Low temperature district heating has significant advantages in energy efficiency, but a huge amount of existing radiators lack the capabilities for low-temperate heating. The innovation of this study is to develop an optimal and techno-economic method to improve the heating power of existing radiator by mounting a small fan considering different hydraulic connection modes. An experimental test rig was designed to study the optimal installation positions and angles of the fan. For a dormitory room in China, a computational fluid dynamics (CFD) model was developed and verified. The model was used to determine the lowest supply temperature of the radiator. Results show that the fan should be placed in a position and angle that blows air over the hottest surface of the radiator i.e. the hot center. The lowest supply temperatures before and after installing the fan are 42.3 °C and 39.5 °C. The response speed is increased by 28%, stability time is shortened by 13%, while the maximum indoor temperature difference is reduced by 15% and the maximum indoor air velocity is reduced by 0.07 m/s. Payback time is 63 days for case study, indicating a good economic feasibility. The method is beneficial to both the heat plant and users.