A ceiling plenum is the pressurized space above a dropped ceiling that is used for the distribution of conditioned air or as a path for return air. In commercial buildings, this space often replaces individual return ducts, allowing the entire area above the tiles to act as one large air chamber. When installing Multi-Zone Ceiling Cassettes, the plenum must be properly sealed to ensure that air is not lost to the building's structure. It is also a common location for installing Fans and other air-moving equipment. Using a plenum for air return can save on material costs, but it requires that all wires and pipes within that space be "plenum-rated" for fire safety. For residential Ductless Mini Splits, the plenum is where the indoor unit connects to the supply and return lines. Proper management of this space is essential for maintaining the overall air quality of a Commercial HVAC environment. It is a key architectural feature in modern building design.