I have a finished basement and would like to get a little heat in it. My house is zoned so I would like to tap in the 2nd floor duct with a 6 inch for the basement and then tap into the first floor duct with a 6 inch for the rec room. The rec. room doors are always open. My question is, does this sound like a good plan, and how many cold air returns should I have. Thank you Barry

Can your furnace and blower handle all this new space to heat before you get into cold air returns?

Yes I was told it could, especially the upstairs duct because the 3 rooms upstairs are much smaller. Barry

The volume of the supply should be balanced by the volume of the return. Some people put a little more positive pressure rather than negative since that will keep air intrusion down. So, 2X6" returns should work. They should be separated from the supply so you get a good air flow and not short looping and located near the floor level. I would also have a test and balance done afterwards.