The article is dedicated to the important modern problem of assessing the energy efficiency of heat supply systems. The author sets a task to elaborate the criteria of assessing the energy efficiency of building heat supply systems taking into account the key thermal parameters. Thermal parameters of heat supply systems and the factors affecting them were observed. The key parameters for energy efficiency assessment of heat supply systems were identified. A new criterion for practical assessment of energy efficiency in heat supply systems was offered using these parameters. The criterion allows obtaining an estimate for the energy efficiency of a system at minimal measured parameters and simple practical calculation. The criterion can be used for either the heat supply system of a single building or a group of buildings. The author offered the method of energy efficiency assessment of heat supply systems based on pattern comparison of real, optimal and reference systems using the criterion. A brief analysis of the existing scientific researches was given. The author performed a preliminary estimate of the energy efficiency of the largest heat supply system of Chita using the criterion. The most typical problems of low energy efficiency of this system were emphasized using additional energy inspection.