In the present work, the formulation of critical manifolds for state controllability is proposed. These critical manifolds are a boundary in the design parameter space that splits it into two regions, one where the controllability is guaranteed and the other one where the dynamic system loses the controllability. We demonstrate that loss of controllability due to model parameter variations induces a one-parametric nonlinear controllability boundary in the two-dimensional parameter space. A heat exchanger is considered to illustrate the results of this work showing that not all the combinations of design parameters guarantee the controllability property.