This Master thesis has been focused on energy retrofit project of an elementary school in Rijeka and the fuel conversion to natural gas with the design of the necessary gas installation and boiler room reconstruction. The building heat load was calculated in compliance to the standard HR EN 12831. The boiler room heating capacity were rated taking into account the heat loads necessary for space heating and how water heating after the energy renovation of the school building . The heating energy demand for space heating and DHW preparation as well as the annual natural gas consumption were estimated following the standard HRN en 13790. The gas installation was designed according to the tehnical regulation for gas installations according to the manual HSUP-P600. The project includes the gas connection to the city gas distribution system, with the dimensioning of the gas connection and gas pipelines, the internal gas installation, the design of the ventilation system, the design and rating of the ventilation for the boiler room along with the selection of the safety equipment, armature and devices for gas consumption measurement. The last part of the work describes the heating system's equipment as well as the technical measures for fire protection and safety at work. The thesis is made as a project, the detailed drafts on the architectural background are attached inside, as well as the scheme of the connection to the natural gas heating system. The costs of equipment and related work were also estimated.