What is a Variable Air Volume (VAV) System?
A VAV system adjust the amount of air delivered by a fan to condition (heat or cool) space based on demand. This can be accomplished in either a single zone or multi-zone application.
In nearly all HVAC systems, the amount of heating and cooling required to maintain desired space conditions are variable. There are many factors that fluctuate impacting the heating and cooling load: envelop load (outdoor air temperatures and construction materials), solar load (sun position and shading), and internal loads (the number of people and their activity, the operation of heat producing equipment, lights, etc.). Of course, the system is sized for the peak (worst design case) heating and cooling load, but if the system were to operate at these peak capacities all the time, the space would be excessively heated or cooled. HVAC systems can address this concern in one of three ways: 1) Constant Air Volume / Constant Temperature, 2) Constant Air Volume / Variable Temperature, 3) Variable Air Volume / Constant Temperature. This definition focuses on the third and most preferred option, VAV, because of its many benefits.
How Does a VAV System Work?
Efficient VAV systems were made possible through the introduction of variable frequency drives (VFD) and have become the industry standard today. A VFD controls the speed of a fan altering the amount of air distributed. When a space experiences part-load conditions, rather than turning the system off or changing the delivery air temperature as done in a constant volume system, the VAV system reduces the amount air delivered to the space enabling it to save energy while still satisfying occupant comfort and ventilation needs.
Types of VAV
Variable volume systems can be classified into two distinct categories - single zone or multi-zone. Knowing the difference is important because they operate differently and are appropriate for different HVAC applications.
Single Zone
Single zone systems are controlled by a single thermostat. The systems are typically smaller because they consist of a single space or very few rooms. A single thermostat will only be able to monitor a single space meaning that the other spaces receive heating or cooling based on the conditions of the room with the thermostat. Consider a building with a single thermostat serving multiple rooms. Only the room with the thermostat is likely comfortable while rooms with more or less window area, different solar exposure, or more or less people, will not be. Since the heating or cooling occurs at the unit containing the fan, filters, and heating and cooling coils, the air volume and temperature is delivered in response to only the room conditions housing the thermostat. More thermostats do not resolve the issue because there is only one fan and a single heating and cooling source. Applying multiple thermostats and averaging the desired temperature or polling to determine the preference of the majority of the thermostats is an option, but ultimately results in a more complex and expensive system producing less than optimal comfort while consuming more energy. Rather than applying multiple thermostats, multiple single zone units are used to address the different thermal demands of a larger building. Using multiple units provides greater comfort and energy savings. This approach also reduces risk associated with failure because if one unit fails, only a small portion of the building will be affected rather than the entire building.
Multi-zone
Multi-zone systems have a central unit containing a fan, filters, and coils that delivers air to a building which has been split into multiples zones (rooms or small groups of rooms that experience common loads) each containing a thermostat (good zoning principles). The difference in this system configuration from the single zone VAV system described above is that the thermostat controls a damper and commonly a heating coil located at the zone level. The central unit containing the fan cools the air and delivers it to all the zones in the building. When the zone thermostat indicates cooling is needed, the zone level control damper (usually contained in a VAV terminal) is opened and cool air is delivered. As the cooling demand decreases, the zone level control damper is closed to reduce the amount of cool air delivered. When the space is occupied, the damper will have a minimum airflow set point to ensure adequate ventilation air is provided to the space (ventilation code requirements). If the space becomes too cool with the minimum air flow, the air is heated by a coil also contained in the VAV terminal. The temperature of the air is increased to maintain the desired thermostat setpoint. If the space temperature is still too cool, the damper is opened to increase the quantity of the warm air delivered to the space (energy code maximum reheat airflow). Each zone is operated independently in response to its zone thermostat. This allows some zones to be heated while others are cooled.
As each zone closes and opens the damper in its respective VAV terminal, the amount of air required to be delivered by the central unit to the system varies. The fan in the central unit utilizes a VFD to adjust the amount of air delivered based on demand from the zones (more supply air from the central unit is needed when the VAV terminal units are mostly open compared to when they are mostly closed). The response of the central fan to the variable demand of airflow allows fan energy to be conserved (energy code control requirements).
The central unit delivers cool air year-round, while heating is conducted at the zone level VAV terminal units. This is true even when outside conditions are cold. Although this may require the majority of the terminal unit heating coils to be active, there is typically at least one zone in a commercial building that requires cooling even on the coolest design day. This zone needs cooling year-round because it is located internally (no exterior exposure) and has heat generating loads (people, lights, and equipment). Reversing this design by providing hot air from the central unit and placing cooling coils at the zone level is not desirable for three reasons. First, cooling is the dominant function in most commercial buildings. Second, there will likely never be a situation in a commercial application that heating is needed in every zone simultaneously. Third, cooling coils in zone level VAV terminal units would introduce the need to address the resultant condensate water. The temperature of the air from the central unit, although always cooling, is reset to be slightly warmer during the winter operation to provide only the necessary cooling to meet demand. This increased cooling air temperature compared to the peak cooling design conditions decreases the amount of energy consumed heating the air at the zone.
