This system is particularly applicable to small to medium size commercial buildings which are divided into multiple use areas with different space or comfort requirements. It uses a single duct with variable volume (individual space control) and variable temperature (heating and cooling) air. The system allows for a constant volume single zone heating and cooling unit to be controlled from more than one thermostat. It can provide simultaneous heating and cooling by alternating the unit operation on a ́time shareî basis. VVT systems are often selected due to their high degree of individual space control and the cost effectiveness of packaged equipment.