Heating, cooling and production of domestic hot water are the dominating energy uses in residential buildings. District heating (DH) provides the thermal energy for these uses to 65% of the Danish homes. However, 50–60% of the DH substations in these dwellings operate with an error, often leading to inefficient operations (i.e., high temperature or high-volume flow of the return heat-carrier fluid in the DH network). This hinders decarbonising the heating sector. The roll-out of smart heat meters allowed access to the hourly heat demand profiles of the DH customers. In turn, this unlocks the creation of data-driven methods for identifying faulty household systems. However, this cannot be done correctly without the knowledge of the fault types occurring in domestic installations. The present study analyzes 382 reports made by the DH technicians during onsite visits to the houses identified as “faulty” customers. The collected onsite information shows that more than 30% of faults stem from wrong control settings in the space heating or domestic hot water installations. These faults can be fixed with almost no additional cost to the building owner and with the immediate decrease of the return temperatures to the DH system.