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Variable Air Volume Systems

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In some mechanical designs, you will find a Variable Air Volume (VAV) system. This is the opposite of a Constant Air Volume (CAV) system. A VAV system will condition (heat or cool) a space by varying the airflow and using a constant temperature, while a CAV system conditions a space with a constant air flow and varying temperature. An Air Handling Unit (AHU) supplies the primary air to the VAV at a constant temperature. The VAV(s) then serves a specific zone or room that can be independently controlled from other VAV(s) serving different zones or rooms. For example, if one zone is by an exterior door in the winter, that area may be kept warmer than a zone in the interior of that building. A VAV system can control those two spaces independently to ensure a space is comfortable for all occupants. Properly calibrating a VAV is key to ensuring the system is operating per design.

What Are the Main Types of Fan-Powered VAVs?

Series and parallel are the two main types of fan-powered VAV boxes. For either type of VAV system, there are four main components:

  • The VAV box
  • A temperature sensor
  • A flow sensor
  • A path for pressure relief

The temperature sensor or thermostat is the primary controller of the system. It modulates the primary air damper or actuator based on the airflow demand for the space. The flow sensor or pressure hose is used to maintain the designed airflow. It is extremely important to CALIBRATE this sensor.  This sensor ensures that the VAV is reading airflow accurately. Lastly, if all VAVs connected to a system are closed, the airflow needs a pathway to relieve the pressure in the system. The differences between the two VAVs can be seen below.

Parallel VAV Box

Parallel Variable Air Volume Systems Box

  • Two air streams
  • Primary air from the AHU bypasses the blower section
  • Typically, the fan only runs on a call for heat
    • Backdraft damper separates two air streams when the fan is off

Series VAV Box

Series Variable Air Volume Systems Box

  • Single path for airflow
  • Primary air from the AHU delivered through the blower section
  • Fan runs continuously (KEY FACTOR)
    • Backdraft damper is not necessary

How Does a Melink Technician Balance a Variable Air Volume System?

Different VAV boxes will have different balance procedures. The most important aspect of balancing a VAV box is ensuring they are calibrated. When balancing a series VAV box, follow the steps below:

  1. Set the terminal to the design max cooling.
  2. Set the fan speed to design airflow by measuring the discharge airflow.
  3. Adjust the primary air damper or return damper to achieve a neutral condition where the discharge airflow is equal to the primary. (This is an important aspect when balancing a series box.)
  4. Calibrate the controller.
  5. Balance the connected load of the VAV box and verify the minimum and heating airflow.

The steps taken to balance a parallel VAV box are similar to balancing a series VAV, but the main difference is you do not need to achieve a neutral condition. When balancing a parallel VAV box, follow the steps below:

  1. Set the terminal to the design max cooling airflow.
  2. Measure the total airflow of the system by traverse or connected load.
  3. Calibrate the controller.
  4. Balance the connected load of the VAV box and verify the minimum airflow.
  5. With the primary air damper at the minimum position, adjust the fan speed to design heating airflow.

Our experienced technicians can assist with your Variable Air Volume Systems. Contact us to get started.

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