Supply Air Temperature Control for VAV Diffusers

Control Strategies to Optimize Your System

VAV diffusers help keep rooms comfortable using an individual thermostat and a built-in damper to create small zones of control. The amount of air provided by the VAV diffuser is based on three factors: the temperature in the room, the room’s set point and the temperature of the supply air.

It’s important to note that the VAV diffusers themselves cannot create more air, nor can they directly change the air temperature – they can only control the amount of air they receive. This is why controlling the supply air temperature is so critical for VAV diffusers to operate optimally.

A thermostat or building management system (BMS) sensor controls the air source, such as a direct expansion (DX) compressor, air-handling unit water valve or heater. If the sensor is in the wrong spot, the system may turn off the heating or cooling too soon. When this happens, the rooms that need the most heating or cooling will not get enough air at the right temperature, and occupants may feel uncomfortable.

Why Supply Air Temperature Matters

Supply air temperature influences the operation of VAV diffusers in several ways:

• VAV systems work best with a consistent supply air temperature. Frequent changes in supply air temperature mean that the diffuser does not know whether to open or close.
• Setting low and high limits for the supply air temperature is essential. The low limit prevents DX coils from freezing when bypass static pressure control is used, and the high limit reduces zone stratification. In cooling, VAV diffusers work well at supply air temperatures as low as 50°F (10°C). In heating, VAV diffuser will work with supply air temperatures as warm as 120°F (49°C), but to avoid stratification, the supply air temperature should always be kept as low as possible to just meet the loads.
• VAV diffusers automatically change from cooling to heating when the supply air reaches a certain temperature. Mechanical diffusers use a fixed range, while digital diffusers adjust dynamically based on the room temperature.

Temperature Sensor Options

There are a few different options for where the temperature sensor can be located:

• A supply air temperature sensor located at the discharge of the source of the heating and cooling is best for keeping a consistent supply air temperature and for meeting design requirements of all spaces.
• A room air temperature sensor is best used for determining when to switch between heating and cooling. It should be placed in the room with the highest load. Once heating or cooling mode has been selected, it is the supply air sensor that controls the source of heating and cooling.
• A return air sensor is not recommended for VAV systems. It senses average conditions and may not meet the needs of areas with the highest demand. It’s more commonly used with constant-volume systems.

With good pressure control and supply air temperature control strategies, VAV diffusers will provide individually adjustable, comfortable spaces for occupants.

For more information about VAV diffuser systems, visit our website or reach out to the Price Sustainable Systems team at sustainable@priceindustries.com.