I also run a dehumidifier in the summer
I keep it at 40% or so. That's also what my home is right now. In the winter here it gets to be about 20%, forced air furnace and a woodburner. I do add humidity in the winter with units in the bedrooms. I think relative humidity and moisture content are related by not necessarily linear. Basements can be especially damp if not conditioned. By circulating the shop air into the duct work you may have increased the airborn dust level and put additional dust into the filters. I wouldn't mix the shop air and house air and keep the shop well sealed off. I ran an exhaust fan in my basement shop when sawing or welding...oops. That probably wasn't good. I cracked an opposite window to keep from sucking air from the house. JMO bill
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
Last edited by woodnthings; 10-14-2010 at 11:50 AM.