I'm guessing that you have a n older house made before A/C or otherwise
doesn't have proper ducting for A/C. In order to get rid of the hot
air, you need a return air in an area where it's hot. If your lowest
return air grill in near the bottom of an upper floow (i.e. in the
floor itself or low on the wall) then you will need a return air placed
higher to draw in the hot air instead or recirculating the already
cooled air.