Need advise on install of roof 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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we put the fans so the top of fan is just lower than the top of the peak, center of roof. get a fan next step bigger than your sq ft calls for and youll be aok with the ridge vent.
http://www.minibite.com/america/malone.htm
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On Wed, 23 May 2007 15:37:51 -0400, Keith Boeheim

crawl space is under the floor. attic is above the ceiling. What are you talking about crawl space for? Were you doing drugs when you wrote that?
humidity and temperature can NOT be regulated if you don't have a closed system.
You don't give a damn where the powered roof vent sucks air from. It will remove more hot air from your attic than ridge vents alone. Do NOT remove the ridge vents. For small hip peaks, you can put ridge vents down the slopes.
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So you think you'll get the same amount of cooling when a powered attic fan is pulling air from a vent that is 2 ft aways and then sending it right back out, as opposed to pulling it up from the soffit vents? In this case, I would locate it in an area away from the ridge vent.
I'd be curious as to what temp and humidity problems the OP is trying to solve. With a ridge vent and soffit vents, there should be no humidity problem, unless there is a source for humidity entering the attic. If that's the case, I'd focus on that first. If the attic is still getting extremely hot with the existing venting, then a powered fan might be in order. But, I'd want to measure how hot it's getting first. And I'd also want to make sure the soffit vents are open. It's common for the insulation to be installed so that it blocks them.
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On 24 May 2007 08:38:54 -0700, snipped-for-privacy@optonline.net wrote:

I set my power vent fan to come on at 105. That occurs on any sunny day when temp reaches 90 degrees. That means that my ridge vents are not moving enouigh air to make up for all the heat from the sun on the shingles.
The net result is that my attic now reaches a maximum temp of about 110 instead of the 125 before I installed the power vent fan.
The power fan shuts off about sunset on a clear day.
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