Fault Detection and Diagnostics for Rooftop Air Conditioners

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normal state variables, 2) steady-state detector, 3) fault detection classifier, and 4) fault
diagnosis classifier. The resulting method is simpler to implement and was shown to have
significantly better sensitivity for detecting and diagnosing faults than the original
method. However, it was not possible to modify the method to handle multiplesimultaneous
faults. Furthermore, the application of this method to the field sites proved
to be difficult because of the requirement for training models using field data. The
method is better suited to implementation in original equipment than for retrofit to field
applications.
A second FDD method was developed to handle multiple-simultaneous faults and
to eliminate the need for model training using field data. The ability to handle multiple
faults was addressed by identifying features that decouple the impacts of individual faults.
The need for on-line models was eliminated by employing manufacturers’ rating data
such as compressor and TXV maps. These data are readily available at no cost and are
generic and reasonably accurate. The performance of the decoupling-based FDD method
was initially tested using laboratory data. A prototype software implementation was
developed and a demonstration was created for illustration purposes using the Purdue
field site with faults artificially introduced. Finally, the FDD methodology was applied
to California field sites to understand the condition of the equipment and highlight the
potential for FDD.
Figure E-1 shows output from the FDD demonstration at a point where four faults
had been introduced. The bar chart in the upper-left quadrant shows individual fault
indicators relative to a threshold for detection and diagnosis. Each of the fault indicators
have been normalized so that full scale (i.e., 1.0) corresponds to an individual fault
causing a 20% degradation in cooling capacity. The graph in the lower-left quadrant
shows impacts of the faults on performance and safety factors as a function of time
during the demonstration. The factors include cooling capacity, COP, and overheating of
the compressor. The capacity and COP are reductions relative to values for equipment
operating normally. The compressor overheating is the difference between the current
and normal compressor discharge temperature normalized by a value considered to be
harmful to the compressor life. The table in the lower right quadrant summarizes the