In dual duct VAV system there is one duct loop for cooling (cold duct) and one duct loop for heating (hot duct). Each interior zone will be served by a single duct VAV box (Terminal Air Unit) which will be connected to the cold duct, as the interior zones have no exterior walls and there will be no heat loss, therefore they need only cooling in all seasons to remove the heat gain from the internal heat sources such as people, computers, printers, etc. Each exterior zone will be serving by a dual duct VAV box (Terminal Air Unit) which will be connected to the both cold and hot ducts as the exterior zones have heat gain in hot season trough the exterior walls & windows as well, therefore they need cooling and as they have heat loss in cold season trough the exterior walls & windows, therefore they need heating as well. The temperature in ach zone will be controlled by the zone dedicated thermostat. The air temperature in cold duct is about 55℉ and in hot duct about 90℉.