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Powered attic ventilator with whole house fan

Hello, I really appreciate this website.

I understand that powered attic ventilators have a lot of problems. But my contractor is suggesting adding one to support a whole-house fan, due to the limited attic ventilation. (I do have some passive ventilation but not quite enough, and the gables are maxed out because it's a flat roof.)

So as long as I always run both fans at the same time, and as long as the cfm of the whole house fan exceeds the cfm of the attic fan, is there still going to be a problem?


Asked by Robert Shelton
Posted Fri, 07/25/2014 - 18:16
Edited Sat, 07/26/2014 - 05:55


8 Answers

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The purpose of attic ventilation (as opposed to whole house fans) is to purge moisture from the attic. In most US climates attic moisture accumulation is due to from air infiltration leaking into a cold attic during winter, but in the humid Gulf Coast states a signficant amount of the moisture burden is from outdoor air entering the cooler attic of an air conditioned building during the summers. Actively powered ventilation doesn't fix either of those conditions, and will often make the problem worse.

Unless you have a VERY tight ceiling (in which case you have no migration of conditioned space into the attic to speak of), depressurizing the attic with a ventilation fan just draws MORE conditioned air into the attic. If the air drawn in from the soffit/ridge/gable/whatever exceeds the volumes drawn through the floor leaks there is a dilution factor, but it still comes with a hefty space heating or air conditioning price tag, since it's also depressurizing the conditioned space, causing outdoor air to be drawn into conditioned space.

If there is an attic moisture problem the first course of action is to air-seal the attic floor. In cooling dominated climates in air conditioned buildings it's usually best to also air-seal the attic from the outdoors. You can then add attic ventilation (unpowered), with about 50% more cross sectional area at the soffits or low on the roof than you have at the ridge, which keeps the pressure difference between the conditioned space and attic floor limiting just how hard the "stack effect" can draw air through that plane. (Gabled vents are usually good enough as-is, but will short-circuit soffit venting.)

On low angle flat roofs adding short unpowered stacks to promote air movement through soffit vents is fine, but don't use a turbine-type whirlybird cap- simple rain-hat caps and stack-effect is good enough, and wind operated turbines are too much- they'll depressurize the attic too. But adding a powered ventilation fan is almost universally a bad idea.

Answered by Dana Dorsett
Posted Fri, 07/25/2014 - 19:02

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You have misunderstood Robert's question.

He wants to use the powered attic ventilator to support the use of his whole-house fan. If he is operating a whole-house fan, he obviously has a (deliberately installed) huge hole in his ceiling, so your admonitions about making sure that he has "a VERY tight ceiling" are irrelevant.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 07/25/2014 - 19:14

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Hi Dana, thanks for your quick and thorough reply. I can see that my question was unclear. I'm asking about heat, not moisture. I live in California, cool nights, very hot days, no air-conditioning. Moving the indoor air outside via the attic, and the outside air into the house, is exactly my intent.

So my question is whether the negative pressure from the attic fan will effectively relieve some of the positive pressure from the whole house fan. Again, the positive pressure would exceed the negative pressure, and there would be additional passive ventilation to release the remaining positive pressure.

Or should I start cutting holes in the roof?

Edit: Oh! Thanks Martin - I didn't see your response when I posted my follow-up.

Answered by Robert Shelton
Posted Fri, 07/25/2014 - 20:10
Edited Sat, 07/26/2014 - 00:23.

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I see no reason (in principle) why the plan wouldn't work, as long as someone performs some calculations to determine whether the powered attic ventilator is properly sized. (For example, if the roof has only 50% of the required vent area, I would imagine that you would want to install a powered attic ventilator rated at 50% of the cfm of the whole-house fan.)

While the plan would work in principle, there are two potential drawbacks to the plan. One is that operating the fans will require more energy than just operating a whole-house fan without the powered attic ventilator.

The other is that the system depends on both fans working, and someone will have to keep an eye on the powered attic ventilator to make sure it is still operating. Eventually, of course, it will conk out, and whoever owns the house at that point will have a head-scratcher when they try to figure out what the designer of the system had in mind.

Answered by Martin Holladay, GBA Advisor
Posted Sat, 07/26/2014 - 06:02
Edited Sat, 07/26/2014 - 06:03.

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A whole house fan discharging into the confined space of an attic has always seemed a little odd to me, however well the attic's ventilated. Wouldn't it make more sense to simply duct the fan's output directly to the exterior?

Answered by James Morgan
Posted Sat, 07/26/2014 - 13:57

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Thanks Martin - the first drawback you mentioned had occurred to me, but the second one hadn't.

James, I know the idea is that a whole house fan will quickly cool off the attic without the problems associated with traditional attic fans, but maybe with enough insulation, cooling the attic wouldn't be such an issue? However, you've given me an idea.

The full story is that I already had a whole house fan installed, but I'm disappointed with its performance. So the contractor suggested getting a more powerful fan in conjunction with an attic fan to make up for the extra air flow. But the manufacturer (QuietCool) does make a fan that exhausts directly through the roof. So maybe I should just add that to what I already have, which would double the performance without requiring additional ventilation.

If anyone is interested (and/or feeling extra generous), you're welcome to view these two videos, one that I saw on youtube, and one of my own, which I made to help communicate the problem to the contractor and manufacturer. Neither of them can figure out why my fan is not performing as well as the other, despite similar ratios of cfm to square feet.

If anyone has any diagnostic input, it would be much appreciated but certainly not expected.

Answered by Robert Shelton
Posted Sun, 07/27/2014 - 14:48

Helpful? 0

Reply to James Morgan:

Discharging to the attic has always been the conventional wisdom because that air helps cool off the attic. That effect comes for free, because it needs no ductwork.


Yes, it looks like the problem has to be a lack of attic venting. Adding some roof vents is the easiest solution, since the QuietCool fan should be able to move the rated amount of air if not restricted. If you are in snow country, the roof vents need to be 12" tall.

Adding the other style fan would help, but it wouldn't solve that bad performance of the fan you already own.

If I were you, I'd check to see if adequate cooling of the house can actually be done with fans blowing fresh air. You could get a at least two $12 box fans and put them in the windows.

Answered by Kevin Dickson, MSME
Posted Mon, 07/28/2014 - 03:43
Edited Mon, 07/28/2014 - 12:34.

Helpful? 0

That's just what I needed to hear, Kevin, I've been telling the contractor I didn't think there was enough ventilation, and now I can tell him a mechanical engineer agrees.

I've tried the box fans and they didn't help much. On the other hand, my whole house fan is currently only marginally better. But your post is encouraging, and fortunately the contractor has been very responsive, so hopefully we can make a significant improvement in my current setup.

Thanks, I really do appreciate it.

Answered by Robert Shelton
Posted Tue, 07/29/2014 - 01:16

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