Single Duct Variable Air Volume Terminal Practical Knowledge Sharing Part 2

In the variable air volume air conditioning system, the single duct variable air volume terminal equipment (VAV-BOX) is one of the most common variable air volume terminal devices.

Its main function is to automatically adjust the supply air according to the temperature requirements of the room so that the indoor temperature remains within the set range to ensure a constant and comfortable space environment.

In the previous chapter, we have gained an in-depth understanding of the measurement principles, common accessories and applications of single -duct variable air volume terminals. In this chapter, we will learn other extended applications of single- duct variable air volume terminals.

Standard applications

Configure a wall-mounted thermostat to read and set the room temperature to control the room temperature

Other applications

1.When it can’t install the thermostat on the wall, there are two solutions:

The first method: The thermostat is installed near the return air diffuser, and the room temperature is set through the BA/BMS system.

Second method:

Each VAV-BOX is connected to an external temperature sensor, which is placed near the return air diffuser to monitor the room temperature. Add a configuration wall controller and connect multiple VAV controllers via RS485 cable to realize functions such as setting the room temperature and viewing the actual room temperature.

2.When there are two supply air VAV-BOXs in a room, only one wall-mounted thermostat is allowed to be installed in the room for control

(Note: This design should be avoided when designing a new project. When a room only allows one thermostat, a VAV-BOX should be given priority for this room.)

First situation

When the set air volume ranges of the two VAV-BOXs are exactly the same, VAV-BOX-01 is connected to the thermostat to adjust the air volume according to the temperature changes in the room;

VAV-BOX-01 outputs one set analog signal (0-10V/4-20mA) to VAV-BOX-02. Make the air supply volume of VAV-BOX-02 equal to the air supply volume of VAV-BOX-01

Second situation

When the set air volumes of two VAV-BOXs are inconsistent, there are two solutions:

The first option:

VAV-BOX-01 is connected to the thermostat to adjust the air supply volume according to the temperature changes in the room;

VAV-BOX-02 is connected to an external temperature sensor to sense the room temperature;

VAV-BOX-01 outputs one set analog signal (0-10V/4-20mA) to VAV-BOX-02. Make the set temperature of VAV-BOX-02 equal to the set temperature of VAV-BOX-01

Second way:

VAV-BOX-01 is connected to the thermostat to adjust the air supply according to the room temperature;

VAV-BOX-01 outputs two sets of analog signals (0-10V/4-20mA) to VAV-BOX-02, make the set temperature value & room temperature value of VAV-BOX-02 are equal to the value on the VAV-BOX-01 controller

3.When a room is equipped with a supply air VAV-BOX and a return air VAV-BOX

The air supply VAV-BOX-S is connected to the room thermostat to control the room temperature; at the same time, VAV-BOX-S will output one set analog signal (0-10V/4-20mA) to the return air VAV-BOX-R, allowing the return air VAV-BOX-R to achieve different return air volumes according to the air volume difference or ratio difference between the supply and return air, thereby maintaining the positive pressure in the room.