Dual Duct VAV boxes are used mostly in hospitals and clinics were the air must be at a constant flow. When the space is calling for full cooling the cold deck damper should go open to its max design cfm while the hot deck damper goes closed. The opposite happens when the space is calling for full heating. What makes this box so unique is when the space temperature is satisfied the two dampers will mix its hot and cold air to produce the constant design flow without over heating or cooling the space. When balancing a dual duct box balance both hot and cold sides as if they were two separate single duct VAV's.
This is done by closing the hot deck damper while calibrating the cold deck side and then doing the opposite when calibrating the hot deck. The following is a list of pro's and con's for this type box:
Keeps a constant airflow thru out the space.
Will not allow air to get stagnant or stale.
Allows more airflow to be filtered through main air handling unit.
Brings in a more constant amount of fresh air (outside air).
The Air Handling Unit will need to produce hot and cold air all year.
A boiler and chiller will have to be ran all year.
Two main ducts from the Air Handler will have to be ran throught the building to supply both hot and cold air, needing more above ceiling space.
