Which of the following items is NOT permitted in a return air plenum?

In the context of HVAC systems, a return air plenum is an important component that helps distribute air back to the air handler in a system. The items commonly found in a return air plenum must adhere to specific building codes and safety standards to ensure efficiency and safety.

Pneumatic tubing is typically not permitted in a return air plenum because it can introduce contaminants into the airflow and is not designed to handle air circulation like other components. Additionally, pneumatic systems use pressurized air which can lead to leaks if not properly maintained, potentially compromising the air quality in the duct system.

In contrast, flexible ductwork, aluminum foil, and air filters are common components used within a return air plenum. Flexible ductwork is specifically designed for air movement and can accommodate various configurations in a space. Aluminum foil is often used for insulation and sealing, ensuring that temperatures are maintained and air leaks are minimized. Air filters are crucial for removing particulate matter from the air before it's recirculated, thus improving indoor air quality.

Understanding the roles and compatibility of each of these components in HVAC systems is essential for ensuring efficiency and compliance with building codes.