Water heating systems each of which is composed of an air-to-water heat pump using CO2 as a natural refrigerant and a hot water storage tank have been developed, and are expected to contribute to energy saving in residential hot water supply. However, the area of tepid water in the storage tank expands because of heat conduction during long time storage, which leads to a degradation of the system performance. One of the ways to reduce the area of tepid water and enhance the system performance is to extract tepid water from the side of the storage tank. In this paper, the performance enhancement of a CO2 heat pump water heating system with such a revised storage tank is analyzed by numerical simulation. The simulation model developed previously for a conventional system is extended by modifying the model for the storage tank and verifying its validity through experiments. Through a performance analysis using a standardized hot water demand, it is clarified how system performance values of the revised system are enhanced, and especially how the trade-off relationship between the system efficiency and the volume of unused hot water is improved, in comparison with those of the conventional system. (C) 2015 Elsevier Ltd. All rights reserved.