Variable Air Volume (VAV) systems are the most widely used types of HVAC air systems for medium- and large-sized commercial building projects (projects larger than 10,000 ft2) because VAV systems are flexible, energy efficient, and provide a comfortable indoor environment. VAV systems deliver variable supply airflow at a constant temperature (typically 55°F) through the primary air duct to multiple VAV terminal units, each of which serves a separate temperature zone. Each VAV terminal unit contains a motor-operated damper that modulates the primary airflow to the zone, an inlet airflow sensor, and, in some instances, a heating coil and a small recirculating air fan.
The supply airflow from the VAV air handling unit is normally modulamd to maintain a constant static pressure within the primary air duct system. This is measured by a duct static pressure sensor, which is typically located two-thirds the way down the primary air duct system. The modulation of the supply airflow traclcs with the needs of the VAV termirutl units; that is, as more primary air dampers in the VAV terminal units open to supply more air to the zones, the static pressure in the primary air duct system decreases and the fan speed in the VAV air handling unit increases to increase the supply airflow delivered by the VAV air handling unit to restore the static pressure in the duct system. Conversely, as the primary air dampers close, the primary air duct static pressure increases and the fan speed in the VAV air handling unit decreases to decrease the supply airflow delivered by the VAV air handling unit to compensate.
The air handling unit for a VAV system is the same as would be required for a CAV system with the exception that there is a means of modulating the supply airflow delivered by the unit. The most common way of modulating the supply airflow of the unit is by controlling the frequency of the signal sent to the supply fan motor through a variable frequency drive (VFD).
The speed of an alternating current (ac) motor is directly proportional to the frequency of the input signal to the motor. Therefore, as the frequency of the VFD output signal to the motor is reduced, motor speed is reduced, and supply airflow is also reduced. The converse is true as the frequency of the VFD output signal to the motor is increased. The maximum recommended frequency of the VFD output signal is that of the VFD line input frequency, or 60 Hz. The supply fan motor will operate at full speed when it receives a VFD output signal of 60 Hz. VFDs can deliver frequencies higher than 60 Hz, but this causes the fan motor to operate above its rated running load amps [i.e., the motor operates in its service factor range (typically between 100 and 115% of the rated running load amps), which is not recommended].
Care should be taken when designing VAV systems that utilize DX refrigerant coils for cooling. Unless the refrigeration system is equipped with adequate capability to unload the refrigeration system capacity, freeze-up of the DX refrigerant cooling coil could occur under low airflow conditions. Unloading of the refrigeration system, adequate control of the discharge air temperature, and incorporation of a VFD into the unit cabinet are all issues that HVAC equipment manufacturers have recently resolved for DX equipment having capacities less than 25 tons. HVAC equipment manufacturers now offer VAV operation for air systems that utilize DX refrigerant cooling coils in capacities as low as 5 tons.
The most common use of a VAV system is in serving multiple temperature zones. Therefore, we will discuss multiple-zone VAV systems first and then discuss the use of a VAV system for a single-zone application.
VAV Terminal Units
Variable primary airflow is delivered to the zones through the modulation of the primary air damper in the VAV terminal units. As the zone temperature decreases, the primary air damper is modulated closed to supply less (55°F) primary air to the zone. Once the primary air damper reaches its predetermined minimum position (usually about 25% of maximum airflow), upon a further drop in the zone temperature, VAV terminal units that have heating capabilities will position the primary air damper to the heating airflow and modulate the output of the heating coil to maintain the heating setpoint of the zone temperature sensor. Fan-powered VAV terminal units are also equipped with a small fan that recirculates air (normally from the ceiling return air plenum) through the heating coil of the VAV terminal unit.
Figure 1 is a schematic diagram of a VAV system serving multiple VAV terminal units.
Dual-duct VAV air systems are about as uncommon as dual-duct CAV air systems. The most likely time an HVAC system designer would encounter a dual-duct VAV air system would be in the case of a system that was originally designed as a dualduct CAV system but was later renovated to function as a VAV system. The HVAC system designer may also have the task of designing the modifications to a dual-duct CAV system to convert it to a dual-duct VAV system.
Dual-duct VAV systems function much the same way as dual-duct CAV systems except the supply airflow to the zones is variable, not constant. The supply fan in the dual-duct air handling unit has to be equipped with a means to modulate its airflow in response to static pressure in both the main hot and cold ducts. The energy efficiency of a dual-duct VAV system would be about the same as that of a conventional VAV system utilizing VAV terminal units.
Care should be taken in converting a dual-duct CAV system to a VAV system to ensure that the zones do not require constant supply airflow to serve as makeup for constant exhaust airflow.
Operation of a single-zone VAV system is similar to the operation of a VAV system serving multiple zones, except there are no VAV terminal units and the supply airflow is modulated to maintain the cooling setpoint of the (single) zone temperature sensor rather than to maintain a constant primary air duct static pressure. The supply air tempemture is maintained at 55°F as long as the zone temperature sensor is calling for cooling.
Once the zone temperature drops below the cooling setpoint (typically 75°F), the air handling unit will operate in the heating mode: cooling will be disabled, the supply fan will operate at the predetermined heating airflow, and the output of the heating coil within the unit will be modulated as required to maintain the heating setpoint of the zone temperature sensor (typically 70°F).
Once the zone temperature rises above the cooling setpoint, the air handling unit will operate in the cooling mode: heating will be disabled, the output of the cooling coil will be modulated as required to maintain the supply air temperature at 55°F, and the supply airflow will be modulated to maintain the cooling setpoint of the zone temperature sensor.
The advantage of a single-zone VAV over a single-zone CAV system is that during cooling operation, the supply air temperature will remain constant at approximately 55°F. This consistently cool supply air temperature will result in a lower space relative humidity than the same area served by a CAV system where the supply air temperature can vary anywhere between 55°F (full cooling load) and 75°F (no cooling load). The higher space relative humidity resulting from the use of a CAV system is exacerbated by outdoor air ventilation in moist climate zones and by a high density of occupants in the areas served by the unit.
Due to recent advances in technology, single-zone VAV systems that utilize DX refrigerant cooling coils can serve areas with a cooling load as low as 5 tons.
HVAC Design Sourcebook - W. Larsen Angel, P.E., LEED AP, is a principal in the MEP consulting engineering firm Green Building Energy Engineers. He has worked in the MEP consulting engineering industry for more than 30 years. Mr. Angel has contributed to the development of design standards and continues to find new ways to streamline the HVAC system design process.