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Old 07-07-2009, 12:39 AM   #1
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Attic ventilation concerns/questions


Hopefully, I've got the right thread for this...

I just installed an attic fan (gable variety) and I'm trying to make heads or tails of whether or not there's any potential pitfalls that could arise from this. I was first concerned because I've seen all kinds of stuff about needing to have proper intake capacity to meet the air flow of the fan. That I know I don't have so it won't operate as efficiently as it might be able to until I get some additional intake area created.

But all the other stuff about moisture and ice dams (that's totally foreign to me) would seem to be like a lot of other DIY info I read about: DNA, as in does not apply. I'm in SoCal a couple miles from the beach and the weather here just makes a lot of other DIY recommendations moot: basement vapor barriers, digging below the frost line, blah blah blah.

Sorry to rub it in.

Anyway, moisture and ice dams, whatever they are, are totally out the window as I see it. Like I said, I will cut out the extra vents some day. I've just got other things I want to accomplish first.

So, other than not being able to cool my attic as much as I might otherwise be able to, does anyone 'in the know' have any good reason why I truly need the additional intake capacity?

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Old 07-07-2009, 07:47 AM   #2
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Attic ventilation concerns/questions


Without the proper intake you should be able to cool your attic better as the fan will pull the air conditioned air (that you paid for) out of your house and into the attic. Seems an expensive way to cool a attic.

Also while putting the house into a negitive pressure you could pull flue gasses back into your house. Very bad situtation.

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Old 07-07-2009, 09:14 AM   #3
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Attic ventilation concerns/questions


If your attic access isn't sealed. Then the additional air the fan needs will be pulled from your house.
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Old 07-07-2009, 12:51 PM   #4
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Attic ventilation concerns/questions


The attic's not sealed tight like with weatherstripping but both hatches are pretty snug. In the front of the house, I have gable vents on either end. One gable vent has the fan, the other would act as an intake. The back of the house also has two roof vents installed about halfway up.

As I see it, my rough math says I've only got about 1/3 the recommended intake. Not great, for sure but at least with that I don't see how the attic will pull air much from the house. There's two access points to the attic and although they're not sealed they're pretty snug and the one open gable vent and the two roof vents should allow enough air flow so that the fan is pulling very little air from inside the house. I'm I way off on that one?

I do see the concern about flue gases but there's no flues anywhere near the vents that would act as intake.
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Old 07-07-2009, 03:18 PM   #5
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Attic ventilation concerns/questions


Your vents are designed for passive movement. That fan will move a much greater amount of air than they can supply. You also have numerous openings into the attic.

Every place where a wire, plumbing stack or flue pipe will have openings around them. Where the top plates (2x4's) have shrunk over the years there will be openings down the walls. The only way I would ever consider using a attic fan is if the entire attic floor was foam sealed. This involves removing the insulation, sealing and then reinstalling the insulation.

The flues do not have to be anywhere close to the attic to cause problems. If you put your house into a negitive pressure the air must be replaced from somewhere and the flues are the easiest openings into your house.

I would recommend that you have a blowerdoor test done on your house which would show you what I am talking about. This would show you where your leaking.
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Old 07-07-2009, 03:41 PM   #6
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Attic ventilation concerns/questions


Light an incence stick, and see if the smoke is drawn through the access or just floats there.

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