Final answer:
The plenum-mounted humidifier is installed in the supply plenum, which is responsible for distributing conditioned air in an HVAC system.
Step-by-step explanation:
The plenum-mounted humidifier is installed in the supply plenum. The supply plenum is a part of a heating, ventilation, and air conditioning (HVAC) system that distributes conditioned air to different areas of a building. A humidifier installed in the supply plenum adds moisture to the air before it is distributed throughout the building.