What type of duct system features a single trunk extending in one or two directions?

The extended plenum duct system is characterized by a single trunk duct that extends in one or two directions, serving multiple branches or outlets. This design allows for an even distribution of air throughout the conditioned space, ensuring that all areas receive adequate airflow. The extended plenum design is efficient for both heating and cooling systems, as it maintains a consistent pressure and temperature across the duct system.

In contrast, other duct systems have different configurations. The flex duct system typically uses flexible materials that can bend and be routed around obstacles, rather than relying on a singular trunk. The radial duct system has branches radiating out from a central point, which differs from the straight extension of the trunk seen in the extended plenum. Lastly, branch duct systems often involve multiple small ducts that split off from a main duct, rather than featuring a singular trunk line. Overall, the structure and airflow characteristics of the extended plenum duct system uniquely define it as the correct choice for this question.