P2-3. AHU and VAV Box Diagnostics > Background

Building HVAC equipment routinely fails to satisfy performance expectations envisioned at design. Equipment and control failures often go unnoticed for extended periods of time. Additionally, higher expectations are being placed on a combination of different and often conflicting performance measures, such as energy efficiency, indoor air quality, comfort, reliability, limiting peak demand on utilities, etc. To meet these expectations, the processes, systems, and equipment used in both commercial and residential buildings are becoming increasingly sophisticated. At the same time, operations staff is stretched thin and are often untrained in trouble-shooting techniques. These conditions necessitate the use of automated diagnostics to ensure fault-free operation. The amount of data that must be analyzed to automate fault detection and diagnostics could overwhelm the communication network of an HVAC system. This argues for conducting as much analysis as possible within local unit controllers. Modern controllers provide a platform that is powerful and flexible enough to perform distributed fault detection and diagnostics (FDD) for the various building systems.
Researchers at the National Institute for Standards and Technology (NIST) focused on developing FDD methods for air handling units (AHU) and variable air volume (VAV) control boxes. The FDD tools for AHUs and VAV boxes were developed with distinct approaches because of the nature of the systems. VAV boxes are simple devices with a limited number of operation modes and possible faults. The VAV boxes typically have little instrumentation and controllers with limited capability. However, VAV boxes are very numerous in a typical HVAC system, resulting in a large amount of data to be monitored for faults. AHUs are more complex and thus susceptible to more kinds of faults. They also tend to have more instrumentation and more capable controllers. The FDD tools for both systems are designed to be robust so that they can adapt to the variety of applications typical of their use.

NIST developed two rule sets:

• The AHU Performance Assessment Rules (APAR) diagnostic tool is a set of expert rules derived from mass and energy balances that can be used to detect common faults in air-handling units. Control signals are used to determine the mode of operation for the AHU. A subset of the expert rules corresponding to that mode of operation is then evaluated to determine if there is a mechanical fault or a control problem. The rules apply to single-duct, variable-volume or constant-volume air handlers, with hydronic heating and cooling coils and economizers. Seven faults can be detected:
  
  • stuck or leaking mixing box dampers
  • stuck or leaking heating coil and cooling coil valves
  • temperature sensor faults
  • design faults such as undersized coils
  • sequencing logic errors" central plant faults that affect supply conditions at the AHU coils
  • inappropriate operator intervention

• The VAV box Performance Assessment Control Charts (VPACC) is a diagnostic tool that uses statistical quality control measures to detect faults or control problems in VAV boxes. VPACC can be applied to most VAV box control strategies. Fault thresholds are determined by statistical analysis of a database of "normal operation" data. The rules require three commonly available sensor measurements: (1) zone air temperature, (2) discharge air temperature, and (3) air flow rate. Faults due sensor drift or control problems can be detected (see Figure 3).

Figure 3 VPACC Example Showing How Signal Noise is Filtered to Show Gradual Fault Over Time


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