Fault Detection and Diagnostics for Rooftop Air Conditioners

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constraints. Economic constraints bring special difficulties and issues, which do not need
to be considered in critical systems.
First, since a packaged air conditioner is relatively inexpensive, the cost to realize
FDD for HVAC systems in terms of software and hardware should be low. Therefore
some relatively expensive measurements such as flow rate cannot be used, and use of
pressure and humidity sensors is limited. This is a particular problem in fault diagnosis
since some faults may have similar symptoms and more sensors can help in
distinguishing them. On the one hand, features as sensitive as possible should be
extracted from limited available measurements, and on the other hand, the diagnosis
method should be as sensitive as possible to isolate several faults with similar symptoms
and insensitive features. Computation should be small enough to be implementable
within a microprocessor.
Second, since HVAC equipment are used in diverse weather and climates, the
behavior of the HVAC plant will vary drastically. In addition, since single-point sensor
placement is generally used, many measurements often are biased and noisy. So the FDD
system should be able to handle biased measurements and be robust to different operating
modes and against noise and disturbances.
Third, unlike critical systems in which faults have zero tolerance, a fault
evaluation and decision step should be added to assess the impact of a fault on overall
system performance and make a decision whether the benefit of servicing the fault
justifies its expense.
Fourth, unlike a critical FDD system which is engineered for a specific large
system, FDD for HVAC systems needs to be adaptive and generic (system-independent)
to the same type of system, or at least to similar models from the same product family.
This would reduce the per-unit costs, which need to be low, compared to the HVAC
equipment price.
Finally, multiple-simultaneous faults are pretty common for air conditioners, so
the FDD technique should be capable of handling them. This feature has been neglected
for previous developments and evaluations of FDD techniques.