# Basic physics question- moving air



## greg273

In relation to a gable mounted attic fan, is it best to have this fan working to push or pull air? Currently, I have it positioned near the eastern gable, facing out, in hopes that it will PULL hot air from the attic. It doesnt seem to be generating much suction, though. I am thinking perhaps it needs to be turned around, drawing outside air IN, pushing warm air out the opposite gable. The gable openings are screened, maybe too fine of a mesh is messing with the airflow?


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## suburbanite

The only reason I can think of that the direction of flow would make any difference is that cold air is denser than warm air and so you move more molecules per fan rotation pushing cold air than pushing hot air.


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## krondor2

it all depends on infiltration of the building envelope.


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## dcross

Hot air goes out at the top, cooler air comes in below.


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## Dahc

I have seen many people install two or more fans. One to push air in one side and one to pull it out the other. This is how my greenhouse is set up. If you have the appropriate fans for the mass of air your trying to move, it's very effective. If the fans you have access to aren't as powerful, you can put more in the trusses (in the attic) to create an inline air flow.

It doesn't seem to me like one fan that fits in a gable vent is going to be effective if it has to do all the work itself.


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## artificer

Does your fan look sort-of like this one?  

If it is, then they have very little or no suction. If you put them in free air, they move a lot of air, but they don't use much power.

How are you determining suction? Putting you hand in front of the fan and trying to feel the air move? If so, sometimes its misleading, since the fans tend to be large/slow in size. The speed of the air isn't that high, but the flow rate is still what's listed. The fan in the above link will move 1100cfm, but the air is only moving 10mph.

The idea behind these fans isn't to move massive amounts of air, but to continuously move smaller amounts of air for little energy costs.

If your opposite gable screen is too fine, and gets dirty, that will stop the air flow. The screen itself, if clean, will not provide much restriction. This assumes that the areas are equal. (source/screened area is as large or larger than the fan area) The larger the source air opening (opposite screened gable), the better.

Push or pull (blow out or in) really doesn't matter. The fans are designed to be installed blowing out, so you don't suck water into the fan. The incoming air restriction is the main factor.

Michael


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## EDDIE BUCK

Not meaning to change thread my question is related. How does the vents all around the eves of the house play a roll when using a gable end exhaust fan. It would seem that if the exhaust fan gets enough air from these then it would not pull from the other gable ends vent. :shrug:


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## Cash

EDDIE BUCK said:


> Not meaning to change thread my question is related. How does the vents all around the eves of the house play a roll when using a gable end exhaust fan. It would seem that if the exhaust fan gets enough air from these then it would not pull from the other gable ends vent. :shrug:


The eaves vents pull in cool outside air as warm air goes out the gable vents. If you have one or more gable fans running, you just pull more air in through the eaves to replace the air the fans are pushing out the gables. 

Greg, it's best to have the gable fan pushing air out the gable vent. If it's sucking air in, you risk pulling in water when it rains. Even if it doesn't eventually short out the fan, you want the fan blowing moisture-laden air OUT of the attic, not pulling it in. 

Being in southern Illinois, where it gets pretty hot in the summer, have you thought about installing a whole house fan? It's a major-sized fan set in the ceiling of the top floor of the house, usually in a hallway, and is used at night after the outside temperatures drop. You open the windows and doors on the first floor, turn on the fan, and it exhausts the heat that has built up in the house during the day and sucks in cooler night air. The fan exhausts into the attic, where the air goes out the gable vents. A friend of mine in Oklahoma City installed one, and he's amazed at the difference it has made in his a/c requirements.


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## greg273

Yeah, I have definitely considered the 'whole house fan' setup. My grandparents had one in thier 70's ranch-style house, and let me tell ya, that thing would MOVE some air in a hurry. And yes, southern Illinois is definitely hot and humid in the summer. Cooling is a way bigger expense than heating, and especially problematic in an off-grid setup. (unless, of course, you invest big bucks up front and go for the earth-bermed house, much better in the long run....) 

