It is often claimed that energy is consumed, not only in everyday conversation but also in scientificdiscussions associated with energy and environmental issues. This claim conflicts with the first law ofthermodynamics stating that the total amount of energy is conserved, even though forms of energymay change from one to another. This is why we need to use the thermodynamic concept, exergy, tofully understand what is consumed.An optimisation of the exergy flows in buildings and the related supply structures, similar to otherthermodynamic systems such as power stations, can help in identifying the potential of increasedefficiency in energy utilization. Through analyses, it can be shown that calculations based on theenergy conservation and primary energy concept alone are inadequate for gaining a full understandingof all important aspects of energy utilization processes. The high potential for a further increase in theefficiency of; for example, boilers, can not be quantified by energy analysis - the energy efficiency isclose to 1; however, this potential can be showed by using exergy analysis (Schmidt and Shukuya2003).This paper outlines the international co-operation work in the general framework of the InternationalEnergy Agency (IEA), the ECBCS Annex 49 Low Exergy Systems for High Performance Buildingsand Communities (Annex 49 2006).