It is often claimed that energy is consumed; this is not only done in everyday conversation but also inscientific discussions associated with energy and environmental issues. This claim conflicts with thefirst law of thermodynamics stating that the total amount of energy is conserved, even though forms ofenergy may change from one to another. This is why we need to use the thermodynamic concept,exergy, to fully understand what is consumed.An optimization of the exergy flows in building, similar to other thermodynamic systems such as powerstations, can help in identifying the potential of increased efficiency in energy utilization. This papershows, through analyses and examples, that calculations based on the energy conservation andprimary energy concept alone, are inadequate for gaining a full understanding of all important aspectsof energy utilization processes. The high potential for a further increase in the efficiency of; forexample, boilers, can not be quantified by energy analysis - the energy efficiency is close to 1;however, this potential can be showed by using exergy analysis.This research work is related to the finished international co-operation work in the ECBCS Annex 37Low Exergy Systems for Heating and Cooling of Buildings and the newly started in ECBCS Annex 49Low Exergy Systems for High-Performance Buildings and Communities (Annex 49 2006).