Problem with calculatesurfaceflux in HT process

Hello !

I was wondering how you can extract an heat flux at the top of a 2D domain with an HT process.
I tried calculatesurfaceflux following this benchmark Tests/Data/Parabolic/LiquidFlow/Flux/2D/square_1.8e1_calculatesurfaceflux.prj · master · ogs / ogs · GitLab with no success (it starts with 0 than I keep having Nan values). Maybe it doesn’t work if you have certain boundary conditions ?

Please let me know if it is possible and how I can achieve it,