In cogeneration units cooperating with District Heating Systems the power rating of the back-pressure turbine should not be chosen to cover the maximum demand for heat. This results from the character of the duration curve of external temperature conditioning the heat demand for space heating and ventilation. The power rating of the back-pressure turbine ought to be chosen according to the optimal coefficient of the share of cogeneration. This coefficient defines the ratio of the maximum heat flux from the back-pressure steam to the maximum demand for heat. The optimal coefficient of the share of cogeneration depends mainly on the ratio of heat required to get hot tap water in relation to heat needed for space heating and ventilation. The paper presents an algorithm which permits to determine the value of the optimal coefficient of the share of cogeneration depending on this ratio. The final result of investigations is an empirical equation describing the influence of the ratio of the heat flux for the production of hot tap water to the maximum flux for space heating and ventilation on the optimal value of the share of cogeneration in district heating systems. The approach presented in the paper may be applicable both in back-pressure CHP plants, as well as in extraction-condensing CHP plants. (C) 2012 Elsevier Ltd. All rights reserved.