The behaviour of the thermal components is commonly described through a set of internal parameters: isentropic efficiencies, pressure ratios, effectiveness factors, pressure losses, temperature relations or differences, et cetera. In this paper a new type of internal parameters, the internal parameters θ (Royo 1994, Royo and Valero 1995) are defined, that are specially adequate for the characterisation of thermal systems in an analytical way.When a component of a thermal plant displays an internal deterioration (intrinsic malfunction) its performance gets worse. This fact is reflected in a variation of the internal parameters describing its behaviour. On the other hand, the remaining components of the plant may be affected (induced malfunction) because they are working under different operating conditions from the usual ones.Knowledge of variations of the internal parameters in each component of a thermal plant does not suffice to determine their performance state under these new conditions. Additional information is needed. The dissipation temperature parameter can supply it (Royo 1994, Royo et al. 1997) as is explained in this paper.Internal θ parameters and the dissipation temperature parameters are appropriate tools for the study analysis and evaluation of malfunctions in thermomechanical systems. Three examples of this are shown, trying to quantify the influence of an intrinsic malfunction on the whole plant. This paper shows a strickingly new and simple vision of the analysis of thermal systems.