There will always be a problem with heating a room whose hot- and cold-air vents are located on the ceiling. The hot air will descend from the vent and almost immediately rise again. As a result, cold air will sit at the bottom of the room. What you have to do is install a cold-air return vent on the floor. It can be anywhere in the room as long as it is as far away from the hot-air vents as possible, and one vent will be enough to solve the problem. The cold-air return will act as a vacuum and suck all the cold air from the room and send it to the furnace. The cold air will disappear from the room and as a result will pull some of the hot air downward.