I am modeling walls to test thermal performance, and I have found that as my model gets more complex the difference between the interior and exterior heat flux gets larger. I applied a film coefficient boundary condition on the interior and exterior of the wall, and on all the edges I put a heat flux boundary 0.
At first I thought it was meshing, but I still had the issue when I made the mesh as small as the software allows. The problem also goes away if I apply the same material to all layers of the system.
Why am I getting a different heat flux on the interior and exterior of my model?
As your model gets more complex, be sure that the mesh quality through the thickness (of each material) remains at a similar mesh quality. It is possible that your more simplistic models had a better quality mesh through the material thicknesses such that we were able to maintain an energy balance more easily.
There is no need to assign a Heat Flux of 0W on the sides/edges, we automatically assume that an exterior wall without any thermal boundary condition will be treated as adiabatic.
Thank you for the Adiabatic tip, that will help speed up the model making process.
I have even tried the simple wall below to show that meshing could not be the issue. I made this super simplified wall model with 1" thick studs with the settings you described above, and I am still getting different heat fluxes on the interior and exterior.