The dynamic thermal interaction between a building and the HVAC systems which service it is still difficult to predict. As this thermal interaction becomes more critical in practice, related knowledge and evaluation tools become increasingly important. It is argued why these need to be based on an integral approach of the overall problem. A research project aimed at development and/or enhancement of building performance evaluation tools for this field of interest is outlined. The work involved expansion of an existing building energy simulation environment on the fluid flow and plant simulation side. Application of this integral simulation system is demonstrated by means of a case study.