Sustainable building heat recovery ventilation is a key factor in modern structural design and production, which can be achieved by the combination of cutting-edge, energy-economical technologies, such as counter flow flat-plats, heat pipes and simple design arrangements like a light well. To achieve low energy use and good indoor air quality, natural ventilation should be applied as often as possible. In this article, an energy simulation study is presented, which is focused on an in-built natural ventilation system of an existing vacant office building. It has been demonstrated that a substantial amount of energy can be saved by utilizing natural ventilation in a building, while it was also proven that obsolete buildings can be energy-efficiently operated, without the need for demolition, if they are cautiously re-designed.