After 30 years, we are remodeling our home (alot of grandchildren). In the living room there is one cold air duct in the North side of the room (21' x 13" 9 1/2"). Just West of the living room is the front bedroom (15' x 9' 8"). In that room there is a cold air duct in the East wall right by the door. When we gutted the bedroom and living room, we found that we "could" have another cold air duct in the living room, if we use the one in the bedroom and use it in the living room too. Should you use one cold air duct for two rooms?

Leave all the original cold air ducts. The ductwork was designed for a specific amount of air to flow back to the furnace. If you move one you need to be sure the new duct will carry as much air as the old one.

If you add a grill to an existing duct, you will increase the heat in the room with the new grill at the expense of the other room. If the duct just goes down through floor in the wall cavity and dumps into a big duct below the floor, it may not hurt the bedroom much at all while making the living room perhaps a little warmer. Most air returns are near the center of the house and are fairly quickly collected into large ducts under the floor.