A new type of air-water heat pump has been developed with the aim to cover an extended application range in high temperature residential heating (for retrofitting existing oil or gas boilers in hydronic heating systems). The base heat output is about 10 kWth. Based on a commercial heat pump, the refrigerant cycle was modified with two main components: A hermetic scroll compressor with a specially designed vapor injection port and an internal “economizer” heat exchanger. Performance tests were carried out in the laboratory over a wide range of injection mass flow rates and at external conditions covering air temperatures down to -12°C. At A-7°C/W60°C an increase of heat output of 28% and a COP improvement of 15% compared to the tests without injection flow have been observed. The heat output at A2° C/W50°C (standard operating conditions without injection) and at A-12° C/W65°C (extreme heating point with injection) are about the same. Considering these promising results and with the aim to further improve the seasonal COP and the heat output curve, a second compressor prototype based on a different compressor model, is being tested in phase two of this project.