The paper presents a method for estimating the energy use efficiency and energy reserve conservation in space heating. The concept of theoretical energy conservation potential is introduced. Estimate of the value of a minimum required heat consumption, which takes into account climatic conditions and sanitary-hygienic standards, is used. Calculations illustrating the elaborated methods have been done for Irkutsk as an example. (C) 2000 Elsevier Science S.A. All rights reserved.