What is defined as a plenum in relation to HVAC systems?

A plenum in the context of HVAC systems refers to a separate space that facilitates the circulation of air within the system. It is typically used to distribute both supply air and return air in a building's heating, ventilation, and air conditioning system. In essence, the plenum acts as a central hub through which air moves, connecting various ducts that lead to different rooms or areas.

The function of a plenum is crucial for maintaining efficient airflow, regulating temperature, and ensuring that conditioned air is evenly distributed throughout a structure. This is why option B is the correct answer. Understanding the role of a plenum helps in designing and maintaining effective HVAC systems, particularly in terms of ensuring adequate air exchange and improving overall air quality within indoor environments.