Up until now, I've left the filter box on the back of my add-on furnace open and just pulled basement air. Lately I've been thinking about hooking the add-on furnace to my existing cold air return. My question is regarding backdraft dampers. Do I really need on in the way of the NG furnace. Attached is a picture of the current duct work. My thought is to just put a 12" return in the overhead duct work. My concern with not doing a damper is that I don't want to pull warm air down over my AC coils, short cycle my system, and not get heat upstairs. (I already have dampers in the supply pipes so that my AC doesn't cool the basement).