attic exhaust fan recommendation?

bytehovenJuly 18, 2006

A Master Flow 1600+ CFM attic fan from Home Depot has failed after just (3) weeks.

Any recommendations for a replacement in the 1600 CFM range?


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Are you really, really, really sure that you need one? I highly doubt it!

The reason I ask is this: most attic floors are not sealed really well to the conditioned space below. When they run, they result in negative pressure in the attic, which then results in even greater leakage of conditioned air from the house into the attic. Through cans, through fixture holes, through the edges of vent boots, etc.

You end up spending more cooling dollars because you accelerate the exit of conditioned air from the house into the attic, and you hasten the entry of unconditioned air into the house below to replace that which is leaving to the attic.

Here is a link that might be useful: Read this PDF on attic fans

    Bookmark   July 18, 2006 at 2:43PM
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I believe he is referring to a whole house fan - commonly called attic fan (at least where I'm from).

Out of curosity, what part failed?

    Bookmark   July 18, 2006 at 4:49PM
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I picked up a unit @ Lowes that is only 16" wide with two small high speed fans. I don't recall the brand name but I have been very pleased with the unit. It has a wireless remote control, five fan speeds and several timer options (1/2 hr, 1 hr, 2 hr, 4 hr and "on"). It was expensive ($400, I believe), but was very easy to install and pulls and enormous amount of air through the house. When it's on it ain't quiet!


    Bookmark   July 18, 2006 at 8:16PM
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dallasbill: Thanks for your post and the .pdf file. Very interesting.

You're right. I have "assumed" I would benefit from powered venting to assist/increase the natural venting of my attic space.

I was able to observe, the extra ventilation did not seem to stop the eventual max temp of the attic space during the day, but it does delay the onset of max temp. But now I think I understand how the fan simply eats up power without any longer term attic temp reduction. Even if it was able to provide a few degrees of temp reduction, maybe I'd be looking at 120* instead of 125*. :)

I did go through the process of chaning all non-ic cans to IC-air tight cans. And then we added extra cellulose insulation in those areas of the house that were transmitting both heat/cold from the attic to the rooms below.

Maybe I should just consider the fan faliure a blessing and skip replacing it.

We have a small house fan to help take the heat off the 2nd floor. We have a raised range setup, and the kitchen and family with vaulted ceiling will often get warmer than the rest of the house. These are the areas we replaced the cans and added insulation. The small house fan also comes in handy when these spaces start to over heat from cooking, laundty chores, etc. This had me thinking the the attic exhasut fan would help the house fan be more efficient.

mschwall: It was the motor that failed, and todya when returning it, I noticed some oil on the output shaft side of the motor housing. The motor still turned freely but did not power up.

Rick: I have thought about a larger whole house fan, but my wife has allergies and we don't ventilate the house with outside area as often as we could. As I result, I have a couple of nice window fans I use when we can. My neighbors have a nice big unit, and it does indeed turn over the air in the house very nicely.

Thanks everyone for the posts. I will reconsider the use of the attic exhaust fan.

cheers RJ

BTW... is there any potential benfit of linking a smaller attic gable fan to come on with our small house fan? I am thinking it could keep the attic from developing positive pressure when the house fan is on. Just a thought...

    Bookmark   July 18, 2006 at 11:38PM
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dallisbill: One question I forgot to ask... How about the use of fans where one pushes air in on one end of the house while another pulls air out on the far end?

I assume the amount of air being moved is still too small to impact the overall heated surface of the roof.

    Bookmark   July 18, 2006 at 11:52PM
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bytehoven... you are correct in your observation. The conductive heat load from shingle-to-subroof-to-joists-to-attic will still be far above what the moving air will do. In fact, gable fans in attics are not recommended anymore, especially if there is more than one gable vent. The negative pressure aspect aside, if there is a second gable vent up there, they simply tend to suck air across from the other gable and do nothing to "vent" the attic itself. It still has the hot air build-up beyond the gable-to-gable air pathway.

When I looked at the typical shrouded fans from Lowes or HD a few years back for my old house, they specifically said not for gable mounting. It's far more effective to add soffit venting and a handyman could help you there. Just search the web on 'soffit venting' for ideas.

