Mass flow controlled district heating with an extract air heat pump in apartment buildings: A practical concept study

•A novel low-temperature district heating (DH) system is constructed and monitored.•Traditional network changed into ring network and it applies mass flow control.•Share of heating energy from added heat pump is greater that of the DH.•New type of control and network planning strategy constructed in practice. Low-temperature district heating (DH) with ring network and mass flow control was constructed and monitored in an existing apartment buildings in Kaarina, Finland. The extract air heat pump was connected in parallel to the area heating. Implementation of the new concept was successful. The study shows the right functioning of the system. The share of DH was 45% in the monitoring period and the rest of the heating was generated by the heat pump. The main result of the study is the practical construction of the new type of control and network planning strategy for the first time in practice after a series of research steps. Mainly, the practical study was in line with the theoretical results.