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Variable Air Volume (VAV) Systems

Variable Air Volume Air Duct Diffuser

Have you ever wondered why some buildings can have multiple heating and cooling zones? Most buildings utilize a variable air volume (VAV) system that varies airflow into different rooms at a constant temperature. The system can temper the air for better heating and cooling within each room. However, before VAV systems, constant air volume (CAV) systems used to supply tempered air into different rooms and were often inefficient. Each room in a CAV system always receives the same air volume flow rate regardless of the room size, so a smaller room can be receiving as much air volume flow rate as much as a larger room. Thus, most building designs prefer VAV systems to CAV systems.

Another problem with a CAV system is that it is usually a single thermal zone. All rooms connected to it will receive the same temperature air even though the temperature load may be different. A room full of people and an empty room of the same size will receive the same volume of tempered air for cooling. It may be possible to have a multizone CAV system with the installation of reheaters or another air handler, but it can be costly.

CAV systems are more common in smaller buildings because they are cheaper to install and simpler to operate than a VAV system. Yet, the energy savings provided from a VAV system may be more economical and provide better comfort in the end.

How does a VAV system work? To keep it simple, a VAV system comprises of the following:

1. Air handler

2. Ductwork

3. VAV box/terminal

An air handler provides the air to the rooms through the ductwork into a VAV box that controls the amount of air volume flow and reheats the air (if a reheater is present) to meet the temperature requirement. The air leaves the VAV box and into the room. Finally, the air returns to the air handler or outside, and the process repeats.

Generally, inside a VAV box consists of the following:

1. Airflow sensor – It senses air pressure in the VAV box and calculates the airflow rate in the system

2. Damper – A plate that stops or regulates the flow of air inside the duct

3. Actuator – This device rotates the damper

4. Controller – Controls the actuator according to the airflow sensor and thermostat

5. Reheater – Heats the air to meet the temperature requirement

6. Temperature sensor for pre-reheated air and post-reheated air – measures the air temperature before and after the air passes the reheater.

Over pressurization in the ducts becomes a concern as dampers close in the VAV system. A variable frequency drive (VFD) controlled fan in an air handler is used to increase or decrease the fan speed as the dampers open and close. The fans speed up as more dampers open and slow down as more dampers close.

VAV systems provide many benefits such as energy savings, reduced wear, precise temperature control, and increased dehumidification. These are reasons to take into consideration when designing a building.

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