Proposition of updating the method used in calculating the heat demand based on a new concept of design outdoor temperature and of building – soil boundary heat transfer

The dimensioning of the heating systems equipping new and existing buildings, in the case of their energy-related upgrading is an extremely important activity in the context of reaching the targets of the European Directive 31 /2010 / UE concerning the Buildings Energy Performance (PEC). The accurate determination, phenomenological based, of the buildings thermal response leads to determining the climatic parameters representative for the climatic zones and for the buildings structure.Unlike the EN 12831: 2003 European Regulation, the design outdoor temperature in the conditions of Romania’s various zones was determined by the identification of the thermal response specific to the transient conditions of the heat transfer through the composite structures of the opaque and glazing closing components with the thermal response in idealized, steadystate conditions; thus, the design outdoor temperature was determined, which is conditioned by an acceptable discomfort during the coldest pentads of a 48 years climatic statistics (1961-2008). The climatic parameter which generates the modeling similitude is the virtual outdoor temperature which allows the use of the steady-state conditions mathematical formalism in issues of heat transfer in transient conditions. A dependency relation between the design indoor temperature, identical to the resulting indoor temperature (different from the operational temperature) and the air volume average temperature is emphasized.Special attention is given to the heat transfer at the building-soil boundary, in the form of various practical solutions (buildings the basement of which is not directly heated, equipped or not with heating systems, directly heated and occupied, as well as buildings on plinths); in all the cases, the solutions approached are specific to the envelope which is or not thermally insulated.