[akpeper]

I am working on finishing my basement and have added two cold air returns in a 13x24 room and one is sucking air and the other is blowing. The one working properly joins into a line coming from our second floor. The second joins into a return coming from the first floor and it seems to just be pulling down the air from the first floor. I verified it was blowing by holding up a tissue. Ironically, when I was doing this my wife opened the basement door and this return started to suck in air. The return on the first floor continued working properly with the door open as well. As soon as we shut the door it begins blowing air again and not sucking. Could this be caused by pressure differences? Any suggestions would be appreciated.