Low or high temperatures may nowadays be both easier to control and healthier to apply. Officially approved by the EU, carbon dioxide presents the ideal "natural" environmentally friendly refrigerant as opposed to synthetical refrigerants (HFCs) that are currently used. According to scientists, carbon dioxide derives from natural sources and from chemical processes. It is thus an inexpensive ‘by-product’ that can be used both for cooling and heating purposes. Due to carbon dioxide's limiting cooling capacity at 31ºC and 70 bars, refrigerant cycles using carbon dioxide become critical. Using the carbon dioxide transcritical cycle, the advantage of the present innovation is that it may well be applied either in the case of an unlimited heat source or a limited heat sink with high temperature smooth variations. The overall purpose of this project is to promote the use of carbon dioxide as a replacement for the potent greenhouse gasses - called HFCs - in domestic applications for hot water heat pumps. Carbon dioxide was originally used for maritime cooling and for air conditioning systems in buildings. Nowadays, the partners of this project are developing test prototypes for carbon dioxide to be used either for hot water and dehumidification purposes in residences and industries as well as for heating aims in space vehicles. To achieve these, high-pressure equipment was used such as compressors, expansion devices and heat exchangers. Car manufacturers for example use compressors to pump the carbon dioxide to a high-pressure gas condition. The gas is cooled by ambient air in the gas cooler located in the front end of the vehicle. The cooled gas is then expanded through an electronic expansion valve and enters the evaporator located in the passenger compartment. The evaporator cools the cabin air. The heated carbon dioxide then re-enters the compressor to complete the cycle. In the hot heat pump system, hot air is achieved through similar routes. The difference is that the compressor sucks carbon dioxide (now in fluid form) as superheated vapour and compresses to high pressure. Calculations show that efficiency may improve considerably in domestic hot-water pumps applying carbon dioxide instead of conventional refrigerant used. Moreover, it is estimated that further use of carbon dioxide in the future will show remarkable improvement in ozone-depleting global warming. The project contains guidelines and reports on carbon dioxide heat pump features and invites European heat pumping equipment manufacturers and system planners to contribute to its further development.