To reduce carbon emissions in buildings, the share of renewable energy for heating must increase significantly. For an apartment building it can be challenging to integrate these renewable energy technologies, due to lack of space in a single apartment unit. Therefore, switching to a collective heating system facilitates the incorporation of renewable sources, but inevitably, increases the distribution losses, reducing the energy efficiency of the system. This case study analysis discusses the theoretical and actual performance of three multi-family buildings with collective hydronic heating systems with a single circulation pipe for both space heating and domestic hot water, connected to an array of solar thermal collectors. Calorimeters at various points in the system allow a detailed mapping of the heat flows in the buildings. The analysis of this monitoring data show that considerable heat losses occur throughout the storage and distribution system, resulting in a low system efficiency. Particularly in summer months, only a fifth of the generated heat is used beneficially, since there is no space heating demand but the circulation pipe continues to pump water at a high temperature to cover the limited demand for domestic hot water. These total heat losses are significantly higher than what was theoretically determined. Moreover, the contribution of the solar collectors is limited and the absorbed solar heat injected in the system does not outweigh the distribution losses. Consequently, the choice for a collective installation must be weighed with care so that it can contribute to an improved energy performance.