No return ducts on the second floor - leaking air in these rooms

I have no return air ducts in my upstairs either. It can get cold up there. I am assuming that you mean that there is cold air coming into those rooms. Possible solutions: Storm windows should be down. Caulk around storms anywhere air could get in. Put clear plastic over windows on outside or inside. Make sure furnace is maintained: have it inspected, blower motor belt is in good condition and tight enough, filters changed once a month in the wintertime. Any doors to the upstairs open so that heat will also go upstairs.