What type of air distribution system has the main trunk reducing in size as branches distribute heat to different rooms?

The correct answer is the reducing extended plenum system. In this type of air distribution system, the main trunk duct starts at a larger size and decreases in dimension as it extends through the building. This design is specifically optimized for delivering heated or cooled air to various branches that lead to individual rooms or areas.

The reducing nature of the trunk duct allows for more efficient airflow management as the volume of air required diminishes along its length, which helps maintain consistent pressure and reduce energy loss within the system. The branches distribute conditioned air effectively, ensuring that each room receives an adequate flow of air based on its specific needs.

This method contrasts with other systems where uniform trunk sizes may not account for varying air distribution requirements, which can lead to inefficiencies either in airflow or temperature control. The reducing extended plenum's design reflects a thoughtful architectural approach to air distribution, promoting efficiency and comfort throughout the space.