The plenum is a term used for the part of the auto A/C system that is located under the dash of the vehicle and is made up of the air ducts, valves and blower assembly.

The plenum also includes air distribution doors that regulate the flow of air between the defrost, dash and floor vents. Additionally, the plenum includes the air blend door which regulates air flow over the heater or A/C evaporator.

Understand that the automotive A/C system is known as a series reheat system. That means all of the air that enters the vehicle first passes over the A/C evaporator, regardless of whether you are calling for heat or cooling. If controls are set for cooling, the air is then directed right into the vehicle. If the controls are set for heating, the air passes over the evaporator first and then is directed over the heater before entering the vehicle.

In cases where the controls are set for a mid range temperature, the blend door is set to direct some of the air from the evaporator directly into the vehicle while the balance passes over the heater. The blend of air temperatures allows for complete temperature control.