The influence of ventilation on the energy performance of a building is generally considered to be quite high. It rises clearly, when a building contains large-volume spaces. A famous example building for that is the Reichstag building in Berlin, housing the German parliament. The huge plenary hall in the buildings centre, which occupies only 4% of the total net floor area but 20% of its net volume, is equipped with a powerful air-conditioning system. The remarkable effects of slightly different settings of this ventilation system on the buildings energy performance are illustrated by example calculations based on the German standard "DIN V 18599: Energy efficiency of buildings". Extending the systems operating time significantly increase the zones energy use by up to 205%, when running the system throughout the whole year, leading to a growth of the buildings total delivered energy of 18%. A clear influence is also shown by the existence and quality of heat recovery, which is analyzed by stepwise improving the recovered heat coefficient from 0 to 45%, 60% and 75%. The latter reduces the zones heating energy by half and that of the whole building by 6%. In conclusion, the analysis clearly demonstrates the high impact of ventilation in large-volume spaces. Consequently, especially for these areas, an energy efficient air-conditioning system should be installed and a reasonable ventilation strategy developed, as realized in the Reichstag building.