Creat surface/ partition

[eroque]

Hi,

So, i´m doing thermal analysis and i need to define a surface to calculate my heat flux across that section.

The anex image represent what i want to do. I already created a partition on that zone but now i can´t select just that rectangle to ask for the flux there. I cant select because is not an external face, and i cant reach it.

Any ideas ?

Thanks !!!!