Let me pose this question, if I were to replace the Alternating Current gable fan with a Solar Powered Roof ventilator (roof fan with small 10watt solar panel), AND a whole house fan, would I be inducing current to flow in the DC motor when the house fan was running? and would that be detrimental?

I'm thinking it would be ok, but wasnt quite sure about that one.


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## tinknal

Which way is the fan blowing in relation to prevailing winds? If the fan is pushing against the wind it has to work that much harder.


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## ace admirer

i could be wrong, but i think the deal is to pull cooler/relatively dry air in at the soffets vents, sweeping the solar load heat from the inner roof sheithing. and also remove any saturated humidity that is generated in the living spaces but migrating to the attic space/insulation. the ideal way to remove would be to pull to the crest.....most modern housed are built with vents along the crest. but with out crest vent,,,the next best is gable pulling. i don't favor the through the roof turbines or power vents as they tend to leak,,,sooner or later. you're looking for a "sweeping" movement, not necessarily a blast of air. the whole house fan is a totally different situtation.....thats a living space confort thing,,,not a solar roof load/moisture thing. 



i vote to increase inlet screen size to limit birds, insects, and other big stuff, set fan to pull.


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## OntarioMan

Not that I have the answers, or the solutions - just an observation : the other day I was up in my hip-roof attic which is well vented, and it was far hotter than I ever expected.

What I'd like to do is use a "one-wire" setup plugged into my PC so I can monitor various temps. in and around my house - to give me an idea of the effectiveness of any changes I make.


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## Cash

greg273 said:


> Let me pose this question, if I were to replace the Alternating Current gable fan with a Solar Powered Roof ventilator (roof fan with small 10watt solar panel), AND a whole house fan, would I be inducing current to flow in the DC motor when the house fan was running? and would that be detrimental?
> 
> I'm thinking it would be ok, but wasnt quite sure about that one.


The solar-powered roof ventilator would be an independently powered unit, not wired into your home electrical system, right? And the whole house fan would be wired into the house AC network, right? If so, I'm not sure why the whole house fan would influence the ventilator, unless there is some weird electrical field thing I'm not aware of. Any professional electricians here?


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## greg273

Cash said:


> The solar-powered roof ventilator would be an independently powered unit, not wired into your home electrical system, right? And the whole house fan would be wired into the house AC network, right? If so, I'm not sure why the whole house fan would influence the ventilator, unless there is some weird electrical field thing I'm not aware of. Any professional electricians here?


 Yes, I would set it up as you described, the solar fan would have its own little 10watt panel. I was just thinking, the whole-house fan, when running, might cause the DC solar motor to spin faster than it was intended, possibly turning that motor into a small generator... Its probably not a big deal, I was just thinking of this as a possibility...


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## greg273

Ok, I am ditching the 120volt AC gable fan, and have just purchased the 12volt roof mounted attic fan with 10 watt solar panel (home depot, $150), next I will get a whole-house fan ($160). The beauty of the solar fan, it is much quieter than the gable fan was, and will run all day with no current draw from the battery bank and inverter. I think I found a good solution. And with the whole house fan, I can ventilate the whole place pretty good in the evening with just a flip of a switch. Should've gone this route in the first place, but hey, live and learn. Maybe one day I will write a book about the mistakes I've made, and possibly save someone else from making them...
Anyway, it is so cool to watch the solar fan working, just spinning away quietly as long as the sun shines, knowing the power is basically free. (minus the initial cost, of course...)


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## dcross

greg273 said:


> And with the whole house fan, I can ventilate the whole place pretty good in the evening with just a flip of a switch.


Where are you going to mount it? Blowing directly outside or into the attic?


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## quietstar

Basic Physics question.. Assuming a 7mph SE wind blowing across a standard roof on which is perched a cupola with 4X4 movable louvered vents on all sides. Now assume that the South and East sides are closed and the North and West sides are wide open. How much hot attic air will be removed by this natural, energy free method when compared to the fans mentioned above. The suction on the leeward side is amazing even with a slight wind. Before cheap electric power, rural folks in the hot, humid South used this method for their homes, dairy barns, etc. Not to mention it's a classic place for that weather vane you will need to monitor wind direction. 
I suspect it may be too low-tech for some here...Glen


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## fantasymaker

Wow Glenn how do you get a 7mph wind in the summer?

