Which type of system's air passages are not classified as air ducts?

Self-contained systems have unique characteristics that distinguish them from other types of HVAC systems. In a self-contained system, air passages are typically integrated within the unit itself and often consist of internal components rather than external ductwork like those found in traditional ducted systems. For example, a window air conditioning unit or a portable heater may have air being drawn in and expelled without relying on extensive ductwork. This is in contrast to heating, ventilation, or cooling systems that typically work with a network of ducting to distribute air throughout the space.

Understanding this distinction is essential in mechanical systems design and implementation, as it helps in correctly identifying system types and ensuring compliance with relevant regulations. In other systems—like heating, ventilation, and cooling—the air passages are designed specifically to function as a system of ducts for effective distribution, thus confirming that self-contained systems do not utilize separate air ducts.