Dual duct systems condition all the air in a central air-conditioning unit and distribute it to the conditioned spaces through two parallel ducts, one duct carrying cold and the other, warm air. In each conditioned space or zone, a mixing valve mixes the warm and cold air in proper proportions to satisfy the load of the space.

Dual duct systems use more energy than single duct VAV systems and require more space for the ducts.

Note: These systems may be designed as constant volume or variable volume system.