Benefits of Variable Air Volume Systems
VAV systems enable simultaneous heating and cooling within the same building providing greater occupant control and comfort which is commonly a high priority in commercial building design. Since fans are the most significant consumer of energy in many HVAC systems, VAV Systems are the best solution for applications prioritizing comfort, reduced energy use, and sustainable design. The constant lower air temperature of this system is advantageous because it enables better dehumidification at part load conditions compared to a constant volume system. This is important because high humidity can result in decreased indoor air quality and introduce the potential for mold growth; not only decreasing comfort but possibly compromising occupant health.
A single zone VAV system’s greatest attribute is that it is a simple solution commonly utilizing packaged units with built-in preprogrammed controls. The simplicity of this system contributes to its being cost effective and easy to maintain. Although multiple units will need to be provided (one for each zone) in a large building, potentially increasing space required and distributing maintenance, the system inherently reduces risk associated with failure. A failure of a single unit will only affect the space(s) served by the unit, not multiple zones.
The multi-zone VAV system is a popular HVAC system solution because of its ability to provide a simultaneous heating and cooling within the same building using a single centralized unit. A centralized unit allows consolidated maintenance because it is a single large unit containing the fan, filters, and coils. This eliminates the need to interrupt or schedule maintenance around the occupant and reduces the required time of facility personnel compared to a distributed system. There are multiple VAV terminals throughout the building as they are located in each zone, but these are basic units requiring minimal maintenance.
Design and Installation Considerations
As with all HVAC systems, proper design and installation are essential for efficient and proper performance. There are two design considerations especially important to VAV systems – zoning and ventilation. Appropriate zoning or grouping of spaces and thermostat placement is key to ensuring occupant comfort and reducing energy use. This requires that factors such as building layout and orientation, occupancy schedules, and space use be considered when defining zones. Although meeting ventilation requirements is critical in all HVAC systems, there are considerations that are unique to VAV systems when ventilation is incorporated into its heating and cooling function. VAV systems reduce airflow to save energy when space heating/cooling demand decreases, but the amount of air delivered to a space cannot go to zero when it is occupied due to ventilation requirements. Calculations are necessary to establish the required ventilation and minimum airflow for each zone. In the case of a multi-zone VAV system, these calculations can be complex and laborious because of the constantly changing airflow at each zone in response to the thermostat. This complexity as well as the potential to further reduce energy savings, contributes to expanded use of dedicated outdoor air systems (DOAS) in conjunction with VAV.
Applications of VAV Systems
VAV systems are a popular HVAC solution due to their customizable thermal control providing enhanced occupant comfort while also prioritizing energy efficiency. VAV systems are most appropriate for applications with fluctuating loads because the system savings are the result of reduced air flow when the loads decrease. This encompasses a significant portion of the commercial building sector applications including but not limited to offices, schools, retail, and healthcare.
Routine Maintenance for VAV Systems
VAV systems like all HVAC systems require regular maintenance to ensure optimum performance and to extend the life of the system. VAV system maintenance is less complicated than other systems because the fan, filters and primary coils are contained in a unit located remotely from the occupied zone. This provides convenient and easy access for the facilities maintenance team without having to coordinate with occupancy schedules. The multi-zone VAV system is a bit more complex because the VAV terminals are located at the zone that contain a damper and a heating coil. These unit’s infrequent attention primarily consists of confirming that the damper and heating coil are functioning correctly and dusting off the heating coil. The multi-zone system also has the need to calibrate sensors that monitor the duct pressure and VAV terminal damper position to ensure the control of the fan is optimized. Implementing a computerized maintenance management system (CMMS) can help streamline maintenance tasks and further preserve the efficiency achieved in the initial design.
Common Issues and Solutions
As good as VAV systems are, there are some inherent issues that must be addressed especially in reference to the multi-zone system. A multi-zone system requires space be available for a larger centralized unit. Traditionally, this has meant consuming building square footage for a mechanical room to house the equipment (usually an air handling unit (AHU)). AAON has addressed this issue by developing a packaged roof top unit that can perform the task saving this interior space. Another drawback to a multi-zone VAV system is the complexity of the controls because of multiple pieces of equipment that must communicate with each other. This can lead to expensive customized controls. AAON has specifically developed unit incorporated controls to accommodate this application decreasing both the complexity and cost that can deter the use of multi-zone systems.
Technological Advancements Make for a Sustainable Solution
The introduction of the VFD has allowed VAV systems to not only provide high levels of occupant comfort but enables them to do so efficiently. Prior to the use of VFDs, variable air volume at the zone level was only possible by diverting the extra unwanted air produced by a constant volume fan away from the space (usually routed through a by-pass duct directly to the return duct). While this provided the desired space temperatures, it was a very inefficient and energy-consuming system configuration and function. The opportunity to make a VAV system efficient by applying a VFD was critical in enabling this technology to be an option when considering sustainable HVAC solutions.
