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Old 06-11-2008, 04:43 PM   #1
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Problem with whole house fan and not enough attic/roof vent area


Hi,
I have a 36" whole house fan installed in the ceiling of a 2 story colonial. Based on its poor performance and calculating the venting capacity of the soffit & ridge vents, the attic venting area appears to be less than half of what the fan requires.

I was initially thinking of adding some fairly large gable vents. But I stumbled on this site and learned that soffit/ridge should not coexist w/ gable venting. Are there other options? What products would be recommended. I'd prefer to add to the existing roof. Would roof vents be OK? Or louvered gable vents (not quite sure if passive or powered louvers exist)?

The fan is a Nutone Model WHV36BD, which is 5600CFM. Their manual states that I need ~18-19sf of vent area in the attic. I have 64 linear feet of soffit vents and 32 feet of ridge. Unfortunuately I don't have better information regarding type. So using a standard vent value of ridge/soffit I calculated ~8sf of vent area.

As far as performance, I notice the fan's louvers (in the living space) open about halfway when the fan is on. They also rattle a bit. If I open the attic access panel, the louvers open fully and its noticeably quieter. Access panel opening is ~7.5sf. So that tells me the #'s are about right and that I need at least 7-8sf of add'l vent space.


If there's any other info I can provide, please let me know. Thanks!!


Last edited by keeena; 06-11-2008 at 05:42 PM. Reason: Corrected ridge/soffit length
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Old 06-11-2008, 05:06 PM   #2
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Problem with whole house fan and not enough attic/roof vent area


Just some basic information for right now and I will see if I have time for a more detailed one later tonight.

After you read through this, please let me know if your square footage of your home allows for the CFM rating that you currently have in place.

Ed



From the Air Vent website regarding CFM calculations:

http://www.airvent.com/homeowner/whyVent/faq.shtml

How much attic ventilation do I need?

- For non-powered attic vents (ridge vents, roof louvers, wind turbines, etc.)

Building codes give minimum attic ventilation requirements. Most attics require more. In fact, research suggests you need 1 square foot of ventilation space, or net free area, for every 150 square feet of attic floor space. Net free area is the total unobstructed area through which air can enter or exhaust a non-powered vent.

For new home construction with a vapor retarder, the minimum is 1 square foot of ventilation or net free area for every 300 square feet of attic floor space.

If your vents are split between ridge vents and intake vents, the minimum requirement is also 1 square foot of ventilation or net free area for every 300 square feet of attic floor space.

- For power attic vents

Power Attic Ventilators are rated in CFM. This number represents the cubic feet of air moved per minute. The higher the CFM, the more powerful the ventilator. According to the Home Ventilating Institute, to determine the correct CFM rating needed, multiply attic square footage by 0.7. For example: 1500 square footage (attic) x 0.7 = 1050 CFM (look for a power ventilator rated 1050 CFM or higher).

When using power vents, be sure to provide sufficient attic intake ventilation to match the exhaust capacity of the fan. To determine this, divide the CFM by 300 for the number of square feet of intake needed. Because most vents are rated in inches instead of feet, simply multiply the number of square feet by 144 to calculate the net free area of intake needed.

- For whole-house fans

Whole-House Fans are also rated in CFM. To determine the right size for your house, calculate the interior square footage of living area (do not include garage, attic or basement) and multiply the total square footage by three. This will give you the CFM number to look for when choosing your whole-house fan.

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Old 06-11-2008, 05:26 PM   #3
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Problem with whole house fan and not enough attic/roof vent area


Thanks for the initial reply. My house is just shy of 1700sf (28x32, 2 floors n/c basement) and the fan is rated for houses up to 1865sf...so no problem there. I have plenty of windows open to provide intake air to the interior of the house. Fan just can't push it out the attic fast enough; attic venting is the bottleneck. I would like to add venting that (a) won't interfere or create problems with the existing soffit/ridge and (b) has at least 8-10sf of venting area (which is quite a bit).

I did find some gable vents which featured a significant amount of venting space, but now concerned about issues when combining w/ ridge/soffit.

Last edited by keeena; 06-11-2008 at 05:38 PM.
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Old 06-11-2008, 05:59 PM   #4
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Problem with whole house fan and not enough attic/roof vent area


If the Gable Vents come with "Hinged" Louvres, they would ope up when the whole house fan is operating and forcing air through them.

When the whole house fan is not operating, the louvres would be closed, so as to not short-circuit the remainder of the air flowage from intake portals to exhaust vents.

That should probably be a quick find with a Google search. If you do not come up with any results, I will try searching for a commonly available product.

