This study intends to investigate the optimal switching modes of a double-capacitive thermal system under different constraints on the state and the control variable, by the application of the Pontryagin's Minimum Principle. Throughout the development, the control effort is assumed to have two modes of state: M or zero and the terminal times being fixed. In the first part of this study, the Principle is discussed under various conditions for this particular problem, with different criterion functions and in the same time imposing a certain constraints; i) on the terminal states, ii) on functions of the terminal states. Depending upon the upper bound value of the control vector, possible driving modes of the states are studied from which particular optimal driving modes are extracted so as to meet the specified constraints and boundary conditions imposed in the problem. Numerical solutions are evaluated for an over0damped, double-capacitive thermal plant and the optimal solutions: the switching mode, the optimal switching time, and the control effort are compared with the analytical results, in the second part of this work, to confirm the development.