Air-to-water heat pumps using CO2 as a natural refrigerant have been developed and commercialized. They are expected to contribute to energy saving in residential hot water supply. The objective of the research is to analyze the performance of a water heating system composed of a CO2 heat pump and a hot water storage tank by numerical simulation. In this paper, the system performance is analyzed under a daily change in a standardized hot water demand, and some features of the temperature distribution in the storage tank and the system performance criteria such as coefficient of performance, storage and system efficiencies, and volumes of stored and unused hot water are investigated. It turns out that the daily change in the hot water demand does not significantly affect the daily averages of the COP, and storage and system efficiencies, while it significantly affects not only the daily change in the volume of hot water unused after the tapping mode, but also that in the volume of hot water stored after the charging mode. The influence of the daily change in the hot water demand on the volumes of stored and unused hot water is clarified quantitatively. (C) 2009 Elsevier Ltd. All rights reserved.