Modern networks and standardized communication protocols enable smoother and more-efficient building operations, enhanced fault detection and troubleshooting, and reduced system-installation costs. Still, prejudice against network connections persists within the industry because they are considered less reliable than hard-wired point connections. Many designers require control points to be hard-wired, allowing only monitoring data to be routed through network connections. This can be confusing to operations-staff members, who may find duplicate points with different values or statuses in databases. Additionally, some equipment and devices may not operate properly with such dual connectivity.

While some control-system products do not meet expectations for reliability in network connections, the primary reason for network-reliability issues in building control systems often is designers providing inadequate standards, direction, and oversight regarding configuration of network connections. Consider the following real-life example.

The Network-Configuration Specification
For a recent office-building control retrofit, the energy-management-and-control-system (EMCS) specification required network connection of all network-capable equipment and devices. The controls were required to employ native BACnet communications. For uniformity and ease of connection, only BACnet devices and equipment were to be selected. The specification provided guidance on control-network configuration, with sample configuration diagrams (Figure 1) provided.

In Figure 1, "EMCS control panel" is any EMCS panel that controls equipment such as air handlers, chillers, and boilers. The blocks labeled "ASC" are application-specific controllers that typically control variable-air-volume (VAV) boxes, while the blocks labeled "Equip/device" represent third-party equipment or devices such as variable-frequency drives (VFDs), chillers, and boilers that are to be network-connected to the system. Because network connections and capabilities vary by EMCS manufacturer, the configuration diagram served to show the intent of the network topology. The specification required each EMCS equipment controller to be interconnected via Ethernet BACnet Internet protocol (IP) and all devices to be connected via BACnet master-slave/token-passing (MS/TP) networks. The specification also required that all EMCS-controlled equipment be on a network originating at the panel providing control of the equipment. A schematic of the actual network configuration with all equipment and communication details was required as part of the EMCS submittal.

The Network-Configuration Submittal
The design went out to bid. A reputable control subcontractor representing an established EMCS product line was awarded the contract. When the submittal was received, the proposed network topology (Figure 2) raised a red flag concerning system reliability.

Note that in Figure 2, the EMCS equipment controllers and the VAV-box ASCs are connected in accordance with the configuration guide, while the chillers, boilers, and VFDs associated with each EMCS control panel are connected to the system on a single network originating at the main mechanical-room EMCS control panel. The contractor argued that the equipment network connections and the points they contained were few enough that they easily could be accommodated on a single MS/TP network. Further, the contractor noted that because of the high-speed Ethernet interpanel communications, there would be no measurable delay in executing commands to or receiving value or status data from the equipment. The equipment, the contractor contended, was "virtually" connected to the panel that controlled it.

Missing from the contractor's arguments was any consideration of system reliability. For example, imagine a building tenant, while independently reconfiguring its office network, inadvertently breaching the equipment network line. Some equipment may remain online, while other equipment fails. The breach and the affected equipment and systems could be on different floors. The building operations staff may not have been aware of the work taking place and, thus, have no idea of the location or cause of the failure. The problem could be very disruptive to system operation and difficult to troubleshoot.