I have a pre-fab home with a forced air furnace in the basement and one cold air return in the dining room. A previous owner finished the basement and added a cold air return in the basement (about half the size of the upstairs one).
I notice that most of the cold air from upstairs simply flows down the stairs (which are only 10 ft away from the cold air duct). As a result my basement is usually cold.
Would there be anything wrong with simply closing the upstairs return and putting a grate directly on the furnace intake downstairs? Thus drawing all of my cold air out of the basement? It seems to me that this would do a much better job of circulating the heat.