Fault Detection and Diagnostics for Rooftop Air Conditioners

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class of high cost fault, accounting for 10% of total service costs. Further analysis
showed that, although most failures in hermetic compressors are diagnosed as a failure in
the motor, those failures usually result from mechanical problems such as overload or
liquid refrigerant in the compressor. Based on their survey and analysis, Breuker and
Braun concluded that five fault types should be considered for systems with fixed
expansion devices: (1) refrigerant leakage; (2) condenser fouling; (3) evaporator filter
fouling; (4) liquid line restriction; and (3) compressor valve leakage.
To evaluate the FDD technique presented by Rossi and Braun, the above five
faults were introduced within a 3-ton fixed orifice air conditioner in well-controlled
environmental chambers under various fault levels and cooling load levels. Results
showed that refrigerant leakage, condenser fouling, and liquid line restriction faults could
be detected and diagnosed before an 8% reduction in COP occurred; compressor valve
leakage was detected and diagnosed before a 12% reduction occurred; and the least
sensitivity was evaporator fouling at 20%. These results are compared with the improved
FDD technique later in the current report.
To keep track of the up-to-date research, Comstock, Chen, and Braun (1999)
performed an exhaustive literature review of FDD in HVAC. This review provided a
solid background and guide for later research.
The fault characteristics on a system with a TXV are different from those with a
fixed orifice for which Rossi and Braun originally developed the statistical rule-based
technique. Chen (2000a) modified and evaluated the original FDD technique for a 5-ton
rooftop unit with a TXV as the expansion device. To simplify the FDD method, two
innovative and easy-to-implement methods were proposed (Chen & Braun, 2000b &
2001). The first method, termed the “Sensitivity Ratio Method”, used measurements and
model predictions of temperatures for normal system operation to compute ratios that are
sensitive to individual faults. The second method, termed the “Simple Rule-Based
Method”, dispensed with any on-line model but used performance indices computed from
raw measurements that are relatively independent of operating state but are sensitive to
faults. Both methods were tested using experimental data for different fault types and