VVT system is for using a constant volume rooftop heat pump unit to provide variable air volume system for multiple zones. In this system the supply air from the rooftop unit will be distributed to all zones, each zone will utilize a dedicated zone damper which can control the air volume for the zone by a dedicated thermostat in that zone. This system cannot provide cooling and heating at the same time, therefore all zones need to be in either cooling only or heating only, while in VAV system some zones can have cooling and some heating at the same time. The main control panel in VVT system will monitor the zones’ temperature and will change the system (heat pump) mode to cooling only or heating only if the majority of the zones request for cooling or heating. Since the rooftop unit is a constant volume system, if the zones’ total air demand changes from the maximum to a lower volume, the rooftop unit (heat pump) will not be able to vary the air flow. In order to solve this problem a bypass damper will be installed between the rooftop unit’s supply duct and return duct to bypass the extra supply air to the return duct and circulate through the unit. This procedure will be controlled by the main control panel in the system. This system is usually being used for the small places as using the VAV system will not be economical for a small place.
