I’m having a problem with my cold air return. When the furnace isn’t running, cold air blows back through the return register. Much of the heat created by the furnace is lost when the cold flows back in. The house is a small single story on a slab, with the ducts overhead and the return in the hallway. The furnace is a 30 year old upflow model located in the garage. The question is, is there anything I can do to stop this cold air from coming in?