We show that the optimum operation of one heat exchanger is the operation that is known since long from experience, namely the one given by certain counter-current flows. The result is according to the principle of equipartition of forces (EoF). The principle of equipartition of forces poses ideal boundary conditions for operation. The boundary conditions may not be realizable in practice, but give a lower bound on the entropy production of the heat exchange process, and thus a measure on the efficiency of the process.