There are two problems with this type install, which is vey common in Northwest Indiana. One is easily fixed the second is rarely ever addressed. The sketch on the right represents the cross section of an outside wall, a first floor cieling, and a second story floor. The larger arrow is the airpath for the hvac system return air, this would be the same even if your returns are high on the wall. The two smaller arrows represent infiltration through the rim joist. When the furnace blower starts it moves air through the house, it also draws air from the house. The return shown on the floor is one of the paths back to the furnace blower. The joist bay shown will go into negative pressure whenever the blower starts, this negative pressure, or slight vacuum is what draws the air. Because the rim joist is not sealed air is easily drawn or blown through the cracks. This is one of the most common causes of infiltration. That cold air will constantly seep into that joist bay causing cold floors and uneven system performance. If your wondering how serious this can be, It is enough to freeze water lines in the middle of the house. The first step in preventing this is a proper air barrier, Tyvek, and insulation in the rim joist. That is an easy fix. The second would be to have all return air ran through duct instead of house cavities. I can only think of one home that was setup that way due to the extra cost.