DIY Home Improvement, Remodeling & Repair Forum > DIY Home Improvement > Roofing and Siding > Relative Position of Gable Vent and Attic Power Fan




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Old 04-07-2014, 09:43 AM  
q0987
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Default Relative Position of Gable Vent and Attic Power Fan

Hello all,

My attic has poor ventilation that is caused by poor building design. Some areas of the attic has no roof static vent and little air in-take.

One of the suggestions is to install attic power fan and gable vent.

Is there any scientific theoy that we have to pay attention when we install the attic power fan and gable vent?.

For example, how far they have to be installed?
etc?



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Old 04-07-2014, 09:48 AM  
Wuzzat?
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You could add more intakes or
place baffles in the attic to properly route the air that is being withdrawn by a fan.

Don't forget the warm air will rise due to the Stack Effect.



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Old 04-07-2014, 10:35 AM  
bud16415
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Great question and something I have been thinking about also. The house I bought last year has gable vents now but without a fan and on the inside there are windows that can be closed. I’m about where you are now wondering is it better to suck air in or blow it out with the fan. My vents are north and south and I wonder what end is better. I also wonder about where to mount the sensor and at what temp to have it come on.

I’m not trying to hijack your thread just that we have a very similar problem and one I haven’t researched as of yet.

Are you thinking of one of the power fans like I see at home depot etc?

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Old 04-07-2014, 02:07 PM  
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http://www.airvent.com/pdf/literature/PAVbooklet.pdf

I think that adding
pdf
to your search terms generally pulls up more credible hits.
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Old 04-07-2014, 11:20 PM  
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If you have prooved you have no roof leak, then the moisture is coming from the home and a fan will only pull conditioned air from the house, running the hvac bills up.

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Old 04-08-2014, 11:15 AM  
Wuzzat?
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I think it's more complex.

If
(the conditioned space has zero infiltration and
there is a big hole in the attic floor and
the attic is vented)
then
(most of the attic air will be pulled in from the vent and exhausted by the fan and the house air will be relatively undisturbed because there is no way the conditioned space air can be replaced).

I'm sure there is an analogy to an electrical circuit and I'm more comfortable in the electrical domain.

I guess you can temporarily put a $16 box fan in the attic with a cardboard baffle
http://www.google.com/search?q=fan+b...03%3B449%3B522
and use smoke trails or candle flames to see what the relative contributions are.

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Old 04-08-2014, 11:47 AM  
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Quote:
Originally Posted by Wuzzat? View Post
I think it's more complex.

If
(the conditioned space has zero infiltration and
there is a big hole in the attic floor and
the attic is vented)
then
(most of the attic air will be pulled in from the vent and exhausted by the fan and the house air will be relatively undisturbed because there is no way the conditioned space air can be replaced).

I'm sure there is an analogy to an electrical circuit and I'm more comfortable in the electrical domain.

I guess you can temporarily put a $16 box fan in the attic with a cardboard baffle
http://www.google.com/search?q=fan+b...03%3B449%3B522
and use smoke trails or candle flames to see what the relative contributions are.
Yes and no, Air will pull from the easiest soarce not circulate thought the attic but the shortist distance, so you are still waiting for air to warm and raise until it gets into the stream.
Warm moist air is pushing against the ceiling of the house looking for gaps and holes aound wiring , light fixtures, plumbing stacks attic access door. Yes we know this happens if there is moisture in the attic that isn't coming from a roof leak.
In the winter it might be safe to argue the air moving with the fan will be much cooler than the air leaking from the house. If the fan pulls warm moist air out, great but what if that air cools as it enters the cool stream before it leaves the house, It will leave the moisture behind, making the problem worse.
http://howtohomeinsulation.com/insul...air_leaks.html
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Old 04-08-2014, 12:00 PM  
bud16415
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Wuz
In my case I see it as a complex circuit with series and parallel paths. So in that case Neal is correct the conditioned side could be viewed as a bunch of very high resistance paths and bad leakage. The other path a very low resistance path being the large opening at the other end of the attic.

One of the questions I had was direction of current flow assuming DC. If I’m blowing in causing a positive pressure in the space the majority of the flow will be across the attic. And a small amount might make warm air enter the conditioned space. Of course a fan blowing out will work in reverse. In both cases the area below should be sealed as best you can unless you want the system to work as a whole house fan. Around here we have very few days we require AC maybe 10 in a summer if we can move outside air thru the house. If we don’t use fans then the number of days AC would be important might jump to 100.

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Old 04-08-2014, 03:10 PM  
Wuzzat?
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Here's the cooling degree days measured at the Erie Airport

Month starting CDD
4/1/13 15
5/1/13 98
6/1/13 126
7/1/13 249
8/1/13 180
9/1/13 87
10/1/13 28
11/1/13 0
12/1/13 0
1/1/14 0
2/1/14 0
3/1/14 0

There might be stations closer to you on this site.
http://www.degreedays.net/

I tried to download the data for my area but the site crashed. Computers are great when they actually work!


Nah, hold on, . . .it worked but the display made me think otherwise.

Month starting CDD
4/1/13 58
5/1/13 139
6/1/13 335
7/1/13 487
8/1/13 363
9/1/13 213
10/1/13 79
11/1/13 5
12/1/13 4
1/1/14 0
2/1/14 0
3/1/14 3
Our worse numbers may be because we are built near a swamp.

Our house uses 3/4 hp on yearly average because we use a box fan for the whole house and a room AC when it gets really bad so we can sleep at night. Norm is 1 hp.



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