A Variable Air Volume (VAV) system is one of the most common HVAC systems used in buildings. The basic concept is that the volume of the air entering the thermal zone changes as the demand for heating or cooling changes, using a variable speed/frequency drive on the supply air fan, while maintaining minimum ventilation requirements. Some of advantages of a VAV system include:
a) Better temperature control
b) Better humidity control
c) Lower system fan energy
d) Less fan noise
Fundamentals of a VAV System
Since the volume of air entering the space is controlled by a fan, we can see that two of the advantages of having a VAV system are directly related to the fan. This has been further explained in the figure below, where we compare the VAV system with a Constant Air Volume system (CAV).
Figure 1: VAV vs CAV systems
From the figure above, we can see:
a) In the “Volume Flow vs Power” section, the CAV is providing air at its design flow rate capacity, whereas in a VAV system the airflow fluctuates between the design flow rate and the VAV minimum flow rate (VAV turndown). As a result, the VAV fan uses less energy than a CAV fan. This is due to the fan laws, or affinity laws, which explains the cubic relationship between fan power and flow rate.
b) Another example of the fan laws is in the “Fan performance curve” section where the fan power is varying with the flow rate for VAV system. The flow rate of the fan varies using a Variable speed/frequency drive (VFD/VSD). In a VSD/VFD system, the speed of the fan modulates to control the airflow into the zone(s).
A frequently asked question about a VAV system is, “What is an acceptable VAV turndown?” Typical VAV turndowns may be between 20% (California’s Title 24) and 30% (recommended by ASHRAE). However, some studies suggest that a VAV system can have a turndown of as low as 10%. The VAV system shown in Figure 1 has a turndown of 20%.
Multizone VAV System Operation in IESVE Software
Multizone operation been explained in detail in the “ApacheHVAC Controllers: Example of VAV controllers” whitepaper. This white paper explains how a Typical VAV operation (Figure 2) is setup in ApacheHVAC.
Figure 2: Typical VAV Operation cooling setpoint of 75 °F (23.89 °C) and heating setpoint of 69 °F (26.56 °C)
In addition, this on-demand learning video HERE explains how the VAV control strategy is implemented in ApacheHVAC.