Experimental investigation of air-to-air multi-function heat pump across operating modes

Air-to-air multi-function heat pumps (MFHPs) integrate space conditioning and domestic hot water (DHW) heating into a single vapor-compression cycle. This paper presents an experimental evaluation and comparison of a 14 kW single-speed MFHP back-to-back with an analogous space conditioning heat pump (SCHP). Dual psychrometric chambers were used under varied environmental conditions to characterize the MFHP across five operating modes (space cooling, space heating, DHW heating, simultaneous space cooling and water heating (SCWH), and defrost). The results demonstrate that this MFHP design achieves nearly identical performance to the SCHP in space cooling and heating modes, confirming that the refrigerant valving for multi-functionality imposes almost no efficiency penalty. This study also introduces three novel methodologies to evaluate the complex SCWH mode, quantifying a 58% increase in combined efficiency and 37% reduction in electrical power in comparison to the dedicated cooling and water heating modes. Furthermore, the MFHP's unique defrost strategy that uses the hot water tank as a thermal source was characterized and compared to the SCHP defrost with and without electric resistance. This strategy results in a rapid (∼2 min) cycle that maintains supply air above 30 °C and avoids occupant discomfort without peak power demands from electric resistance. This work validates the MFHP's benefits but also identifies key design limitations and considerations, including refrigerant charge imbalance in various modes and relatively inefficient DHW heating, providing a clear pathway for future work in residential MFHPs.