depending on how far too the pipe is from the laundry tub, you may need a condensate pump or you may already have one if you have high efficiency condensing furnace. then you can just tie into the existing pipe, as for the cold air problem reaching up stairs, have your a/c inspected, i recommend every year, just like your furnace, as well, as what most of the other guys have said, duct work is a very tricky thing to size to start with, remember that in the fall you will have to back and balance all the round pipe again. most hardware stores carry a booster which can be be installed with very little effort in your basement providing it is unfinished. all it is is a section of pipe with a fan in the center, i have them in my house ( a rental) for the upstairs heat in the winter time. Another thing you may want to try is putting a dehumidifier in the upper units, this will make a difference on its own. also if you have a humidifier attached to your furnace, shut it off and close the louver on it. Good luck