As the title of the thesis indicates “Feasibility study of modifying Gävle Folkets Hus's ventilation system to improve efficiency”, the target of this project is to suggest improvements on the ventilation system of the building in question in order to enhance energy efficiency of it, focusing on reducing energy losses and optimizing energy consumption. Implicitly, the modifications are also channeled to create an adequate indoor air quality. “Gävle Folkets Hus”, is located in Södra Centralgatan 10, 80250 Gävle, Sweden. The edifice was built in 1946 and some modifications were made in 1980. Inside this 5584 m2, 5 floors and 300 rooms building many activities are performed: restaurant, theater, offices, etc. The major parts of the project are explained as follows. Firstly, the whole energy balance of the building is calculated: gains from people, appliances, lighting, sun and district heating and losses from ventilation, transmission and hot tap water. Regarding the calculations of the energy balance, many aspects have been studied such as the ventilation, tap water and heating systems, the analysis of the architectural planes, the thermal transmission coefficients of the materials used in the building, the level of occupation and activities to which the building is submitted and the climatological conditions to which the building is exposed.The total quantity of energy inputs reaches 909,64 MWh and the quantity of energy outputs is 826,41MWh. Inputs varies 10% from outputs which is a reasonable value. Subsequently, the whole ventilation system is measured so as to identify if the air distribution is adequate. The capability of the ventilation system is compared with the ventilation requirements for new Swedish buildings according to the Swedish Code BBR 2008 and Socialstyrelsen recommendations. High quality equipment from the company SWECO has been used to do these measurements. Finally, a feasibility study of improving the ventilation system is done. Economic studies of a new ventilation installation and a heating pump are done. It is also considered the possibility of installing presence detectors and electronic fan controllers. The investments for the three improvements are 1.000.000 SEK, 200.000 SEK and 20.000 SEK respectively. These studies improve the energy efficiency and therefore, produce benefits. The whole new ventilation installation has a Net Present Value (NPV) of 169.933 SEK, being the payback in 11 years. Concerning the heating pump, NPV is 15.448 SEK and its payback is 8 years. Finally, the installation of detectors has a NPV of 60.528 SEK and its payback is only 1 year.