Measurement of refrigerant mass distribution in a propane air-to-water heat pump under partial load and controlled operating conditions

The phase-down of high-GWP refrigerants and PFAS-related concerns are accelerating the adoption of propane (R-290), although its flammability requires low-charge designs. This study quantifies component-level refrigerant distribution in a domestic R-290 air-to-water heat pump using the Quick Closing Valve Technique. Tests covered standardized seasonal conditions and variable compressor speeds at approximately constant saturation temperatures. Across the variable-speed tests, the condenser held the largest refrigerant share, averaging 42% of the total charge, while the compressor and evaporator each accounted for 22%. Refrigerant in the compressor free volume represented 48% of the compressor inventory and 11% of the system charge. Under specific conditions, reducing compressor speed by 30 rps increased condenser inventory by up to 57%. Detailed heat-exchanger models indicated that this increase was consistent with an upstream displacement of the condensation boundary, involving contraction of the two-phase region and expansion of the subcooling region, thereby increasing liquid retention. In the evaporator, uniform-circuit modeling explained only part of the measured trends; the remaining differences were consistent with additional multicircuit effects not represented by the model, whose individual contributions could not be discriminated using the available experimental measurements. These results provide component-level data for charge-model validation and the development of regulation-compliant low-charge heat pumps.