Here fan-powered variable air volume (VAV) system, each zone is conditioned by a box with a small fan that draws-in plenum return air and mixes it with supply air from a primary system. The primary system is similar to variable air volume (VAV), with the primary air supplied at a fixed temperature. transition to duct , control valve , union , flexible connector , control valve , strainer , get valve , round and rectangular duct transition to vav primary air connection , duct transition to vav secondary air connection , balancing cock When compared to a single duct terminal unit, the fan-powered versions have a fan that enables the induction of plenum air into the airflow. During heating, the supplied conditioned air can be reduced to the minimum ventilation rate and the heating airflow is supplemented by induced plenum air. for more details of Fan powered vav box download this file.