Monitored data from a low exergy building at Wentzinger Hof Freiburg was investigated for optimal energy operation. The first step was the investigation which involved the sub-division of building envelope into four balance boundaries and each boundary consists of its own active components such as geothermal bore well system, reversible heat pump system, buffer storage, heat exchanger and distribution system. The purpose of the analysis was to refine the monitored raw data to calculate the thermal energy consumption, auxiliary energy consumption and performance along the balance boundaries. The second step was the optimization and rating of the building envelope. The analysis yielded the performance and energy consumption for each boundary level. It was suggested from the optimization analysis that a considerable amount of energy can be saved through the optimized control strategies of active components, seasonal dead-band control and the operation of buffer storage. (C) 2013 Elsevier B.V. All rights reserved.