How to fix it--- you may need to check to see, make your best guess, where is the hollow return passage going to. And follow it until you found any open spaces, where you can see the covered joist, 2x4, Stud cavities, in most case, builder used only thin gage sheet metal to cover the joist cavity. You can use duct tape seal all the gap where sheet metal and joist nailed together. (Actually. down in your basement you may be able to spot the thin sheet metal covered return passage)
How big the family room is?
Any windows, door?
How many floor air registers?
From my opinion, it is not a problem to cover the air return outlet any time, because the floor air register will create the air pressure, circulates the air in the family room. When blower fan is running.
How big the family room is?
Any windows, door?
How many floor air registers?
From my opinion, it is not a problem to cover the air return outlet any time, because the floor air register will create the air pressure, circulates the air in the family room. When blower fan is running.