Decoupling features for diagnosis of reversing and check valve faults in heat pumps

Recently, a decoupling-based (DB) fault detection and diagnosis (FDD) method was developed for diagnosing multiple-simultaneous faults in air conditioners (AC) and was shown to have very good performance. The method relies on identifying diagnostic features that are decoupled (i.e., insensitive) to other faults and operating conditions. The current paper extends the DB FDD methodology to heat pumps. Heat pumps have all the same faults as occur for air conditioners with additional faults associated with components that accommodate heating mode, including reversing valve leakage and check valve leakage. Decoupling features were developed for these additional faults and laboratory evaluations were performed to evaluate diagnostic performance. It was found that check valve leakage could be detected and diagnosed before the heating capacity degradation reached 5% for a system with a fixed orifice expansion (FXO) device and 3% for the same system retrofit with a thermal expansion valve (TXV). Furthermore, the feature for check valve leakage is very insensitive to other faults and operating conditions. The decoupling feature for reversing valve leakage could successfully detect and diagnose faults for a TXV system before the heating capacity degraded 6% and was also insensitive to other faults and operating conditions. However, this feature did not work well for a system with an FXO in heating mode because the refrigerant exiting the evaporator and entering the reversing valve was typically a two-phase mixture. Fortunately, it was possible to diagnose this particular fault at many operating conditions in cooling mode for the system with an FXO.