The capacities of performance improvements of low and mid-size heat pumps are studied in this project. For the new buildings market, the components of the heat pump match already well the requirements for high performance and monovalent heating. Minor improvements are to be expected on inverter driven compressors and on the integration of the domestic water in the heat pump cycle. This study is focused on the retrofit market (replacement of existing oil burners). For this case the existing air-water heat pumps are not economical and performance decrease rapidly at low air temperatures. A two-stage heat pump with two phase injection and an economiser vessel has been built in collaboration with Termogamma. It has been compared to a single-stage configuration (Pompe à chaleur biétagée à haute performance, Phase I). The measurements at steady state conditions show significant improvements on heat output and coefficient of performance for the refrigerants R22 and R407C. In phase 2 of this project the tests are focused on a new defrosting scheme using energy stored at low exergy in the economiser. This method is compared to the reversed cycle defrosting at critical conditions A2/W50/93r.h. Both defrosting methods have equivalent performance reduction. Relative heat output reduction is -12% COP is reduced by -4% comparing to the steady state conditions (evaporator not frosted). Combining the two methods, reversed cycle and economiser, the overall performances are further improved (heat output -7% and COP -3%). The complete cycle length was limited by oil migration effects. The mechanisms of oil migration in two-stage heat pumps will be examined in a starting project at the Laboratory for Industrial Energy Systems (LENI). Several types of refrigerants are compared in the single-stage and two-stage case by simulation. The simulation includes real compressor and exchanger characteristics. The results show the important decrease of the coefficient of performance and the reduction of the heat output at low air temperatures. At higher temperatures the single stage mode is more adapted for the existing compressors. COP crossing occurs between 0°C and 5°C external air temperature. For the retrofit application, traditional heat pump design cannot be explored efficiently in the case of monovalent heating. This two stage concept is very promising, but oil migration problems have to be solved for commercialisation. The refrigerant cycle can be simplified, according to the refrigerant type used. The economiser did not show the expected improvement for the defrosting cycle. The major part of the energy potential could not be explored in the tests. For an air-air heat pump the increased refrigerant charge can be justified with the important improvements on the defrosting (only hot gas defrosting can be implemented in this case).