Using bath fan to vent attic space .

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Davebones

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Was asked about installing a attic vent fan in a small attic space . I found a online calculator that called for 120 cfm . Does anyone know of a attic vent fan rated this small ? I thought that maybe a bath vent fan would work for such a small space . Is there any reason you wouldn't want to use a bath fan for this ? All the attic fans at the big box store's are way to big for such a small space .
 
Davebones said:
Was asked about installing a attic vent fan in a small attic space . I found a online calculator that called for 120 cfm . Does anyone know of a attic vent fan rated this small ? I thought that maybe a bath vent fan would work for such a small space . Is there any reason you wouldn't want to use a bath fan for this ? All the attic fans at the big box store's are way to big for such a small space .
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I think I remember reading on bath fan instructions that they are only for bath areas. I do know they say not for kitchens. I could be wrong about my first statement though.

Edit:
I have seen small solar powered attic fans but had no storage capabilities.
 
Little Bill said:
I think I remember reading on bath fan instructions that they are only for bath areas. I do know they say not for kitchens. I could be wrong about my first statement though.

Edit:
I have seen small solar powered attic fans but had no storage capabilities.
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And no storage is logical. You want the fan on when the sun is shining.
 
zbang said:
With typical attic temperatures, I suspect the bath fan would cook in less than a season. Yes, I know they're poked up in to attics all the time, but they don't usually get the brunt of the heat in their design usage.
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Good point. When intalled in a bathroom the fan motor is at the ceiling or wall. Move that to the roof deck or anywhere in the attic space and things change. I would not expect it to have a very long lifespan.

OP: Can we get a little more info on why this area is so small? It it like a small hip over a wing of the building and you're trying to lower the temp in the room? What are the horizontal dimensions of this attic space?
 
That would be an awfully small attic but I would use an inline fan like a fantech. They are the workhorse of a machine
 
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My concern would be that it isn't listed for this application. I would install the smallest attic fan available. IMO it is ok if it is greater than 120 cfm, but it would cycle more often. I would probably set the t-Stat a bit lower so it wouldn't short cycle as much. I am also curious as to how the 120 cfm calc was derived. Unless this is a very small space it seems a bit low.

Bob
 
If it were my home two choices are my priority on something like this.

One, natural convection ventilation - you need inlets at eaves and outlet at a high point in the roof or a gable, convection will naturally ventilate the area.

Two, spray foam insulation on underside of roof - I have seen that done more and more lately and is supposed to eliminate the need to ventilate some of these small spaces. See it done in old attics that they want to convert to living space where there will be sloped ceilings because it is essentially the underside of roof, or even in newer construction with cathedral ceilings where loose fill insulation will not be as thick as normally desired, they spray underside of roof and this prevents moisture/condensation problems in the space that ventilation was normally put in to mitigate in years past, then they can fill the rest of the space with loose fill instead of leaving a small gap to ensure ventilation.
 
Dennis Alwon said:
That would be an awfully small attic but I would use an inline fan like a fantech. They are the workhorse of a machine
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mgookin said:
Looks like good stuff. Probably won't get called back on that one.
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These are excellent fans with outstanding longevity. They build up great vacuum. But two things you should know.

1 -They're best at sucking air, the closer they are to the air discharge the better.
2 - If water may possibly reach fan through air pipe, drill a weep hole into fan, otherwise it'll stall and possibly burn out.
 
delectric123 said:
These are excellent fans with outstanding longevity. They build up great vacuum. But two things you should know.

1 -They're best at sucking air, the closer they are to the air discharge the better....
.
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This makes no sense to me. An in-line fan, or any fan for that matter, has to be better at pushing air than it does at sucking it.
 
Davebones said:
Was asked about installing a attic vent fan in a small attic space . I found a online calculator that called for 120 cfm . Does anyone know of a attic vent fan rated this small ? I thought that maybe a bath vent fan would work for such a small space . Is there any reason you wouldn't want to use a bath fan for this ? All the attic fans at the big box store's are way to big for such a small space .
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I would go with the in-line fan. A bath fan might hold up a year in an attic. If the heat doesn't get to it dust will.
 
If the space is extremely small - is there access to the fan? If using powered ventilation you may need externally mounted fan of some type if there is no interior access.
 
Dennis Alwon said:
That would be an awfully small attic but I would use an inline fan like a fantech. They are the workhorse of a machine
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+1, i swapped out all of my bathroom fans (cheap crap) with Fantech's in my attic. what a world of difference. Fantech also make metal versions used as dryer boosters which may be better suited for the application. but as others noted, ~130cfm for an attic may be pointless if it cant move the required cfm to do the job. Fantech does make bigger units too.
 
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Personally I would not use any kind of bath fan to vent an attic. Even the fantec types.

It is highly doubtful they are designed for that type of long cycle time. Basically on 6 to 10 hours a day every hot day.

Use a true exhaust fan made for continuos duty, that also eliminates any listing issues that might arise.
 
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