The core must be a 3D mesh with a cross-section that is approximately rectangular in shape.

The primary fluid streamwise direction (see Equation 6–1) must be aligned with one of the three orthogonal axes defined by the rectangular core.

The pass-to-pass plane must be perpendicular to the primary fluid streamwise direction.

The two dimensions of the pass-to-pass plane can each be discretized into multiple macroscopic cells (macros), but in the direction perpendicular to this plane the macros cannot be subdivided.

Flow acceleration effects are neglected in calculating the pressure loss coefficient.

For the simple-effectiveness-model, the primary fluid must have a capacity rate that is less than that of the auxiliary fluid.

Auxiliary fluid phase change cannot be modeled using the ntu-model.

The macro-based method requires that an equal number of cells reside in each macro of equal size and shape.

Auxiliary fluid flow is assumed to be 1D.

The pass width has to be uniform.

Accuracy is not guaranteed when the mesh is not structured or layered.

Accuracy is not guaranteed when there is upstream diffusion of temperature at the inlet/outlet of the core.

Non-conformal meshes cannot be attached to the inlet/outlet of the core. An extra layer has to be created to avoid it.