Effects of simultaneous soft faults in a reversible and variable-speed air-to-water heat pump

Abstract

The widespread adoption of air-to-water heat pumps is driven by their high heating efficiency, with some systems offering reversibility for cooling applications. Despite their benefits, these systems are susceptible to soft faults such as undercharge, overcharge, and outdoor unit airflow restrictions (condenser fouling in cooling mode or evaporator fouling in heating mode). Detecting and addressing these faults before they escalate into hard faults is critical. While previous experimental studies have focused on residential heat pumps with imposed faults, most were conducted on air-to-air systems, primarily in cooling mode and with fixed-speed compressors. This study addresses the gap by investigating a reversible air-to-water heat pump employing R290 and a variable-speed compressor. The experimental work consists of imposing faults of undercharge, overcharge, condenser and evaporator fouling, individually and in combination. The investigation analysis considers the system’s behavior under these faults in cooling and heating modes and how the control strategy (subcooling or superheat) and a compressor suction accumulator affect it. In cooling mode, the combination of 85% undercharge and 50% condenser fouling worsens the EER up to 39% if the superheat is controlled and up to 10% if the subcooling is controlled. In heating mode with subcooling control, 85% undercharge combined with a 50% evaporator fouling can worsen the COP by 32%. However, there are cases where those same fault levels only impact the COP by 4% due to the charge requirements of the condition and that the excess charge is stored in the compressor suction accumulator. As compressor speed can significantly affect the refrigerant charge requirements of the system, this work shows its potential for fault detection, particularly for undercharge. These findings can provide a basis for developing Fault Detection and Diagnosis methodologies for reversible air-to-water heat pumps with variable-speed compressors.