per pound of dry air. An indoor fan or a window AC for dehumidification can help ensure comfort in July.
But you may need a large fan. Lasko's 2155A 16" window box fan has a thermostat and sliding mounts to fit windows and it's efficient, moving 2470 cfm with 90 watts on the highest of 3 speeds, and it's fairly quiet. About $50 at ACE Hardware stores.
A house with an open window or two upstairs is a natural cool air trap. At night, warm house air will rise up out of the window and cooler night air will slide in and replace it. During the day, air stops moving. More air and heat can flow with a fan that runs at night, eg an upstairs window with a 1-way damper outside (eg plastic film, hinged at the top edge) and an downstairs window with a damper inside and a differential thermostat or an outdoor 70 F thermostat that runs the fan when outdoor air is cooler.
It can never reach the daily min, but a larger fan can keep it closer. A Q cfm fan is like a 1/Q F-h/Btu thermal resistor or a Q Btu/h-F conductor between outdoor air and the house mass. This may be the bottleneck, if the exposed mass surface is large, with a 1.5 Btu/h-F-ft^2 airfilm conductance.
If a house has infinite thermal mass (most don't :-) and G Btu/h-F of conductance and a Q cfm fan and a constant indoor temp Ti and 8-hour night and 16-hour day temps Tn and Td and the fan moves 8Q(Ti-Tn) Btu at night and the house gains 16(Td-Ti)G Btu/h during the day and these heatflows are equal, Ti = (QTn+2GTd)/(Q+2G), approximately.
For example, G = 800 and Q = 100 and Ti = 80 and Tn = 70 makes Ti = (100x70+2x800x80)/(100+1600) = 79.4. G = 400 and Q = 2470 makes Ti = (2470x70+2x400x80)/(2470+800) = 72.4.
We might model a house with C Btu/h of thermal mass and G Btu/h-F of conductance and a Q cfm fan and Tmin and Tmax indoor temps like this, viewed in a fixed font like Courier:
1/Q (closed at night) ---www--- \\--- | | -------------- | 1/G | Tmax . . . Tn/Td ---------www--------*--- Tmin/Tmax . . . | Tmin . . | ------- ------- --- C night day --- | | -
RCn = C/(Q+G) and RCd = C/G and Tmin=Tn+(Tmax-Tn)e^(-8/RCn) and Tmax = Td+(Tmin-Td)e^(-16/RCd), and Tmin = [Tn+(Td-Tn)e^(-8/RCn)
-Tde^(-16/RCd-8/RCn)]/(1-e^(-16/RCd-8/RCn), if I did that right.
For example, C = 6000 and G = 400 and Q = 2470 and Ti = 80 and Tn = 70 makes RCn = 2.1 hours and RCd = 15 hours, so...
20 C=6000'house thermal mass (Btu/F)
30 G=400'house conductance (Btu/h-F)
40 Q=2470'fan cfm
50 RCN=C/(G+Q)'night time constant (hours)
60 RCD=C/G'day time constant (hours)
70 TN=70'night temp (F)
80 TD=80'day temp (F)
90 TMIN=(TN+(TD-TN)*EXP(-8/RCN)-TD*EXP(-16/RCD-8/RCN))/(1-EXP(-16/RCD-8/RCN))
100 TMAX=TD+(TMIN-TD)*EXP(-16/RCD)
110 PRINT TMIN,TMAX'min and max indoor temps (F)
Just for the record -- I have ~1600 sq. ft. in two stories. At night, I put a box fan in one window, set on medium, blowing out. It matches the window size almost exactly, so I don't bother to block the sides. It is in a spare upstairs bedroom. By the time I walk back into my bedroom, also upstairs, I can already feel the difference in temp. I don't know how long it takes, since I just leave it on all night, but when I get up in the morning, the entire house is cool.
Sometimes simple solutions will get you where you want to go.
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.
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%?
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