TECHNOLOGY BASICS: What are Fault Detection and Diagnostic Systems?

Fault detection and diagnostics (FDD) is concerned with automating the processes of detecting faults with physical systems in buildings and diagnosing their causes. Approximately 15%–30% of the energy used in commercial buildings is wasted due to faults and errors in operations. Automated fault detection and diagnostics technologies or tools can address this waste by identifying deviations from normal or expected operations, and determining the type of problem and its location, minimizing the need for complex manual analysis of operational data.

Modern FDD technology can be applied to nearly every building system for which data is available, although it is typically applied to HVAC systems. FDD tools integrate HVAC operational data from the building automation system (BAS), and often meter data, leveraging common protocols such as BACnet or Modbus. Integration with other systems, e.g., lighting is less common in modern FDD technology, but is possible.

The basic building blocks of FDD systems are the methods for detecting faults and subsequently diagnosing their causes. Several different methods are used to detect and diagnose faults. The major difference in these approaches is the information used for formulating the diagnostics. Diagnostics can be based on a priori knowledge (e.g., models based entirely on first principles) or empirically driven through analysis of large amounts of historical data on the physical systems.

Today’s FDD offerings feature large libraries of algorithms that can be applied to detect a wide diversity of faults across many systems and available data points. A brief list of some of the types of detectable faults are given below.

• General faults
• Sensor drift, bias or complete-failure faults
• Valve and damper sticking or leakage faults
• Scheduling fault, i.e., HVAC use outside of intended hours of operation
• Actuators (e.g., fan and damper) hunting fault due to poorly tuned control loops
• Manual overrides in place
• Leaking air distribution duct fault
• Fouling coil

Chilled water plant

• Chilled water supply temperature setpoint reset
• Chilled water plant lockout
• Hydronic differential pressure setpoint reset
• Cooling tower leaving-water temperature setpoint reset
• Chiller short cycling

Air handling unit

• Under or over economizing
• Excessive outdoor air intake
• Unnecessary simultaneous heating and cooling
• AHU supply air temperature setpoint reset
• AHU static pressure setpoint reset
• Dirty filters
• Leaky medium pressure ductwork

Packaged rooftop unit

• Compressor fault/failure
• Condenser fault/failure
• Overcharged/undercharged refrigerant

Terminal units

• VAV minimum supply airflow too high (causing reheat)
• VAV supply airflow constantly at maximum flow due to insufficient cooling supply
• Zones are outside an acceptable space temperature range
• Space heating and cooling setpoints: insufficient dead-band or night setback
• Rogue zones driving the AHU system to inefficient operation
• Air Leakage in Terminal Unit or Downstream

Boiler plant

• Hot water plant lockout
• Hot water supply temperature setpoint reset
• Hydronic differential pressure setpoint reset
• Boiler short cycling

Depending on the scope of the FDD implementation, and the data that are available, additional faults may also be included, extending to problems such as building pressurization requirements, recirculation of exhaust air into air handler intake plenum, and faults associated with the control and operation scheduling of lighting loads.

In addition to detecting the occurrence and physical location of a fault, FDD tools attempt to diagnose, or isolate the cause of a fault. To complement the FDD findings, the tools offer the information on recommended action, or the impact of the fault in terms of energy use, cost, equipment health, or occupant comfort. This information can be used to guide and prioritize corrective action. In addition to recommended actions and fault prioritization, some FDD tools may offer supplementary capabilities such as the ability to add notes to faults, assign a responsible party for follow up, and integrate with a maintenance management (work order) system.