Performance comparisons for variable-speed ductless and single-speed ducted residential heat pumps

• Construction of a baseline model of conventional fixed orifice or thermostatic expansion valve system from a variable-speed system for fair comparison.
• Introduction of process efficiencies which decouples the second law efficiency and provides insight on the irreversibility in different parts of systems.
• Performance comparison between variable-speed and conventional systems under different ambient temperature and building loads.

Residential variable-speed ductless heat pump (DHP) systems are claimed to operate more efficiently than conventional single-speed ducted systems because of fewer on-off cycles and smaller fan power consumption. However, few methods exist for quantifying the energy-saving benefits of DHP technology because of complicated and proprietary control strategies employed in the DHP system. In this paper, a model in the literature is used to assess the energy savings relative to a baseline ducted system with the same heat exchangers and rated capacities. Process efficiencies are introduced to highlight the sources for differences in performance between systems. Analyses of process efficiencies at different building loads and ambient temperatures show that DHP systems perform better than conventional systems primarily because of lower fan power consumption associated with the use of variable-speed fans. Greater benefits exist at lower heating loads because of lower air flow requirements and the cubic dependence of fan power on flow. In contrast, conventional heat pumps typically operate with on/off fans. Heating seasonal performance factors (HSPF) were determined for different climatic regions and the DHP system was found to perform 22%–107% better than a conventional system, depending on the climate.