need advise on instal of roof mounted vent fan

I have a 7 y/o single story home with a hip roof in Western New York. I would like to have a powered roof mounted fan with thermostat and humidistat to regulate temp and humidity in the crawl space. I have a ridge vent (short in length because of the hip) and soffit vents. Roof trusses are 24" oc and the soffit vents are every other space with foam baffles. Local home repair show on radio Saturday mornings says that an HVAC contractor is the trade to install the unit. Is this correct? Also, if the fan is mounted high on the roof will it draw from the ridge vent and not the soffits and if this is a problem should it be mounted midway up the roof or do I have to remove the ridge vent?
Thanks for any advise
kb
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Keith Boeheim wrote:

Yes, the V in HVAC is ventilation. I'd ask a good contractor if they expect any issue with cross draw from the ridge vent, but I expect it won't be a huge problem. I do know that gable vents typically aren't recommended in conjunction with ridge vents for this very reason.
Matt
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If that's true, then the 4' spacing of each eave vent on a hip roof with a ridge vent is overdone. This noted on the original post. Too much supply, air movement will be much slower. 4' spacing of eave vents is common a gable type roof with a ridge vent. Dave
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Dave wrote:

Too much supply won't slow the air movement. The air movement will be dictated by the weakest link in the system. The issue is that having gable vents in conjunction with ridge vents effectively destroys much of the pressure difference between the ridge vent and the eave vent, which lowers the airflow through the eave vents.
Matt
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You just re-worded what I said regarding the difference between hip and gable roofs with same soffit vent spacing. Thanks for the confirmation. Dave
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Dave wrote:

OK, I thought you meant too much air through the eave vents. I think I see your point now. Yes, too much air from the gable vents can destroy the air flow pattern in the attic.
Matt
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You can never have too much eve vent ...the limitation is at the ridge vent. I have a continious eve vent metal strip with small holes in it all around my 2550 sf house...including the garage which is part of the home. Like I said before, to work properly the system is a designed system not one pieced together.
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A ridge vent system is designed to move a certain amount of air/heat out of the attic which is balanced by the amount of air available from under the eve ventilation. The size of the eve ventilation and the amount of ridge vent is a calculated amount which forces the heat to rise to the roof top and vent. Any other vent combined with this system can and will cause major problems. For example: If you install a powered or any other device in the system it will take some of the available air provided by the eve ventilation and it will leave the ridge vent without enough air to move air. The ridge vent depends on the flow of air from under the eve to the ridge vent to force out the heat and at the same time keep rain from entering at the ridge vent. The mechanical system may even be powerful enough to draw moisture into the attic from outdoors through the ridge vent. This will definately cause mold problems which I know you do not want.
Ridge vent depends on the eve to supply the appropriate amount of air to cause the warm air to rise and vent through the ridge vent. There are calculations made to provide for this system to work properly.
I have Cor-A-Vent brand on my home and it was specifically designed by the Cor-A-Vent personnel to work properly. You can check some examples on their web site. (you should contact the mfg of your ridge vent before doing anything).
I would NOT install a mechanical system or any other combination of systems with a ridge vent system. If you wish to install a machincal system of sorts, I would remove the ridge vent entirely.
I hope this helps you! It is better to be safe than sorry.....
Keith Boeheim wrote:

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A ridge vent system is designed to move a certain amount of air/heat out of the attic which is balanced by the amount of air available from under the eve ventilation. The size of the eve ventilation and the amount of ridge vent is a calculated amount which forces the heat to rise to the roof top and vent. Any other vent combined with this system can and will cause major problems.
For example: If you install a powered or any other device in the system it will take some of the available air provided by the eve ventilation and it will leave the ridge vent without enough air to move air. The ridge vent depends on the flow of air from under the eve to the ridge vent to force out the heat and at the same time keep rain from entering at the ridge vent.
A mechanical system may even be powerful enough to draw moisture into the attic from outdoors through the ridge vent. This will definately cause mold problems which I know you do not want.
Ridge vent depends on the eve to supply the appropriate amount of air to cause the warm air to rise and vent through the ridge vent. There are calculations made to provide for this system to work properly.
I have Cor-A-Vent brand on my home and it was specifically designed by the Cor-A-Vent personnel to work properly. You can check some examples on their web site. (you should contact the mfg of your ridge vent before doing anything).
I would NOT install a mechanical system or any other combination of systems with a ridge vent system. If you wish to install a machincal system of sorts, I would remove the ridge vent entirely.
I hope this helps you! It is better to be safe than sorry.....
Keith Boeheim wrote:

