Both single-duct and fan-powered VAV terminal units have the same purpose, to provide heating, cooling, and ventilation to the temperature zone (group of spaces controlled by a single zone temperature sensor) that each VAV terminal unit serves.
Physical Characteristics
Primary Air Inlet Single-duct and fan-powered VAV terminal units have one (typically round) primary air inlet
Primary Air Damper and Controller The primary airflow for single-duct and fan-powered VAV terminal units is modulated by an automatic damper mounted within the primary air inlet. The primary air damper actuator is typically electric and is controlled by a direct digital control (DDC) system controller mounted on the VAV terminal unit.The primary air damper actuator and controller are normally housed within a terminal box mounted on the outside of the VAV terminal unit
Inlet Velocity Sensor The primary air inlet is equipped with a velocity sensor consisting of multiple pitot tubes that constantly measure the total pressure and static pressure of the primary airflow at multiple locations across the primary air inlet. This type of sensor is called a multipoint averaging sensor because the sensor averages all of the total pressure measurements and all of the static pressure measurements of the pitot tubes. The velocity pressure, which is the difference between the average total pressure and average static pressure, is read at the controller mounted on the VAV terminal unit. The controller is connected to the inlet velocity sensor by two tubes, one for the average total pressure and one for the average static pressure measured by the inlet velocity
sensor. From the velocity pressure, the controller is able to continuously calculate the primary airflow (in cfm). The inlet velocity sensor ensures that the airflow called for by the space temperature sensor is what is actually delivered by the VAV terminal unit (regardless of the static pressure within the primary air duct system) and also ensures that the primary airflow delivered by the VAV terminal unit remains within the preset minimum and maximum primary airflows for the VAV terminal unit.
Outlet Single-duct and fan-powered VAV terminal units have one rectangular outlet to which the downstream ductwork is connected.
Heating Coil A hot water or electric heating coil can be specified as an accessory to a single-duct or fan-powered VAV terminal unit if heating is required by the zone.
Terminal Box The primary air damper actuator and the unit controller for both singleduct
and fan-powered VAV terminal units are normally housed within a terminal box
mounted on the side of the terminal unit.
