How Does a Trane BS Box Work: A Comprehensive Guide

A Trane BS Box, also known as a Single-Zone Variable Air Volume (SZVAV) system, is a heating, ventilation, and air conditioning (HVAC) solution designed for commercial buildings. It works by controlling the amount of outdoor air that enters the system based on the supply airflow. The system consists of several key components, including a return-air grille, ceiling plenum, return ductwork, filter, coil, supply ductwork, diffusers, and a mixing box.

Understanding the Mixing Box

The mixing box is the heart of the Trane BS Box system, where outdoor air mixes with the recirculated air. Due to the pressure drop through the return-air path and damper, the static pressure inside this mixing box is negative, causing outdoor air to be drawn into this mixing box.

Modulating the Outdoor-Air Damper

During system operation, the controller modulates the position of the outdoor-air damper in proportion to the change in supply airflow, which is determined from the signal sent to the variable-speed drive on the fan. This ensures that the same quantity of outdoor air enters the system, regardless of the supply airflow.

The controller uses a feedback loop to monitor the supply airflow and adjust the outdoor-air damper accordingly. The feedback loop typically includes the following components:

  1. Airflow Sensor: Measures the actual supply airflow and sends a signal to the controller.
  2. Damper Actuator: Receives a control signal from the controller and adjusts the position of the outdoor-air damper.
  3. Controller: Compares the actual supply airflow to the desired setpoint and adjusts the damper actuator to maintain the correct outdoor airflow.

By precisely controlling the outdoor-air damper, the Trane BS Box ensures that the desired amount of outdoor air is introduced into the system, regardless of the current operating conditions.

Reducing Supply Airflow during Part-Load Operation

how does a Trane bs box workImage source: Flickr by Jackie Bese

The Trane BS Box is designed to reduce supply airflow during part-load operation, which results in several benefits:

  1. Reduced Pressure Loss in Supply Ductwork and Diffusers: When the supply airflow is reduced, there is less pressure loss in the supply ductwork and diffusers. This, in turn, reduces the pressure that the fan needs to create at the fan discharge.

  2. Reduced Pressure Loss in Return-Air Path: Similarly, moving less air through the return-air path also results in less pressure loss, causing the static pressure inside the mixing box to increase.

Adjusting the Outdoor-Air Damper

To ensure that the same quantity of outdoor air enters the system during part-load operation, the outdoor-air damper needs to be opened farther when supply airflow is reduced. This is because the static pressure inside the mixing box has increased, and the outdoor air needs to be drawn in more forcefully to maintain the desired outdoor airflow.

The controller continuously monitors the supply airflow and adjusts the outdoor-air damper position accordingly, ensuring that the correct amount of outdoor air is introduced into the system at all times.

Maintaining Optimal Indoor Air Quality

By precisely controlling the outdoor airflow, the Trane BS Box system ensures that the indoor air quality is maintained at the desired level. This is particularly important in commercial buildings, where occupant comfort and health are critical considerations.

The system’s ability to adjust the outdoor airflow based on the supply airflow also helps to improve energy efficiency, as it reduces the amount of energy required to condition the outdoor air.

Troubleshooting and Maintenance

To ensure the Trane BS Box system continues to operate efficiently, regular maintenance and troubleshooting are essential. Some key maintenance tasks include:

  1. Inspecting and Cleaning the Filters: Dirty filters can restrict airflow and reduce the system’s efficiency. Regularly inspecting and cleaning or replacing the filters is crucial.

  2. Checking the Damper Actuator: Ensure the outdoor-air damper is moving freely and the actuator is functioning correctly. Calibrate the actuator if necessary.

  3. Verifying Airflow Sensor Accuracy: Periodically check the accuracy of the airflow sensor to ensure the controller is receiving accurate feedback.

  4. Monitoring System Performance: Regularly review the system’s operating parameters, such as supply airflow, outdoor airflow, and energy consumption, to identify any potential issues.

By following the manufacturer’s recommended maintenance procedures and addressing any issues promptly, you can ensure your Trane BS Box system continues to provide efficient and reliable HVAC performance for your commercial building.


The Trane BS Box, or Single-Zone Variable Air Volume (SZVAV) system, is a sophisticated HVAC solution that precisely controls the amount of outdoor air entering the system based on the supply airflow. By understanding the key components, such as the mixing box and outdoor-air damper, as well as the system’s design principles, you can ensure your Trane BS Box system operates at peak efficiency and maintains optimal indoor air quality for your commercial building.