Both supply and return vents may need to be replaced to ensure your duct work system is working at its best in heating/cooling your home. It's important to understand that when your AC is on during those hot L.A. summers, it's not only blowing cool air through the ducts and into each room, but it's also drawing air out of each room at the same time. You can't neglect either type of vent - supply vents that bring the conditioned air and return vents that keep the air circulating can both get dirty and may need to be replaced. Return vents and supply vents are both covered openings in your walls, but the return vents are generally quite a bit larger. Air moves in faster but out slower, in through a small vent but out through a large one. This keeps things balanced and keeps the air temperatures as even as possible throughout a room. It often happens that either a supply or return vent is so dirty as to be "clogged" like a slow-moving drain. Or, it's also common for homes to simply lack a sufficient number of these vents or the right kind for optimal performance. Your airflow and comfort level, as well as your energy bill and environmental footprint, all depend on proper ventilation, so it's well worthwhile to clean, repair, or replace all problem vents and duct work.