most definetly need a return installed and maybe an additioinal supply. hard to say w/o seeing. a low installed return will help "pull" the heat which comes from the ceiling, helping spread the air more equally throughout the space. also returning some of that stale air into the system so it can be filtered. be nice to have a little filtration down there. returns also help balance the room pressure. no returns means your pressurizing your room. not usually a problem but can be in some situations. leaving your door open allows all the cool air from your house to fall to your basement. heat rises cooler air which is less dense sinks.