A plenum is an unconditioned area above a conditioned space that holds ductwork returning air from the space to the coil. Plenums are special because they contribute both to the space load and to the equipment load. A plenum receives heat from several sources: conduction through its walls and roofs, the lighting from the space below, and heat from the return air. This causes the plenum to raise itself to a temperature higher than its adjacent spaces, and so conduction is transferred to those spaces. Similarly, this heat contributes to a load that the equipment must take in to account as the return air is circulated back.
Within the engine itself, the plenum values are highly dependent on the airflow (the CFM) coursing through the system. Therefore, most of the other loads and psychrometrics must be calculated first, then the plenum values, and then the psychrometrics a second time to take into account this additional load.