This work describes the system for the preparation of hot water in the industrial plant of Gredelj in Zagreb. A block diagram of the entire existing heating system is presented together with some important components. The components contain the technical specifications and show the consumption of the required heat energy for the preparation of hot water. The current system utilizes natural gas and LNG as heating fuel. Taking into account the real need for the hot water, for the existing system has been made a calculation of the conceptual solution for the modernization of plants using renewable energy sources i.e. solar energy. The modernization of the industrial plant for the preparation of hot water in Gredelj is presented with the introduction of solar collectors. Modernization is shown by the scheme and by defining the components of the whole system with the corresponding technical specifications and calculations. A simulation of a modernized system consisting of several hydraulically connected parallel collectors has been performed. This simulation uses information about availability of solar energy in the city of Zagreb. The obtained results with the simulation are following - the generated energy by the use of solar collectors, the heat transfer to the heating system by the collector, the energy saving for the preparation of hot water in percent, an overview of the availability of the collector solar energy with a monthly view throughout the year, the maximum collector temperature throughout the year, CO2 emission decreasing and the entirely system efficiency.