In single duct VAV system there is only one duct loop for cooling (cold duct). Each interior and exterior zones will be served by a single duct VAV box (Terminal Air Unit) which will be connected to the cold duct. 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. The exterior zones have heat loss trough the exterior walls & windows in cold season, therefore they need heating. Heating for exterior zones can be provided by the electric re-heat coil built in the VAV box by the manufacture. The temperature in ach zone will be controlled by the zone dedicated thermostat. The air temperature in cold duct is about 55℉ and the electric coil will warm up the cold air from 55℉ to about 90℉ for heating mode. The minimum air flow is required by the manufacturer to pass through the electric coil at all the time to avoid the risk of fire.