A warm-air supply duct that runs _____ from the furnace plenum to a riser is called a leader.

The term "leader" specifically describes a warm-air supply duct that is installed in a horizontal layout from the furnace plenum to a riser. This design allows for the efficient distribution of warm air through the ducts to different areas of a building. Horizontal ducts can easily accommodate the layout and airflow needed for effective heating, ensuring balanced temperature throughout the space.

In contrast, other orientations like vertical or diagonal ducts may be used in different contexts, but they do not align with the definition of a leader in this specific application of HVAC systems. Curved ducts might be utilized in some systems to navigate around obstacles, but they wouldn't typically be identified as leaders. Understanding this terminology is crucial for comprehending how HVAC systems are configured and how they function effectively.