In order to extend the application range of air-water heat pumps to high temperature residential heating systems, new small hermetic compressors with remodeled injection ports for vapor injection have successfully been tested in laboratory. The injection flow has been controlled by means of either an electronically regulated expansion valve or of a capillary tube. Extreme load and operating conditions could be achieved (air –12°C / water 65°C), using R-407C, while keeping the limits in discharge temperature and pressure of the manufacturer. Using an economizer heat exchanger in the mid pressure stage of the cycle, the heat output in very low external temperatures could be raised up by 30%, compared to the reference cycle without injection. The COP could be improved on a broad range of operating conditions. Based on the experiences of tests with three distinct injection flow paths, a new developed generalized compressor model is presented and the geometric design aspects are discussed in order to further improve the system efficiency.