Which of the following can be included in a duct system?

Including ducts, dampers, and fans in a duct system is correct because each component plays a vital role in the airflow and ventilation within a building. Ducts serve as the primary pathways for conditioned air to travel from the air handler to the intended spaces. Dampers are essential for regulating airflow, allowing for the control of air volume and direction within the ducts, which helps maintain comfort and efficiency in heating or cooling systems. Fans are crucial for moving the air through the ducts, providing the necessary pressure and flow to ensure that the air reaches its destinations effectively.

Other options focus only on a single component, which would not represent a complete and functional duct system. A system made up of only ducts would lack the necessary mechanisms to regulate and control airflow, while a system that includes only fittings or air handlers would not have the means to deliver air throughout the space. Therefore, the inclusion of ducts, dampers, and fans provides a comprehensive overview of a functioning duct system that supports effective HVAC operation.