    Bookmark   July 19, 2006 at 12:28PM
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When I started my business, the fad of the time was ceiling fans. Many people claimed that they did not add any comfort. Some people claimed they were great.

Ceiling fans have been around for several hundred years. I find it hard to believe that a fan is worthless. With this said, I also installed exhaust fans in commercial applications. I tweaked exhaust fans, for maximum effectiveness.

One deciding point, was a cleaners that wanted an exhaust fan and evaporative cooler to help cool the employees. The main assembly area, that was manned by a front counter worker, had an average temperature of 120F degrees. When the door was opened to the cooled side, it caused the cooled side to instantly get hot.

I cut a 30 inch opening in the roof, and the 150+ degree air felt like a high pressure fan. I could have put a ball into the air flow and it would have risen. Without any forced air, the air flow was tremendous for 30 minutes. When I started the fan, it did not make much difference that day, but the following day, it reduced the temperature at the assembly area by 20 degrees. The exhaust fan was 30 feet away. When I added the evaporative cooler, the building was cooler than outside by 15 degrees. It was not really cooler, but the wind chill made the air feel cooler.

I have questioned the need for power vents, for many years. I added a power vent 10 years ago to my house. My AC bills dropped. During the last 12 years, my energy consumption has dropped enough to maintain the same overall cost of electricity. Partly from conservation habits, and from changing lights to CF. Squirrils have removed a large portion of insulation during this time and I need insulation in the attic. Since I do not have the cash to change my living conditions, I am studying the effects.

IMO, the attic fan has helped to save electricity, by reducing the temperature in the attic. I have observed temperatures about 135 degrees in my attic during the summer. I have a good passive ventillation system. When I added the fan, I closed 1 of 3 gable openings for the fan.
This summer, the fan stopped. I have noticed a significent difference in electricity. I do not feel up to the task of resealing the gable, I already repaired the fan and turned it off, but know that I need to do so.

Quantative analysis shows a $40-50 dollar difference in the cost of electricity. Comparing it with similar usage numbers and adjusting the cost for increased costs, then attempting to compare with weather from past years, shows this cost. My increase from last year is about $80.

    Bookmark   July 20, 2006 at 12:56PM
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1) anecdotal, anecdotal...

2) what does "an observed temperature" mean? It is compared to what other pre-fan observed temperatures? Over what period of time?

3) the cost of electricity in N. Texas, where you live, is 25% higher than last summer. Your dollar figures mean nothing. It's KwH consumption that counts.

    Bookmark   July 20, 2006 at 2:09PM
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I have posted this before to previous questions regarding powered attic fans. It is a long read but worth it nontheless. I am still remodelling but when I had my A/C contractor out I asked them to install a fresh air makeup vent to the airhandler. This should supply about 10% outside air to provide a positive pressure on the living space to keep out dust and humidity. My research indicates that ridge/soffit venting is the most effective/cost effective method out there. I will let everyone out know how well the fresh air vent works or if anyone else has one tell me how good it is.

Here is a link that might be useful: Humidity control in the humid south

    Bookmark   July 20, 2006 at 4:57PM
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That's a great article and does a great job of distinguishing between latent heat removal and sensible heat removal. Thanks!

How are you, yourself, going to accomplish latent heat removal on your 10% fresh air supply? Any idea what your calculated payback is?

    Bookmark   July 21, 2006 at 9:51AM
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Would a dedicated dehumidifier with an exterior air intake system be the best method?

When I was researching some of these from Therma-store and Aprilaire, they run in the $1000 for the main hardware + and extra goodies to hook it all up.

    Bookmark   July 21, 2006 at 9:05PM
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Those are some of the worst articles I've read in a while. Especially that one posted by Dallasbill. Maybe because I just finished an upper division class on resarch and statistics and I look at the written word more carefully now. Anyone could have written that drivel and posted it on the interweb, there are no references or any corrolating data.

However, the garbage caused me to excute my own search and I found a great, published article with well documented data that is believable.

