The aim of the paper is to discuss the possible use of Combined Heat and Power (CHP) plants and to highlight some abnormal effects, generated by the available support mechanisms, both in sizing and management and to propose guidelines for defining optimal operational strategies of CHP power plants. Some composite indicators are proposed to give a more comprehensive assessment of the performances of a plant, for the design and optimization phase and for a possible approach to support mechanisms. A new composite indicator is introduced in order to assess the benefits of different scenarios. The method is tested with reference to a case study: a medium size district heating system, powered by a CHP plant supported by conventional auxiliary boilers. Data coming from a real plant equipped with a remote monitoring system are analyzed. Operating data of a typical month are used in order to test the approach for the reference system. The paper shows how the use of the defined composite indicators can modify in a meaningful way the operating strategy of the CHP, increasing a lot the share of thermal energy produced with the CHP unit with respect to the conventional boilers. (C) 2017 Elsevier Ltd. All rights reserved.