Fault Detection and Diagnostics for Rooftop Air Conditioners

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Figure 4-26 Histogram bar plot of the normalized fault indicator for compressor valve
leakage
In summary, the system has three faults, low refrigerant charge, liquid line
restriction and condenser fouling. To assess the impact of the diagnosed faults on the
overall system performance, Figure 4-27 plots the cooling capacity degradation. It can be
seen that the system cooling capacity was degraded 23~45% and the average is about
32%, which is coincident with the value indicated by refrigerant charge fault indicator.
The cooling capacity degradation can be confirmed by investigating the return air
temperature and system running time. It can be seen from Figure 4-28 that the average
return air temperature is around 78 F and the highest is 88 F, which does not satisfy the
comfort criteria. From Figure 4-29, it can be seen that the system kept running
continuously for a long time (average is 2.5 hours and maximum is up to 9 hours) in
order to remove the heat load. So, from the comfort criteria, service should be done to
correct the diagnosed faults in order to maintain comfort.