Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Results from Simulation and Laboratory Testing of Air Handling Unit and Variable Air Volume Box Diagnostic Tools



Natascha S. Milesi-Ferretti, J Schein, Cheol D. Park, Michael A. Galler, Steven T. Bushby, J House


Control of building systems is increasingly more intelligent and complex. This development both necessitates the use of automated diagnostics to ensure fault-free operation and enables diagnostic capabilities for the various building systems by providing a distributed platform that is powerful and flexible enough to perform fault detection and diagnostic (FDD). Most of today's emerging FDD tools are stand-alone software products that do not reside in a building control system. Thus, trend data files must be processed off-line, or an interface to the building control system must be developed to enable on-line analysis. This is a cumbersome process and it does not scale well because all of the data must be obtained at a single point. A better approach would be to develop algorithms that can be embedded in commercial controllers so that the fault detection can be done as close to the source of the fault as possible. Only the result of the analysis needs to be conveyed to an operator or supervisory controller.AHU Performance Assessment Rules (APAR) is a diagnostic tool that uses a set of expert rules derived from mass and energy balances 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. 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. This report describes the results of a research study to determine the effectiveness of these tools in detecting commonly found mechanical faults and control problems.The research involved a complementary set of laboratory experiments using commercial AHU and VAV box controllers under both normal operating conditions and operation with known faults, computer simulations, and emulations using the NIST Virtual Cybernetic Building Testbed (VCBT). The APAR and VPACC tools were both found to be successful at finding a wide variety of faults. It was also found that some faults could not be detected under certain operating conditions because the control system was able to mask the problem or because sensor data needed to detect the fault is not commonly available in commercial systems. Both tools appear to be suitable for embedding in commercial control products.
NIST Interagency/Internal Report (NISTIR) - 6964
Report Number


BACnet, building automation and control, cybernetic building systems, direct digital control, energy management systems, fault detection and diagnostics


Milesi-Ferretti, N. , Schein, J. , Park, C. , Galler, M. , Bushby, S. and House, J. (2003), Results from Simulation and Laboratory Testing of Air Handling Unit and Variable Air Volume Box Diagnostic Tools, NIST Interagency/Internal Report (NISTIR), National Institute of Standards and Technology, Gaithersburg, MD, [online], (Accessed April 22, 2024)
Created January 31, 2003, Updated November 10, 2018