A brine/water heat pump with the auxiliary liquid subcooling cycle with a heating capacity of 9 kW at -7/50°C was built and a series of measurement were conduced. The heat pump is designed for the monovalent heating in the retrofit market. For this purpose, air is planned to be used as the heat source in field operation. The refrigerant R407C was used and a new R22 retrofit mixture was also evaluated. The measurements conform to the conditions defined at the heat pump test center of Töss (Switzerland). Results with the auxiliary cycle show an increase of the heating power supply by 22% and an improvement of the coefficent of performance by 5%, compared to the heat pump without additional liquid subcooling cycle. Due to the high performances shown by zeotropic refrigerant, the COP improvement is low, but improvements of 10% are expected with azeotropic or pure refrigerant (e.g. propane). Measurement results were used to calibrate the developed simulation model. The single stage application requires high pressure ratios and a mean compressor with higher built in pressure ratio was chosen (figure 2.2). Therefore the exergetic efficiency can be maintained at high level through the seasonal application range (figure 7.8). The auxiliary cycle improves the seasonal COP by 11% (SCOP = 2.74 with auxiliary cycle / SCOP = 2.44 without auxiliary cycle; table 6). These results are calculated with the calibrated model. Following assumptions were made: a) heat curve (issued at A-12/W60) with constant hot water mass flow; b) heating temperature defined by nominal return temperatures and with a temperature lift of 10 K at A-12/W60. The condensors’ temperature lift is proportional to the heat output of the heat pump. Therefore the COP curves (with/ without auxiliary cycle) diverge and a better improvement of the seasonal COP is found. A further improvement of the SCOP can be achieved by optimising the heat exchangers (+12%) and by variation of the heating water mass flow (+7%). A promising refrigerant for the high temperature application is Isceon 59 (R134a/R125/R600 [46.5/50/3.5]), which was tested on the same unit. The measured COP was found to be the same and a reduction of the heat output of 15% compared to the R407C was measured. The related advantages are the lower condensation pressure level (same as propane) and the reduced compression output temperatures. Contrary to R407C the point A-12/W60 can be achieved within the limits in pressure (25 bar) and temperature (120°). The control of the expansion valve is not a critical point and the cascade control is not needed in this context. Improvements of the regulation in transitory conditions are proposed in chapter 6 of this report.