First of all there is no cold air return, it is called supply air to the furnace. If your hot air is in the floor around the premature of the home. To properly mix the air, and balance the temp. from hi to low for both heating and cooling, you should take 80% return air from ceiling or hi in sidewall, and 20% from floor or low in the side wall. This way you will not get cold air drafts across the floor and you move that hotter air that is up around the ceiling .