A central heating or cooling system processes air by sucking it in the return vents and blowing it out the supply registers, which are strategically located in each room. The return vents are there to ensure that the system conditions air from inside the house -- not outside -- and to ensure indoor air pressure remains equalized.
Heating and Cooling Air Returns
The best location of the return vents depends on whether the system is heating or cooling the air. If it's cooling, the return vents should be near the ceiling, where the warm air collects, but if it's heating, the return vents should draw cool air from the floors. When designing a system that both heats and cools, it's necessary to split the difference, and the choice often hinges on whether it's more difficult to heat the house or to cool it.
Return Vent Maintenance
To ensure optimum pressure equalization and increase cooling efficiency -- especially with newer, more efficient HVAC systems -- it's better to place a return vent in every room than to rely on one or two centrally located ones. The return vents occasionally include filters that remove dust and particulate matter from the air and protect the HVAC system, as well as a grill that blocks dirt, fur and hair. These filters must be periodically cleaned or replaced, or the system starves for air and won't function properly. For the same reason, you should avoid placing furniture and other obstacles in front of them.