The subject of this project is to develop a high performance air water heat pump which is able to be introducted in the retrofit market. The system requirements of this market are heating temperatures of 65°C, at ambient temperatures descending until -12°C. To provide higher base performance ratings, the choice of the thermodynamic system is a two stage heat pump with economizer exchanger. This type of heat pump has improved performances over a large range of application. For heat output adjustment it can be switched to different heating modes (single stage and two stage). For the two stage cycle, the lubrification of the compressors is a major aspect to be studied. A new online measurement method is applied to evaluate the oil quantity which migrates through the cycle. The absorption spectre of the refrigerant - oil mixture is analysed with a Fourier Transform Infrared (FTIR) spectrometer and oil quantity can be extracted with high precision. (The analysis method has still to be refined). The infrared spectometers also allow to control the composition of the refrigerant mixture. A single capture of the spectre needs about 30 seconds. The measurements have to be referred to a regularly updated background spectre, which requests to vacuumize and clean the measurement cell. The oil migration is simultaneously measured with a high precision densitymeter. The output signal allows a real time follow of the oil quantity and the transmitter will give reliable results after calibration over the application range. Any change of the refrigerant composition cannot be captured by the densitymeter. The resulting density shift would therefore not be attributed correctly to the amount of oil. For the analysis of oil distribution in the whole heat pump, the measurement method to determine the oil level in the compressor crankcase, and the observation of the oil retention in the evaporator are also discussed in this project. The heat pump design has been made, based on simulations of the thermodynamic cycle and using existing parts in the laboratory, such as the evaporator and the condenser. The design aim is to avoid oil trapping and to ensure oil transport in the vapour lines of the heat pump. free download : www.waermepumpe.ch/fe/berichte.html