In the introductory part of this paper, natural gas has been introduced as an energy source since the very beginnings of its use. In short, its features and advantages over other sources of energy and possible ways of using are shown. Below, the principle of the condensing gas boiler is explained in detail, since it is precisely this that is selected as a source of energy in this project. The basic terms on gas systems are briefly explained. Calculation of heat losses and required heating power for sanitary hot water was carried out on the basis of which heat source was selected, as well as annual energy budget calculation and annual gas consumption for the public utility building located in Rijeka. Most of the calculations were executed using computer programs and the process has been briefly explained. Then the dimensioning of the gas pipeline, the heating pipes, the boiler ventilation and the gas boiler chimney was done. All the results are satisfactory according to the rules and regulations in force. In the last part of the work, the selection of equipment with manufacturer specifications and technical characteristics was performed for all system elements. Selection includes equipment from domestic and foreign manufacturers. The entire project was supplemented with the relevant draft documentation with detailed drafts and schemas on the obtained substrate and the cost of equipment and related works.