High-temperature R290 heat pumps offer a promising solution for decarbonizing existing buildings that are equipped with traditional hot water heating systems. Compared to conventional combustion boilers, air-to-water R290 heat pumps can both reduce energy consumption and abate greenhouse gas emissions, especially if the heat pump is driven by green electricity. By leveraging the efficiency of R290 as a natural refrigerant, these heat pumps can provide high-temperature heating (up to 70 ℃), making them suitable for replacing existing boilers without compromising comfort levels. This can be obtained without requiring major renovations on the envelope and secondary systems of the building. However, the discrepancies between expected and actual performance emerging from the monitoring of current heat pump technologies and the limited available dataset of real data highlight the need to investigate what are the performances under real operation of such primary systems considering the annual dynamics of energy demand for space heating and integration with DHW and how the working conditions (e.g. design temperature, radiator sizing, outdoor climate) can affect their final efficiency. In this research, we investigate the energy savings that can be obtained by R290 heat pumps on real buildings of different sizes compared to the most efficient condensing boilers. In particular, how the operation and control of modular heat pumps can be optimized in large systems can also be derived. The primary energy savings and the environmental benefits are evaluated for different climates.