Experience shows that often theoretical heat demand and real building heat use varies, and in some cases a significant percentage. This can be explained by the fact that the identification methods of rooms’ heat demand are inaccurate or incorrect use. One issue for discussion is the building's heat gain use size of the overall balance of the building's energy needs. Currently, both legal instruments and computer tools take the heat gains use 100% or nearly for building heat needs to cover what in the real building is almost impossible to achieve because of heating system efficiency.