This article focuses on low-energy District Heating (DH) systems operating in low-temperatures such as 55°C in terms of supply and 25°C in terms of return in connection with existing buildings. Since the heat loss from the network has a significant impact in case of supplying heat to low-energy houses in a traditional DH network, the aim in this paper was given to reduce the dimensions of the low-energy DH network as much as possible. Hence, the performance analyses of the inhouse radiator heating systems equipped in existing buildings were carried out for low temperatures of supply and return. The response of the radiator heating systems at different levels of supply temperature was used to form the operational planning of the low-energy DH system, which determined the design parameters of the low-energy DH network in terms of overall mass flow requirement and the return temperature from the buildings. Since the existing buildings were considered to be renovated to low-energy class, the operational planning was simultaneously modelled for both present high-demand and future low-demand situations of the same case area.