Heat flux through a boundary

Hi guys

Ive been roughly following this tutorial to model a heat exchanger www.youtube.com/watch?v=7U5ue03K7gQ

Using different geometry and BC's

It uses the expression intop1(nitf.qwf_u) to calculate the total power exchange

Im having some trouble getting this to work in my model, I understand that intop1 means it is integrating nitf.qwf_u over the boundary but I just dont know what nitf.qwf_u means and what the equivalent in my model would be.

Any help is appreciated

Thanks