The Laboratory of Industrial Energy systems has evaluated a series of high performance air-water heat pump concepts with the aim to retrofit existing fuel fired boilers for residential hydronic heating systems. The results are published within the framework of the Swiss Retrofit Heat Pump project, which is conducted by the Swiss Federal Office of Energy. The two-stage heat pump cycle has shown the best efficiency prospects. Oil migration between the upper stage and lower stage compressors is the main obstacle for commercialization in the low heat rate range. The oil mass concentration is measured by the Fourier Transform Infrared Spectrospopy (FTIR) method with an attenuated total reflection cell (ATR). This method was calibrated on a separated test loop and compared to measurements of mass-sampling and of density by a Coriolis effect densitymeter. Infrared analysis gives a precision of < 0.1% of oil concentration and the detection limit was identified at about 0.3%. Density measurements cannot be used in the ordinary temperature range of the liquid line in the heat pump. Mass sampling and the FT-IR measurements give corresponding results. An air-water heat pump (10 kWth, R-407C) was build in the laboratory to perform comparison tests in the single-stage and two-stage heating modes. The main elements of the heat pump are: two scroll type compressors, a brazed plate condenser, a finned tube evaporator, an economizer heat exchanger and two electronic expansion valves. Defrosting is done by cycle inversion with a 4-way reversing valve. Measured improvements on the two-stage heating mode of 30% for the COP and 24% for the heat output comparing to the corresponding test point (A-7/W50) in the single stage configuration have been shown. Important reductions of the maximum system temperature at the outlet of the compressors enable an extended range of application (including the point A-12/W65). In the two-stage heating mode an oil transfer rate of 5 - 10 g/min from the high pressure stage to the low pressure stage compressor was detected in all tested conditions, which is strongly limiting the continuous service of the heat pump. For long term running tests an additional balancing system composed of an oil pump and an oil vessel has been installed. The circulating oil concentration is between 0.2% and 0.4% for the two-stage configuration as measured with the FT-IR method. In single-stage mode the concentration is lower than 0.2% and only the mass sampling technique provided reliable results. Sensibility analysis in changing the regulation parameters did not give concluding results in order to design a stable configuration of the two-stage heat pump. In absence of integrated two-stage scroll compressors intermediate solutions have to be considered. These consist in the implementation of a booster compressor or the implementation of an oil separator at the outlet of the high pressure stage compressor coupled with a oil return management or a periodic shut down of the heat pump with external balancing of the oil level in the crankcase. An adapted heat transfer model for refrigerant-oil mixtures was developed, in order to investigate the theoretical impact of oil in the evaporator.