So I agree with the idea that the attic fan has the potential to be a bad idea.....and I would not have believed it unless someone wrote that poorly written article posted thanks.

Here is a link that might be useful: Published Article on Attic Fans

    Bookmark   July 24, 2006 at 12:48AM
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This is not a literary site, but rather a site for information about do-it-yourself electrical wiring. Not all of our contributors are grammarians or poet laureates. However, most of us write in full sentences, and spell words like correlate correctly. Even if we don't, we are still welcome to contribute, as long as it's done in a positive manner.

    Bookmark   July 24, 2006 at 12:31PM
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Frank has a point. The DOM article is a bit hand-wavey (that is - here's what we found, you needn't be bothered with HOW we found it). We're expected to take its content on the strenth of the source's (that is, Dominion Energy's) reputation.

OTOH, the UCF piece does cite its data, which puts us in a better position to judge its validity. But it has its own downside - it's based on only one data point, while the DOM paper claims to synopsize multiple studies.

I think the salient point here is that several sources suggest that one's money is better spent on insulation, radiant barriers, and light-colored roof shingles than on attic fans. (But you're right that it's off topic. ;-)

    Bookmark   July 24, 2006 at 1:56PM
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I have several articles saved on construction methods for my area of the country (far SE) and most seem to agree. Where do you live Frank?

    Bookmark   July 26, 2006 at 2:58PM
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lourock: I live in Los Angeles.

Kurto: You miss the point. Thanks to that unreferenced garbage, I was forced to look elsewhere for real data...and found real information. I have now turnd my attic fans off! This post is valuable, and has helped me.

If anyone else can glean some good from this, let it be known not to believe stuff on the web at face value too often.

    Bookmark   July 30, 2006 at 12:24AM
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So, let me get this straight: you guys seem to be saying that a powered attic vent will:

1) possibly cause a negative pressure in the attic space, also possibly drawing cool air from the living space

2) not decrease the attic temperature significantly

I'm thinking you are using some seriously fuzzy logic/math here. As for the "research" posted... the pdf I read was not even worthy of being posted. The other link has expired.

Consider these points:

1) According to building codes, an attic space should already have soffit vents and ridge vents at a rate of one pair per 150-300 sq. feet of attic space. So that equals at least 5 square feet of wide open venting, not including a gable vent at the opposite end of the attic. If this is the case, there is NO WAY that a significant negative pressure will occur on the living space unless you have some serious holes in your ceiling.

A 1600 cfm powered gable vent is going to pull it's air from these existing vents, or, ideally, from a gable vent at the opposite end of the attic. While ridge vents are indeed designed to exhaust air, they will also allow intake of air. It's just a hole.

So this talk of backdrafting in furnace units in the basement (two floors away?) is just silly. Adequate attic venting as required by standard building codes is enough to eliminate this risk. If this venting is not already present, this will of course need to be remedied. But to assume that a negative pressure situation will be typical situation if a power vent is used is erroneous.

2) As for not reducing temperatures significantly, if the fan thermostat is set at 90 degrees, it WILL reduce attic temperatures and keep them lower. Let's say you have about 1500 sq. feet of attic to vent, or roughly about 9000 cubic feet of air volume. A 1600 cfm fan will completely exchange the attic air every 6 minutes or so. If the air enters the attic at 90 degrees from outside, it is NOT going to go up 30 degrees in 6 minutes. That just makes zero sense.

The key is to have the vent come on at a lower temperature to ensure that temps don't have a chance to climb to 120 or more.

As far as electricity consumption, a powered attic vent pulls considerably less than an a/c unit.

I don't have any links for you. Just using common sense.

    Bookmark   April 16, 2007 at 4:30PM
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Ok, heres my 2 cents worth. I agree with Lourock on the benefits of soffit/ridge venting. I live in the Southeast and I can tell you that convection does occur between these two vents, you have cooler air going in low and very hot air exhausting up high. Its a basic flue system regardless of how its shaped. And yes, if your home isnt well sealed between living space and the outdoors and attic it will draw your conditioned air from indoors. (If you feel like getting very hot and experimenting, get some weightless smoke from your ac supply house and use it in various areas of the attic)

IMO Gable vents reduce the effects of properly installed soffit/ridge vents by pulling air in at the very outside edges of the attic and exhausting the same close to the edges leaving the center portion with low air draw and a hotsot.

