Performance degradation and diagnostic features of a residential split heat pump under multiple simultaneous faults using a physics-based model

Operational faults can significantly degrade the performance of residential air-source heat pumps, yet the combined effects of simultaneous faults remain insufficiently understood. In particular, systematic and reproducible assessments of combined refrigerant-charge and airflow faults across both cooling and heating modes remain limited. This study investigates the impacts of refrigerant charge, indoor airflow, and outdoor airflow faults, applied individually and in combination, on a residential split heat pump operating in cooling and heating modes. A 4-ton R-32 system was modeled in the public-domain and validated against manufacturer data, with prediction errors generally within 6% for capacity, 5% for cooling power, and 10% for heating power. The results show that the coefficient of performance (COP) is more sensitive to faults than capacity in both modes. Under combined faults, the maximum reductions reached 14.7% in cooling capacity and 18.1% in cooling COP, while the corresponding heating-mode reductions were 12.8% and 28.1%, respectively. In heating mode, several cases with 120% refrigerant charge and reduced airflow caused the compressor discharge temperature to exceed 130 °C. Subcooling, condensing temperature, and discharge temperature were the most informative diagnostic features and were used to develop a virtual refrigerant charge sensor with R 2 values of 0.879 in cooling and 0.913 in heating. The results demonstrate the usefulness of a public-domain physics-based model for evaluating multiple-fault interactions, identifying heating-mode compressor thermal-risk conditions, and supporting interpretable heat pump fault diagnostics.