What are Dual Duct Variable Air Volume (DD-VAV) systems?

When there are two air handlers: one provides central cooling and ventilation to multiple zones through a single duct run and the other provides central heating to the same zones through a separate (second) duct run. In some cases, the air handlers share a common fan.

Each zone has two dampers, which are controlled by separate actuators. So either the hot or cold deck are open, not both.