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In this report different air-water heat pump cycles under the operating conditions of the retrofit market and in the low heating power range are examined. The already existing heat pump models show excessive power and efficiency loss at very low ambient temperatures. Due to the variations of the source temperatures the heat pump system has to be improved over a relatively wide application range. The existing perfomances are insufficient and new solutions have to be found. The introduction of a supplementary compressor, as well as the efficient use of the liquid subcooling are two examples, which will determine the selection of the cycles (see pages 4 - 7). By simulation of the suggested cycles, like the heat pump with the auxiliary liquid subcooling cycle, as well as the two stage cycles (with intermediate expansion or intermediate injection) it can be shown that these cylces operate with an improved seasonal coefficient of performance up to 10% - 15% (see table 4.4). Heat pump cycles with more than one compressor permit a better adjustment of the heat output. The seasonal coefficient of performance of these heat pump cycles is calculated with the temperature distribution of the climate in Zurich and a given heat curve. The cycle components are simulated on a purely theoretical base, with the exception of the compressors characteristics and inlcuding the real heat output from the heat pump. The cycles are calculated with pure refrigerant and with zeotropic mixtures. The results with propane show equivalent seasonal coefficients of performance on all suggested cycles. The heat pump cycle with liquid subcooling shows good performance in the middle range of the air temperatures. The main compressor will then work at its best conditions. At low external temperatures the two-stage cycles with two serial connected compressors, show very good performance. And at low temperature differences between the air and the heating water single-stage heat pump are the best with the actual compressor types. With zeotropic refrigerant mixtures (R407C, R290/R600a), the advantages of the proposed cycle with liquid subcooling in the condensing part are not significant and the losses, due to the low performance of the auxiliar compressor. will lower the coefficient of performance under the level of the other two-stage cycles (see table 4.3). The simulation results show that several solutions can be considered for this application. An evaluation of the costs and the resulting technical problems can only be mentioned at this time.