Seven years later you are still looking for a variable speed switch? I put mine in 30 years ago. Works well.
On high, the fan sucks so much air it sucked my pet gerbil up and tossed them around and finally spit them out the soffits.
Sure sounds like "better than nothing" fits.
I guess you haven't seen many attics. Where did that 5F limit come from? I have yet to see any attic where the attic temp in summer is just 5F above ambient outside temp. Typical is more than several times that. Which is what insulation is for. 5F is nuts and is never going to happen with passive, even with fans it would require moving a lot of air when it's
85F+ and sunny.
We are in SW Florida and still benefit from whole house fans for about half the year. I agree with Trader, even with two 22" fans running on high, I never get the 1600 sq/ft attic below that mythical "5 degrees above ambient" if the sun is shining. As for variable speed, the best way is with multi speed motors. You size the blade to the motor running on high and it will work OK at the lower speeds. If the blade is not properly sized, you will burn up the motor (too much pitch) or simply waste energy with it not moving enough air (too little pitch). The Grainger catalog has charts that will help you with this selection. One of my fans is 2 speed, one is 4 speed. (probably overkill since we still only use two).
There are many places where even with "excessive" ventiung the attic will be WAY over 5 degrees over ambient. If I open the 6
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