Abstract: A plenum and diffuser for use with an HVAC system for distributing air through an opening in a ceiling, the plenum having a large diameter bottom opening by which air is passed through an opening in the ceiling, and having an air inlet opening. A hanger bar extends through the plenum housing over the large diameter opening. A junction box is secured to the hanger bar for supporting a light fixture or ceiling fan. A diffuser is in the form of a planar, circular structure of thin, rigid material having a circumferential outer edge and a central opening dimensioned to accommodate a junction box. The diffuser has a plurality of radially extending fins and a radially extending air slot opening in conjunction with each fin.