In the last decades, there is a lot of discussion about climate change and energy shortage. In building industry great amounts of energy are spent for heating and cooling which in return produce large quantities of CO2 emissions. Therefore, it is self-understandable that efforts towards more effective and sustainable solutions have to be made. This thesis deals with Thermally Activated Building Systems (TABS), which refer to temperature - controlled surfaces that heat and cool indoor temperatures by adding or removing sensible heat and where more than half of heat transfer occurs through thermal radiation. Aim of this research is to describe, analyze and evaluate the performance of this system with the help of Building Energy Modelling Programs (BEMPs). This report consists of two main parts. TABS are modelled with the help of Building Energy Modelling Programs (BEMPs) and specifically EnergyPlus and TRNSYS. The first part of the report is focused on comparisons of the results between EnergyPlus and TRNSYS. Several models have been tested in the two programs, initially simple growing progressively more complicated to determine the accuracy of the results in several cases. In the final step, models equipped with TABS are designed in EnergyPlus and TRNSYS and their results, regarding inside temperature, are compared. The study has shown that EnergyPlus and TRNSYS lead to significantly different results for building simulations under the given conditions. Additionally, possible causes of these differences have been found in the modelling processes of the programs. In the second part of the thesis a combination of two radiant systems for heating and cooling is introduced as shown in the figure below. The first system is concrete core activation (CCA), meaning that heating and cooling is provided by means of circulating water running in tubes embedded in the floor slabs. CCA which is slow response system is coupled with a fast delivery system which is radiative panels. The systems operate in a change-over configuration - CCA operates during night-time and radiative panels during occupation. Additionally, a 2-pipe distribution system is used which is cheaper but may lead to thermal discomfort. This part also provides a guideline or framework on the optimization this system's performance by lowering the energy consumption while maintaining the provisional thermal comfort in acceptable range. The parameters which influence the performance of the system are analyzed and a way to optimize them is provided. The evaluation of the system has shown that the radiative panels do not contribute significantly in the thermal comfort of a 1-zone model in contrast to CCA.