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I want to install a "whole house fan" inside on the ceiling at the top of the uppermost stairs in my house. I have just created a cut out hole in the ceiling drywall already- I'm evicting a red squirrel. The idea is that the heat rises, but when the temp is lower outside the house than on the uppermost floor, you can open windows and suck the hot air out by pumping it out the roof vents and soffits. Theres an insulation issue, and you need a power line, but install looks simple. It would be really great to have a powered fan at the (outside) roof level too, as I suppose you are suggesting. What if anything can you tell me about installing one of these too. I am thinking cheap and simple, at the same time as the whole house fan - just to make sure it doesn't suck- maybe two if necc. Can it fit into a std vent, I already have a couple of them - the square 10" x 10" already existing. I am not even sure that is what you are talking about. Here's what I'm talking about:
(the bottom pic was the HD whf) http://www.cobshomes.com/news.html?date 96606800
(how to install) http://www.eere.energy.gov/buildings/info/homes/wholehousefan.html
The homedepot link is gone ($189.00), and there are a lot of these around, and technologies, and prices. http://www.homedepot.com/prel80/HDUS/EN_US/diy_main/pg_diy.jsp?CNTTYPE=PROD_META&CNTKEY=misc/searchResults.jsp&N)84+3734&cm_mmc=hd_over-_-Search-_-D-27X-_-bid20546567-whole_house_fans&ysmwa=KCQtJ0ec2N-WrZDlZVYLz8WDFj8O3fNsFV-4NYa_dlbky9HyDPyF4TVEdLCMF31-
-
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Master Flow Model WHFS24M, was $189.00 at HD. Don't know why the link (US avail. only?) is gone. I'm in Toronto, and I never got anywhere past the homedepot.com website, not the .ca, but I don't know whats up now. It looks like the perfect unit though.
-
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These look interesting: http://www.quietcoolfan.com /
A friend of mine has one of these, and really likes it: http://www.tamtech.com/wholehousefanhv1000.htm
And there are others, as found with this search: http://www.google.com/search?&q=insulated+whole+house+fan
I'm contemplating the same situation -- 50 year old hip roofed house, I want to increase attic ventilation and also have a whole house fan... I'm wondering about two linked fans, one in the ceiling and one on a vent... and maybe capturing three existing little vent fans (bathroom, kitchen, and closet) so they are vented more directly to the outside, rather than into the attic...
-Kevin
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First, your mechanical vent fans (baths and kitchens) should be vented to the outside the roof (to the exterior). Reason is that the moisture will condensate in the insulation and lower the R value (dramatically). Consider you will never get the R value published in the reports (that value is under perfect conditions) and under normal conditions your fiberglass or cellulose may only yield 1/2 it's published value during winter conditions. Add excess moisture to that and you are wasting a lot of money through the reduced insulation values. (Not to mention that the moisture will attract vermin). So get the fans vented to the exterior if you do nothing else. You can pick up a roof flange w/damper and some metal flex ducting at your local building supply.
Should only take a hour to install. Go up on the roof, pry up & remove some shingles in the area where the cap is going to go, cut a hole (3-4" usually), nail down the flange interleaving the shingles as you replace them (will need some roofers cement to cover the nails), go in the attic and hook the metal flex duct to each fan exhaust and roof termination. Your home will be MUCH warmer next winder and easier to cool.
Second, you will love having a whole house fan. Raise the bedroom windows 1" and you get a nice cool breeze blowing through the room. The fans themselves should tell you how much ventilation you need in the attic, if your house was built to code there's no problem.
As far as mechanical attic ventilation, these are usually installed in attics where there's insufficient ventilation. The fan is set to trigger on either by excessive heat (usually set between 100-110 deg.) or humidity (usually 50% or more.) Don't worry about where the air is coming from, once they run they will depressurize the attic and the air will be sucked in from any opening. It doesn't matter if you have ridge vents or gable, either will work, don't believe you should have both however as they will interfere with the design of the other. (Easy to close off the gables in that case however).

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