If you set your attic exhaust fan @ 90 degrees where I live it will run 24/7 for months.

These vents also serve a purpose in the winter by helping exhaust humid air from the attic. The humidity level is high here even in the winter.

I do agree that a good quality roof mounted exhaust fan properly located is cheaper to run than an ac unit and will help lower attic temperatures and humidity levels when it also has a humidistat.

    Bookmark   April 16, 2007 at 8:57PM
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This is interesting, and I hate to put a damper on your enthusiasm, but why resurrect this ancient thread now? The thread was last addressed 9 months ago. The person who originally asked has probably long since dealt with the problem and moved on.

    Bookmark   April 17, 2007 at 2:45PM
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I may be in the minority but I do more reading of both recent and old posts than I do advice giving, although I will give my opinion if I think it can help.

    Bookmark   April 17, 2007 at 8:43PM
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I recently became concerned that my attic fan was going to seize up because it is making a lot of noise. I'd rather not replace it because I worry about an attic fire, even from a new one. I'm not comfortable with an electric motor running close to 18 hours a day in my attic. I'm not too concerned with paying some more on my electric bill if the house A/C runs more, so I guess I have two questions:
1. Are attic fans known to have caused house fires?
2. Is a hot attic a fire hazard from spontaneous combustion?


    Bookmark   January 17, 2009 at 4:55PM
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I' m installing an attic fan that has both temp and humidity settings. What temp/humidity settings would be best? I live in SE Ohio

    Bookmark   April 18, 2011 at 8:00PM
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Boogiesan.... In 2004 I installed 2 of the BEST "Roof Mount Ventilators" that MasterFlow makes (Good, Better, Best)... a division of GAF - Lifetime Warranty. Recently 1 of these experienced a Motor Freeze-up - the other is still working fine. No Fire or other adverse results - it just stopped working.
I feel these have SIGNIFICANTLY reduced my AC bills, made my Upstairs bedrooms more livable, and Significantly REDUCED the Heat-Stress on my New roof shingles.
No "Scientific Proof"... just my humble opinion.

Note: Previously (with my Old roof) I had 16ea 12" Square can roof vents with 32ea 4x16 Under-eave vents - and my shingles Literally got COOKED to death. I eliminated the 16ea Square roof vents, and installed these 2 "Roof Mount Ventilators". A GREAT improvement.

    Bookmark   June 20, 2011 at 10:56AM
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well this is an interesting thread.

i'll offer a slightly different point of view. I use my powered attic ventilator to keep the attic humidity down in the winter. how? I hard wired the thing and manually switch it on whenever my attic humidity gets above 70%. and believe you me - it makes all the difference in the world.

I live in Chicago, and when temps get below 10F my attic will be at 21F & 80%+ RH without the powered fan. that is enough to cause frost in the attic, which I have had from time to time. the powered attic ventilator eliminates the problem.

now - I may have some other issues to deal with - e.g. why is the attic getting so humid to begin with??? without getting into too much detail, I have addressed all potential sources of air/vapor migration I can think of, and have upgraded to R49 insulation, but still have the high humidity (without the fan). I also have the requisite number of passive vents and soffit venting.

the house does not have a platic vapor barrier in the ceiling (only kraft faced batting).

I have reached the point where I believe it is impossible to prevent high humidity in an attic during very cold temps(unless, perhaps, if you have the plastic barrier in the ceiling that newer homes seem to have. mine is 27 years old).

    Bookmark   January 22, 2012 at 12:16AM
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I recently.bought a exhaust fan through an online company called Carl's Fan and its a newer fan but It's more technology forward so to speak. It's from a newer company called progress lighting.

Here is a link that might be useful: link to site

    Bookmark   July 16, 2012 at 11:01PM
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I'll bet this is spam. I will give generous odds. Any takers?

    Bookmark   July 17, 2012 at 6:09PM
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