I am finishing a basement and the intake/exhaust pipes for my furnace hang down more than I want. The pipes are sloped and I'm guessing its to direct the condensation back toward the furnace to a drain pipe that is attached to the intake pipe. I would like to raise the pipe a bit to get them above a drop ceiling. My question is, how much slope do I really need? The span is about 15' and the pipe is hanging down roughly 4-5" from the joist at one end and tapers to touching the joist where it exits the wall.