In this Master's thesis the performance of a heat delivery system of an energy efficient single-family house located in Zwettl, Lower Austria, is investigated. Over the course of ten months, in which the building was inhabited, monitoring data of the heat delivered to the floor and brick walls was collected. Using these data, a detailed analysis of the building's performance is presented. In particular, it is compared to the simulation model that was created during the design phase of the house. It turns out that the design phase model is rather inaccurate and often does not match the actual behavior. Consequently, this simulation model is investigated and enhanced. Particular improvements concern the following input parameters of the building regime: set temperature points, natural ventilation, infiltration, internal heat gains and weather data. Most of these are based on the actual user behavior, which was derived from additional sensors in the building (e.g. temperature, CO2 concentration, etc.). The results are presented by comparing the improved simulation to the design phase model and to the actual monitoring data in a monthly evaluation. The particular influence of each mentioned parameter is analyzed and discussed separately. It is shown that the enhanced simulation much more accurately models the reality.