VAV system efficiency has been further advanced though the incorporation of more sophisticated and advanced controls. These HVAC controls are commonly connected to a building automation system (BAS) allowing the system to not only monitor the HVAC function within the building but also the other building systems such as lighting, security, and fire alarm. Monitoring of other systems enables the HVAC system to make real time adjustments to save additional energy. An example of this is the ability to modify function and set points based on real time occupancy data. Prior to the system integration enabled by a BAS, the HVAC system was commonly controlled based on a schedule of occupancy that was not always accurate or requiring manual system overrides. With a BAS, information from the security system or the occupancy sensors used for lighting control can be used to confirm occupancy or lack thereof allowing zone thermostat set points to be adjusted or the amount of outdoor air to be modified without occupant intervention. This real time response enables appropriate operation when the space is occupied ensuring health and comfort while providing significant energy savings and reduced wear in equipment when the space is unoccupied.
Efficiency Tips for VAV Systems
VAV systems can be an efficient and sustainable HVAC solution, but it is important to design and operate the system correctly to reach optimum performance. The following are items to consider to achieve these efficiency targets.
Fan speed reduction with the use of VFD is critical, but the importance of fan selection cannot be overlooked. Selecting a fan to provide best efficiency over the entire system operation is unique to a VAV system because a spectrum of design flow conditions must be considered. Selecting a fan for constant flow system simply requires a single static pressure and air flow rate at peak conditions. This is not the case for a VAV system. The VAV fan selection must account for these same peak design conditions, but this represents a very small percentage of the actual fan operation. Most of the fan operational hours will be at part load requiring lower airflow. A high efficiency fan selected at high static pressure and high airflow does not transfer to a high efficiency fan at other static pressures or lower airflow. Therefore, it is essential that during the fan selection process the range of operating conditions be considered. The topic of fan performance is also addressed in many energy codes with the most recent requirements defining that fan efficiency conform with minimum fan energy index (FEI) values.
Direct digital control (DDC) systems used today to control HVAC systems are capable of monitoring multiple points (sensors and system components) simultaneously. In a multi-zone VAV system, the status of each zone can be individually checked and reported back to the central control system. This provides enhanced system efficiency compared to systems of the past that depended on a single static pressor sensor located in the duct to dictate the speed of the fan. Using a single VAV static pressor sensor often resulted in inaccurate information because the location of this sensor was incorrect to get a representative reading. The result was wasted energy due to a fan running more than necessary and uncertainty regarding adequate airflow at the zone level. The individual zone level input with DDC allows the system to optimize the air flow to the space with much greater confidence and accuracy ensuring the best energy savings at the central fan.
Multi-zone VAV systems should be designed with the supply air temperature set so when the system is at lowest part-load only a single VAV terminal remains in the wide-open positions (100% peak design flow). When one zone routinely prevents the system from reducing the fan airflow or increasing air temperature this should be monitored and corrected. This is especially important if the zone that is the culprit is not one where comfort is critical such as a storage space, corridor, or utility space. Increasing the supply air temperature from the central unit results in more fan energy to satisfy the zones requiring cooling, but this is offset by the reduced zone heating as well as the amount of cooling production required. Zones with constant load year-round (no external exposure or high internal loads) should be designed for the highest central supply air temperature to ensure they do not prevent system reset correction at building part-load conditions.
One of the greatest consumers of energy in an HVAC system is the conditioning of the outdoor air especially during the most extreme summer and winter design conditions. Multi-zone systems, if not correctly designed, will commonly bring in excess outdoor air increasing the system energy use. Automated monitoring of the required outdoor air at the zone level by the DDC system is a way to combat this waste. When programmed correctly, the DDC system can adjust the intake of outdoor air to the lowest acceptable value resulting in decreased heating and cooling. Incorporating a BAS system to confirm zone occupancy as part of the programming, further enhances the energy saving potential. Another factor that should not be overlooked as part of this design is the grouping of spaces or rooms that compose a zone. Making sure rooms within a zone have similar schedules of use and outdoor air requirements will also lead to greater energy savings.
Like all systems, VAV systems require good design, proper installation, and regular maintenance to provide best performance over the life of the system operation.
Conclusion
Variable Air Volume (VAV) systems offer numerous benefits, including improved energy efficiency, precise temperature control, and reduced energy costs. By understanding how VAV systems work and implementing proper design, installation, and maintenance practices, building owners and managers can optimize their HVAC systems for improved performance and efficiency.
FAQ Section
What is a VAV in HVAC?
A Variable Air Volume (VAV) system regulates airflow to optimize energy use and comfort in HVAC systems.
How does a VAV system differ from a CAV system?
VAV systems adjust airflow and temperature based on room requirements, unlike CAV systems, which maintain constant airflow.
What are the benefits of VAV systems?
VAV systems offer energy efficiency, better comfort through zoning, and scalability for various building sizes.
What are the challenges of using VAV systems?
Issues include potential humidity problems under low-load conditions, higher installation costs, and complex maintenance.
Where are VAV systems commonly used?
They are widely used in commercial buildings, residential setups, and industrial applications.