| Summary: | Fault detection and diagnosis (FDD) has become an important subject in heat pumps due to its potential for energy savings. However, research on multiple faults occurring at the secondary fluid side of heat pumps is rare in the open literature. This study experimentally examined single secondary fluid flow rate faults (SSFF) and multiple-simultaneous secondary fluid flow rate faults (MSSFF) and their effects on the performance of a heat pump unit, which is a core component of ground source heat pump systems, and proposed FDD methodology to detect these faults. The secondary fluid flow rate faults were simulated in cooling mode by varying the evaporator and condenser secondary fluid flow rates at 60%, 80%, 100%, 120%, and 140% of the reference value according to varying outdoor entering water temperature conditions. Condenser secondary fluid flow rate faults affected the heat pump coefficient of performance(<i>COP</i>) significantly more than the evaporator secondary fluid flow rate fault in SSFF. Cooling capacity was highly dependent on the evaporator secondary fluid flow rate fault while <i>COP</i> was greatly affected by the condenser secondary fluid flow rate fault in MSSFF. The FDD methodology was modeled using correlations and performance trends of the heat pump and can detect SSFF and MSSFF within an error threshold of ±1.6% and ±6.4% respectively.
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