The coefficient of performance (COP) for a residential CO{sub}2 heat pump water heater system is heavily influenced by the inlet water temperature to the gas cooler. During the tapping and charging of a domestic hot water (DHW) tank, there will be mixing as well as conductive heat transfer between the hot and cold water reservoirs in the tank. This will in turn increase the average inlet water temperature to the gas cooler. A possible way to reduce the internal heat transfer and mixing losses in single DHW tanks is to use a movable insulating plate to separate the hot and cold water reservoirs in the tank. A full-scale test rig was therefore constructed in order to test the performance of such a device. The test results demonstrated that a 50 mm EPS plate with a balancing weight of about 10 kg had the required functionality and thermal performance, and reduced the heat transfer and the mixing of hot and cold water in the DHW tank to a very low level.
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