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Heat pumps are expected to play a central role in decarbonizing heat supply, but face challenges in existing buildings due to high temperature requirements of existing radiator systems. This paper links the performance analysis of heat pump systems with methods to reduce temperatures of the space heating circuit. Field data and system simulations of air and ground source heat pumps show a linear correlation between the seasonal performance factor SPF and the mean heat pump temperature over a wide temperature range. Every Kelvin of reduced heat pump temperature increases the SPF by 0.10–0.13 points. Applied methods to reduce heating temperatures are demonstrated at existing multi-family buildings. Thermal insulation reduces the building's heat load, allowing a reduction of heating temperatures with the existing radiators. A further temperature reduction is achieved by analyzing the required heating power per room and identifying critical, undersized radiators. In a studied building, the selective exchange of only 7% of all radiators is sufficient to reduce heating temperatures from 75 °C/60 °C–55 °C/45 °C. This corresponds to a reduction of the electricity consumption by 40–42%. However, the potential of these methods is specific for each building and depends particularly on its renovation state and installed radiator capacity. Nonetheless, an energy- and cost-efficient operation of heat pumps retrofitted in existing radiator heating systems is viable, if following the proposed system design method linking heat pump performance and reduction of space heating temperatures. • Performance analysis of retrofit heat pumps by field tests and system simulations. • Effect of heating temperatures on heat pump performance is analyzed. • Measures to reduce temperatures are applied in existing multi-family buildings. • Selective exchange of 7% of all radiators reduces electricity consumption by 40%. • Methods improve energy and cost-efficient operation of heat pumps in existing buildings. [ABSTRACT FROM AUTHOR]