I suspect what Gregs problem was is very simple with the gable fan running it wasnt powerfull enough to require all the venting to be used so just the vents close to it were drawing air and the farther away areas were building up stangnant air that was heating up.


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## greg273

Well, actually the main problem was that the gable fan was drawing 5amps AC, which is a killer on my limited electricity budget. The house isnt even drywalled or insulated fully yet, so its hard to tell if it was doing its job or not, but 5amps was just too much electricity to spare, especially considering in the summer months, it would probably run 75% of the time. In my case, I think the solar option is the best way to go.

And Quietstar, I like the cupola idea, but my house is already framed, tall enough as it is, and it sits low enough in the valley that the winds arent reliable. Nothing against low tech, in fact low tech and simple is my preferred method of doing just about everything, EXCEPT when it comes to the Photovoltaic/Electrical system of my house... but then again, the elegance of electricity from sunlight is beautiful in its own technological way.


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## fantasymaker

In that case Id just go with a good ridge vent and close the gable ends . Is the floor of the attic (ceiling of the house)well insulated?


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## Jim S.

Attic or gable-end fans should pull air in from the eave vents, so they are exhaust fans. The newer styles are so neat, too, as they incorporate a solar panel so you don't even have to wire them. Self-contained.

My house is very adequately passive vented now, and it has been 100-106 for the past 8 days. That has made my interior ceiling HOT, even with a lot of attic insulation, so my a/c runs all day long. I will install a powered fan setup this winter, while it is cooler to do so. It'll save me a/c money.

Greg, we have a whole house fan. It is MARVELOUS in spring and fall, not too great in humid summer here. We love ours, which was installed when we bought, but if I were to get another, I would look for a belt drive unit or one with real good motor isolation. Ours fan blade is direct-mount, and it does roar.

Saves a lot on a/c bills, though, and we'd much rather have the windows open, as long as the neighbors aren't burning trash! :flame: But that's another story...sigh...


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## PyroDon

first be sure you have eave vents and that they are open . the gable vents should thermosyphon even when the fans are off . though you would do better to install a ridge vent the full length . the gable fans should force air out .
If you dont have clear eave vents you'll find the suction when you open them up or cut some in .


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## fordy

greg273 said:


> Ok, I am ditching the 120volt AC gable fan, and have just purchased the 12volt roof mounted attic fan with 10 watt solar panel (home depot, $150), next I will get a whole-house fan ($160). The beauty of the solar fan, it is much quieter than the gable fan was, and will run all day with no current draw from the battery bank and inverter. I think I found a good solution. And with the whole house fan, I can ventilate the whole place pretty good in the evening with just a flip of a switch. Should've gone this route in the first place, but hey, live and learn. Maybe one day I will write a book about the mistakes I've made, and possibly save someone else from making them...
> Anyway, it is so cool to watch the solar fan working, just spinning away quietly as long as the sun shines, knowing the power is basically free. (minus the initial cost, of course...)


.....................There , is NO WAY a 10 watt solar panel is going to produce enough power for a 12VDC fan to even come close too the 120 volt AC fan . i.e. , 5 amps(assumed) x 120 volts = 600 watts power consumption . 1hp = 746 watts , now : volts x amps = watts , so 10 watts divided by 12 volts = .83 amp !! And that is ignoring resistive losses . Conclusion is that the 12VDC fan is going to produce A whole Lot less Air volume . , fordy


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## greg273

fordy said:


> .....................There , is NO WAY a 10 watt solar panel is going to produce enough power for a 12VDC fan to even come close too the 120 volt AC fan . i.e. , 5 amps(assumed) x 120 volts = 600 watts power consumption . 1hp = 746 watts , now : volts x amps = watts , so 10 watts divided by 12 volts = .83 amp !! And that is ignoring resistive losses . Conclusion is that the 12VDC fan is going to produce A whole Lot less Air volume . , fordy


 Well, it is rated for 1000CFM. And that .83 amp is free, and running the whole time the sun is shining.
The 120AC whole-house fan uses the same motor as the AC gable fan! 5AMPS... I'l have to check on that, but I believe that was the amperage... Now, someone on grid power, with money (and electricity) to burn can afford 5AMPS continuous draw, but not this homesteader. 
The size of the rotor would have an effect on the CFM rating, the solar attic fan has relatively large blades compared to the AC model.


