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Attic Gable Fan(s)
Posted by: mikebw
Date: July 23, 2020 09:25AM
I know this topic could lead down all sorts of rabbit holes with cans of worms to be opened, but I'm going to try and keep it short.

I have an attic with a gable vent on each end. There is also a fan on each end. The roof has a ridge vent, but no other soffit vents or other airflow features.

This week I discovered that only one fan was working (they are both on separate thermostats and should come on when it gets hot up there). This morning I determined that the one fan which was not working was not getting power. There was a wire inside an electric box that had come out of the wire nut, so after shutting off the power I was able to put that back together and now it works. At the moment both fans are moving air from the attic, to the outside.

My question - is it better for both fans to be sucking air from the attic and blowing it outside, or have one reversed so it pulls in outside air sending it through the attic and then the other fan moves it outside?

I am sure there are some air leaks into the attic from the house, so with both fans sucking they are probably bringing up cooled air from downstairs, which seems undesirable.

If that is true, then my next question is can I just reverse the wiring on one fan to achieve the air flow through the attic, as above? ~115V AC.

Or, just leave one fan off?
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Re: Attic Gable Fan(s)
Posted by: macphanatic
Date: July 23, 2020 09:46AM
If you have enough venting, it shouldn't be an issue to have both fans exhausting. However, I doubt that a ridge vent alone provides sufficient venting for both fans and may not be enough for one to operate properly.

Typically, a gable fan is positioned on the downwind side or as close to as possible. A gable vent or more is positioned opposite the fan.
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Re: Attic Gable Fan(s)
Posted by: decay
Date: July 23, 2020 10:25AM
I would think they're sucking the warmest air from downstairs, since heat rises. Am I wrong?



---
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Re: Attic Gable Fan(s)
Posted by: mikebw
Date: July 23, 2020 10:39AM
Quote
decay
I would think they're sucking the warmest air from downstairs, since heat rises. Am I wrong?

You're not wrong... cold air is more dense so it tends to fall, leaving the less dense hotter air up top.

However that leads me to also think that they are sucking air in through the ridge vent, which is at the peak of the roof in full sun, so potentially hot air depending on the time of day. I have always thought the purpose of a ridge vent was to get air flowing up from the soffit vents and out the top of the ridge, but again I do not have any soffit vents, so...
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Re: Attic Gable Fan(s)
Posted by: Speedy
Date: July 23, 2020 10:48AM
One in, one out.



Saint Cloud, Minnesota, where the weather is wonderful even when it isn't.
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Re: Attic Gable Fan(s)
Posted by: Dennis S
Date: July 23, 2020 12:37PM
Meanwhile, call your power company and see if thy have a free "energy audit."
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Re: Attic Gable Fan(s)
Posted by: mikebw
Date: July 23, 2020 01:34PM
Quote
Dennis S
Meanwhile, call your power company and see if thy have a free "energy audit."

thumbs up
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Re: Attic Gable Fan(s)
Posted by: davester
Date: July 23, 2020 01:47PM
You really should have at least one low-mounted intake vent. The ridge vents' purpose is to exhaust hot air that rises within the attic. Given the absence of soffit vents I would suggest disconnecting the fan on the upwind gable and using that as the low vent, then using the exhaust fan on the downwind gable. I definitely would not recommend blowing air into the attic from the outside. Only bad things can come of that (i.e. blowing hot air laden with insulation particles down into the house).



"In science it often happens that scientists say, 'You know that's a really good argument; my position is mistaken,' and then they would actually change their minds and you never hear that old view from them again. They really do it. It doesn't happen as often as it should, because scientists are human and change is sometimes painful. But it happens every day. I cannot recall the last time something like that happened in politics or religion." (1987) -- Carl Sagan



Edited 1 time(s). Last edit at 07/23/2020 01:48PM by davester.
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Re: Attic Gable Fan(s)
Posted by: Dennis S
Date: July 23, 2020 01:59PM
Quote
davester
You really should have at least one low-mounted intake vent. The ridge vents' purpose is to exhaust hot air that rises within the attic. Given the absence of soffit vents I would suggest disconnecting the fan on the upwind gable and using that as the low vent, then using the exhaust fan on the downwind gable. I definitely would not recommend blowing air into the attic from the outside. Only bad things can come of that (i.e. blowing hot air laden with insulation particles down into the house).

Or you could pressurize the attic and pull the Sheetrock nails loose.
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Re: Attic Gable Fan(s)
Posted by: davester
Date: July 23, 2020 06:07PM
At first glance I actually thought this thread was about Clark Gable fans who hang out in the attic.



"In science it often happens that scientists say, 'You know that's a really good argument; my position is mistaken,' and then they would actually change their minds and you never hear that old view from them again. They really do it. It doesn't happen as often as it should, because scientists are human and change is sometimes painful. But it happens every day. I cannot recall the last time something like that happened in politics or religion." (1987) -- Carl Sagan
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Re: Attic Gable Fan(s)
Posted by: modelamac
Date: July 24, 2020 09:21AM
As Speedy says below, you need to have one bringing air into the attic and one taking it out, especially if you have no soffit vents to bring air in. You have to be careful about which fan you reverse or turn around to create the intake. You don't want it turning on and sucking in rain. The chances of having the heat in the attic remain high enough to have the fan on when it rains is fairly low, but dependent on the thermostat setting.

Quote
mikebw
Quote
decay
I would think they're sucking the warmest air from downstairs, since heat rises. Am I wrong?

You're not wrong... cold air is more dense so it tends to fall, leaving the less dense hotter air up top.

However that leads me to also think that they are sucking air in through the ridge vent, which is at the peak of the roof in full sun, so potentially hot air depending on the time of day. I have always thought the purpose of a ridge vent was to get air flowing up from the soffit vents and out the top of the ridge, but again I do not have any soffit vents, so...



Remember how when you were little
you could just rip off your diaper and
just run around naked and everyone
thought it was so cute and funny?


Anyway, I need bail money.
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Re: Attic Gable Fan(s)
Posted by: Bill in NC
Date: July 24, 2020 12:51PM
the first thing is to do the energy audit and air sealing between the attic and conditioned space below.
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