The present project has the following content: The project investigates a heat generator replacement in two multi-family houses, where the oil boilers are replaced by indoor propane heat pumps with the two heat sources air and a peak load coverage via borehole heat exchangers. By designing the ground source for peak coverage, existing space and drilling depth constraints are overcome and the heat pump is operated without fossil fuel peak coverage. This is also supported by the high possible flow temperatures of the propane heat pump of 70 °C. Therefore, monitoring of the heat pumps over three heating periods serves not only for operational evaluation and optimization, but also for verification of simulation models, which are used to develop advanced design and planning recommendations for multi-source systems. Furthermore, an evaluation of indoor propane heat pumps in terms of efficiency and economy will be performed. The goal is to document a best practice system for multi-source use and indoor propane heat pumps.