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## greg273

Jim S. said:


> Attic or gable-end fans should pull air in from the eave vents, so they are exhaust fans. The newer styles are so neat, too, as they incorporate a solar panel so you don't even have to wire them. Self-contained.
> 
> My house is very adequately passive vented now, and it has been 100-106 for the past 8 days. That has made my interior ceiling HOT, even with a lot of attic insulation, so my a/c runs all day long. I will install a powered fan setup this winter, while it is cooler to do so. It'll save me a/c money.
> 
> Greg, we have a whole house fan. It is MARVELOUS in spring and fall, not too great in humid summer here. We love ours, which was installed when we bought, but if I were to get another, I would look for a belt drive unit or one with real good motor isolation. Ours fan blade is direct-mount, and it does roar.
> 
> Saves a lot on a/c bills, though, and we'd much rather have the windows open, as long as the neighbors aren't burning trash! :flame: But that's another story...sigh...


 Thanks, Jim. Yes the model I bought was the smaller of the two, the direct drive version. Louder, but I dont anticipate using it for more than 30 minutes or so at a time. My grandparents had one, and man it would pull your socks off if you let it!
And I sympathize with your trash-burning neighbor situation, I've had to deal with that also. I finally offered to pitch in on a dumpster, that helped.


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## fordy

greg273 said:


> Well, it is rated for 1000CFM. And that .83 amp is free, and running the whole time the sun is shining.
> The 120AC whole-house fan uses the same motor as the AC gable fan! 5AMPS... I'l have to check on that, but I believe that was the amperage... Now, someone on grid power, with money (and electricity) to burn can afford 5AMPS continuous draw, but not this homesteader.
> The size of the rotor would have an effect on the CFM rating, the solar attic fan has relatively large blades compared to the AC model.


..............Your definitely saving money , but if your 120vdc fan doesn't move an equivalent volume of air your interior airconditioner will probably have to work harder due too the residual heat left in your attic due too a lower volume of air being removed . A supposition on my part . , fordy


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## DrippingSprings

The one we had as I was growing up was a real fan. Most nowadays arent near powerful enough. We had it in one end of the house and you could open a window on the far end of the house and it would make the heavy drape stand straight out. This with only a small 2x2 ft vent in a hallway leading up to the attic. 

We once had one that used an old wooden prop off a plane. If you did not open a window is would actually pull the doors shut on the other end of the house


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## Al. Countryboy

We have a two story house and have a whole house fan in the hall at the top of our stairs. We do not run it alot during the day because temps. here have been in the 100's lately, but in the afternoon and up until we go to bed we run ours. Since we have had unually hot weather here in the south we have run ours almost all night for the last few nights. Like some of the others said, it will make the curtains stand out and make the doors slam to. We also use our windows by opening some more than others to cause more of an air flow in some rooms more than others. We sometimes close off one of the up stair bed rooms that is never used if we want a stronger breeze going through our down stairs. The only thing that I wish that we had done different is to have the fan switch down stairs instead of at the top of the stairs. Some times in the early hour mornings, I have to climp the stairs and turn off the fan. Night before last I decided to just flip the breaker which is down stairs. These old bones just do not like to climb stairs like they use to. :nono:


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## quietstar

Anyone who really inquires will understand that contrasting temps represent natural energy eager to move large volumes of air if allowed. Compare inlet air temp to roof ridge temp and do the math. If you have a large stock holding in your local electrical Co. you might want to buy a big fan rather than allow nature to do the job. Low tech can be fun and save your money for other projects...Glen


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## gallowglass

When I build it will incorporate something like this: http://www.motherearthnews.com/gallery.aspx?id=67220&seq=8


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