Return registers are placed high on interior walls because they draw air efficiently from the room into the system while keeping the return path inside conditioned space. The air in rooms tends to rise, so a high-wall location captures a large portion of the air without being affected by the coolest air that sits near the floor or by drafts near doors and windows. This position also helps keep the return away from exterior leaks and outdoor intrusion, supporting stable temperature control and better filtration. Placing a return on the floor near an entry can create drafts and pull in air that isn’t well mixed, reducing comfort and energy efficiency. A return on an exterior wall risks pulling in outdoor air through cracks and leaks, wasting energy. Ceiling plenums are used in some commercial designs, but for typical residential systems the interior-high-wall location remains the standard and simplest to install and maintain.