Here is my dilema and I'm hoping someone can offer a few suggestions to help. I have a downstairs bedroom that was originally a attached garage. The previous owners converted it to a bedroom and built a new attached garage next to the room. This room has 2 ceiling supply vents but no cold air return and the door seals the room pretty tight. This room has been extremely cold in the winter and unbearable in the summer since I bought the house a few years back. The house has a relatively new heater (gas) and my heating/cooling bills have been outrageously expensive. So I climbed in the attic (access from the garage goes over the bedroom and kitchen ceiling). I found that I have 2 6" supply lines to feed the bedroom vents. One line was totally disconnected and laying on the insulation. The other line was rigged up with duct tape and wasting major money. One line goes direct to a vent in the front of the room. The other line tee's off to a vent above the door but on the kitchen side and proceeds to the vent at the rear of the room. I have read several responses to questions on cold air returns. I do not have any idea of how much air the heater is designed to move. The heater is actually placed into a closet accessible only from the outside of the house and intake and exhaust protrude over the doorway. The fit of the heater into this closet restricts most access and would allow me to retro a 4" return back to the heater from the bedroom. Would this assist in helping to warm the room when I have the door closed? Would a 4" line be acceptable or should I try to increase the size to a 6 or 8" return? The door is closed 99% of the time to keep the dogs out. Any help or suggestions would be appreciated.