In this article, we would like to share about our experience in designing HVAC system. Between HVAC professionals, especially in designing an HVAC system of a building, it is common to discuss about VAV. What is a VAV? variable air volume (VAV) systems are a type of HVAC (Heating, Ventilation, and Air Conditioning) system that effectively regulate and control how air is delivered to different spaces in a building. Unlike traditional constant air volume (CAV) systems, VAV systems can adjust the amount of air they supply based on the heating or cooling requirements of each zone.
If a particular room requires less cooling, the VAV system will reduce the airflow, which helps save energy. Typically, each zone has a thermostat that measures the temperature. When the room temperature goes above or below the set point, the VAV system adjusts the airflow to help maintain comfort.
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Components of Variable Air Volume Systems
Commercial buildings, offices, schools, and other large facilities often use VAV systems to match airflow rates with changing cooling or heating loads. These systems consist of several parts that work together to distribute air efficiently and on demand.
VAV Box
A VAV box is a terminal unit that controls the volume of air entering a particular zone. It has dampers that adjust airflow based on the zone’s temperature needs.
Many modern VAV-based HVAC systems include this key component to provide flexible and efficient airflow control for various areas.
The VAV box changes airflow to a space by adjusting its damper position. If the zone needs more cooling, it opens the damper wider; if less cooling is needed, it closes it further. A thermostat in the zone signals the VAV box to increase or decrease airflow.
Inside a VAV box, you will find dampers (which can be motorized or spring-loaded) and actuators. Actuators can be pneumatic or electronic, and they move the dampers according to signals from the building management system or the thermostat. Some VAV boxes also have airflow sensors to ensure accurate air delivery.
Dampers
Dampers help achieve energy savings by reducing airflow when demand is low. They adjust automatically to maintain the desired temperature in each zone. By working with the thermostats, the dampers keep room conditions comfortable without wasting energy.
Air Handling Units
An air handling unit (AHU) is the central piece of equipment that heats or cools the air and then pushes it through ductwork to the VAV boxes. Typically, it is located in a central mechanical room or on the rooftop, depending on the building design.
AHUs also clean the air by filtering out contaminants, and they can change humidity levels by adding or removing moisture. By using coils or heaters, the AHU can adjust the supply air temperature to match the requirements of each zone.
Using Heat Pumps with Variable Air Volume Systems
While there are many VAV systems rely on boilers and chillers for heating and cooling, a heat pump can also be used. In that case, the heat pump would replace or supplement traditional equipment to supply warm or cool air to the AHU. By reversing its refrigerant cycle, a heat pump can provide both heating and cooling through the same device.
When paired with a VAV system, a heat pump helps to manage energy consumption since it can operate efficiently across different load conditions. The VAV boxes still modulate airflow in each zone, but the source of heating or cooling would come from the heat pump instead of (or in addition to) boilers and chillers. This integration can be especially beneficial if the building is in a region with moderate heating and cooling demands.
Types of Variable Air Volume Systems
VAV systems are well-known in HVAC for adjusting air volume based on real-time demand. Different types are available, each with its own benefits.
Single Duct VAV System
This is the most common type. It uses one duct to deliver conditioned air to multiple zones. Each zone has its own VAV terminal unit that regulates airflow according to that zone’s temperature setting. This design is popular in offices, schools, and hospitals.
Dual Duct VAV System
This system has two ducts: one carrying hot air and the other carrying cold air. A mixer box in each zone blends the two air streams to reach the desired temperature. Although it can be more complex and use more energy, it provides precise temperature control, which can be essential in certain applications.
Reheat VAV
In this system, air can be reheated before it enters a zone. The VAV terminal unit has a reheat coil, which warms the air if the zone still needs some heat even when the main system is providing mostly cool air. This is useful in spaces that require airflow but not a lot of cooling at certain times.
Fan-Powered VAV System
A fan-powered VAV system features a fan in the terminal unit to supply additional airflow when necessary. There are two main types:
- Series Fan-Powered VAV: The fan runs continuously in series with the main airflow.
- Parallel Fan-Powered VAV: The fan typically stays off during cooling and operates only when extra airflow or heating is required.
Thermostatic VAV System
This type uses a simple thermostatic valve to control airflow. A thermostat in the zone adjusts the amount of air entering the space based on the temperature setting. It is a simpler, sometimes cheaper, approach for buildings that still need some level of precise temperature control.
Decentralized Control VAV
This setup gives each zone its own controller, allowing very flexible control of temperature and airflow. Spaces like conference rooms or labs often need localized control, making decentralized VAV a good option.
Conclusion
Variable air volume systems provide significant energy savings and comfort compared to constant air volume designs. By tailoring airflow to each zone’s real-time needs, VAV systems help maintain ideal conditions in diverse building environments.