"Location of returns should be towards the interior of the home. Air supplied at the perimeter cools or heats, and then returns towards the center. If you can cool or heat the perimeter, then you can cool or heat the home."
"Whether or not this return air duct is adequate depends on how many such returns were installed in the building and whether or not the return air capacity is balanced with the air handler and air supply ducting."
"During the heating season, the warmer indoor air rises up through the building and escapes at the top either through open windows, ventilation openings, or leakage. The rising warm air reduces the pressure in the base of the building, forcing cold air to infiltrate through either open doors, windows, or other openings and leakage. During the cooling season, the stack effect is reversed, but is typically weaker due to lower temperature differences."