How Does a DELLA BS Box Work: A Comprehensive Guide

A DELLA BS box, also known as a Branch Selector Box, is a critical component in Variable Refrigerant Flow (VRF) systems, specifically in heat recovery applications. It works by managing the flow of refrigerant between the outdoor units and the indoor units, allowing for simultaneous heating and cooling operations. This comprehensive guide will delve into the intricate workings of a DELLA BS box, providing you with a detailed understanding of its functionality.

Refrigerant Distribution

The DELLA BS box is the central hub where the refrigerant lines from the outdoor units converge. Typically, there are three pipes that connect the outdoor units directly to the BS box. These pipes carry the refrigerant, which is then distributed to the indoor units based on their heating or cooling demands.

Inside the BS box, you’ll find a network of solenoids and Electronic Expansion Valves (EEVs). The solenoids are electromechanical devices that control the flow of refrigerant, while the EEVs regulate the superheat of the refrigerant, allowing for precise control of the cooling or heating output.

The fan coils in the indoor units determine the mode of operation, but the actual change of operation mode occurs within the DELLA BS box. This is where the solenoids and EEVs work in tandem to manage the refrigerant distribution, ensuring that the indoor units receive the appropriate refrigerant flow to meet their heating or cooling requirements.

Solenoids and EEVs

how does a DELLA bs box work

Solenoids are the key components that control the flow of refrigerant within the DELLA BS box. These electromechanical devices open and close based on the signals received from the system’s control board, directing the refrigerant to the appropriate indoor units.

The Electronic Expansion Valves (EEVs) play a crucial role in regulating the superheat of the refrigerant. By precisely controlling the opening and closing of the EEVs, the system can maintain the optimal superheat levels, ensuring efficient cooling or heating operation.

The combination of solenoids and EEVs within the DELLA BS box allows for the seamless management of refrigerant flow, enabling the VRF system to operate in either heating or cooling mode, or even simultaneously in both modes, depending on the demands of the indoor units.

Communication Wiring

VRF systems, including the DELLA BS box, are communicating systems. The indoor units, outdoor units, and the BS box exchange information through a daisy-chain of communication wiring. This communication network allows the system to coordinate the operation and monitor the performance of the various components.

The communication wiring used in a DELLA BS box system is typically 18/2 non-shielded stranded. It’s essential to follow the wiring diagram provided in the submittal documents from your sales engineer to ensure proper installation and communication between the components.


The VRV product line, which includes the DELLA BS box, utilizes Polyvinyl Ether (PVE) oil as the lubricant for the refrigerant system. PVE oil is chosen for its outstanding miscibility at low refrigerant velocities, ensuring efficient lubrication of the compressors and other moving parts.

It’s important to note that PVE oil is more hygroscopic (absorbs moisture) than Polyol Ester (POE) oil. The key difference is that PVE oil can be dehydrated by pulling a vacuum, whereas POE oil cannot. Additionally, when PVE oil interacts with moisture, it does not produce acids through hydrolysis, but instead turns the oil into sludge.

Proper maintenance and handling of the PVE oil are crucial to ensure the longevity and optimal performance of the DELLA BS box and the overall VRF system.

Heat Pump vs. Heat Recovery

There are two main types of VRF systems: heat pump and heat recovery. Understanding the differences between these systems is essential when working with a DELLA BS box.

In a heat pump system, there are only two pipes running between the outdoor and indoor units: a liquid line and either a hot gas or suction gas line. The mode of operation (heating or cooling) is determined by a master thermostat, which is the only component in the system that can change the mode of operation.

On the other hand, heat recovery systems, like the DELLA BS box, have three pipes running from the outdoor units to the indoor units. This configuration allows the system to operate in both heating and cooling modes simultaneously, with the DELLA BS box managing the refrigerant flow to meet the individual demands of the indoor units.

Understanding the differences between heat pump and heat recovery systems is crucial when designing, installing, and maintaining a DELLA BS box-equipped VRF system.


The DELLA BS box is a crucial component in VRF heat recovery systems, responsible for managing the flow of refrigerant to the indoor units based on their heating or cooling demands. Its intricate network of solenoids, EEVs, and communication wiring, combined with the use of specialized PVE oil, enables the system to operate efficiently and provide simultaneous heating and cooling capabilities.

By understanding the detailed workings of a DELLA BS box, HVAC professionals can ensure proper installation, maintenance, and troubleshooting of these complex systems, ultimately delivering optimal performance and comfort to end-users.


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