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

Abstract

Operational faults can significantly degrade the performance of residential air-source heat pumps, yet the combined effects of simultaneous faults remain insufficiently understood. 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 DOE/ORNL Heat Pump Design Model 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 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 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 R2 values of 0.879 in cooling and 0.913 in heating. The results demonstrate the usefulness of a validated physics-based model for evaluating fault interactions and supporting heat pump fault diagnostics.