Ed

Last edited by Ed the Roofer; 06-13-2008 at 09:44 AM.
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Old 06-13-2008, 07:56 AM   #5
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Problem with whole house fan and not enough attic/roof vent area


Unfortunately I haven't found anything yet on builder supply sites. A google search for "hinged gable vents", "hinged vents", "hinged louver vent", etc...hasn't turned up anything yet. A product may exist, but I might not have the right name for the product. I have seen the powered fan systems w/ louvered vents (for venting a garage/shop/attic), but haven't come across a larger, standalone vent which has movable louvers/shutters. The only kind I did find was basically like a dryer vent...very flimsy plastic louvers which I highly doubt would seal very well (it appears to be intended to be attached to a pipe exh system).

Last edited by keeena; 06-13-2008 at 08:20 AM.
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Old 06-13-2008, 09:46 AM   #6
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Problem with whole house fan and not enough attic/roof vent area


Neither did I. But, as you said, I know they are available with the powered ventilator varieties for sure. I wonder if you can contact one of those manufacturers and see if you can purchase just the hinged gable louvre portion, without the fan and motor.

Ed
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Old 06-13-2008, 10:10 AM   #7
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Problem with whole house fan and not enough attic/roof vent area


I found a couple options, but they are industrial vent shutter type things, not pretty residential gable vents

Examples:
http://www1.mscdirect.com/CGI/NNSRIT...61152&PMT4NO=0
http://www.google.com/products?num=3...ckout-restrict
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Old 06-13-2008, 11:31 AM   #8
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Problem with whole house fan and not enough attic/roof vent area


I also posted a link to this thread topic on another Roofing specific forum, so maybe one of the other guys can help out who are not regulars over on this site.

Ed
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Old 06-14-2008, 07:02 AM   #9
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Problem with whole house fan and not enough attic/roof vent area


Ed- Very informative. My house has trouble staying cool. I was considering adding a powered attic fan (possibly activated by thermostat) to clear out the heat in the attic, I'm hoping that will help.

Attic is 1344 sq. ft (48x28). Currently I have 2 big louvered gable vents (18"x24") and 5 roof (round) vents (12" x 12"). My eaves are wood all the way so I guess I have no intake vents.

Your formula above mentions to take the CFM required (in my case 940, although I notice most come 1050 or higher), divide by 300 and that determines the amount of intake sq. footage you need to support the fan. So I'd need about 3.5 sq. feet of intake?

What exactly constitutes intake? Do I need to cut out my eaves and install a couple of soffit vents on each side of the house? Or is the gable louver on the opposite side of the house able to double duty as an intake as well?

Thanks!!!
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Old 06-14-2008, 11:55 AM   #10
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Problem with whole house fan and not enough attic/roof vent area


Spup345, Please Start a new topic discussion for your concerns. Briefly, there is a different calculation for using a PAV, (Powered Attic Ventilator), and also, yes you will need to add more soffit intake ventilation.

Regarding the Original Poster, a poster by the name of Tar Monkey on the Roofing dot com Forum provided a link. I fould powered and hinged versions, but the price was inexpensive enought to just remove the motar an let the hinged louvres flip open from your whole house fan activation.

The Google Search Term Link is here:
http://www.google.com/search?hl=en&s...er&btnG=Search

The inexpensive powered and hinged version was found here, but I only searched a few of the results. You may be able to find exactly what you are looking for by clicking on more results on the page.
http://doitbest.com/Attic+ventilator...sku-114138.dib

Ed
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Old 06-16-2008, 09:26 AM   #11
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Problem with whole house fan and not enough attic/roof vent area


Ed - Thanks for the links! I see a couple that look promising, though I'll need to figure out a way to mount them. Most appear to be flush mount (no built in trim ring), so I'll have to figure out a way to get it to work w/ my vinyl siding. Probably just need to get some edge trim for the siding and build up the ply sheathing to allow the vent to sit flush.

Last edited by keeena; 06-16-2008 at 09:37 AM.
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Old 06-16-2008, 11:59 AM   #12
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Problem with whole house fan and not enough attic/roof vent area


I have the EXACT same problem as Keeena. 36 inch fan, 2 story colonial, 1900 square feet. Ridge and soffit venting. But fan is bottlenecked in the attic....air not escaping fast enough.

I did fine this on the Home Depot site...

http://www.homedepot.com/webapp/wcs/...ctId=100092056

But, they are part of a power vent and not sure how to seal against vinyl siding....

I think one on each end gable would do the trick though....

Any suggestions or comments on this item?


Last edited by wrogalski; 06-16-2008 at 12:48 PM.
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