Heat balance calculations based on the interior surfaces of boundary zones will be incorrect.
The reason is because of the fact that in FLUENT the values on the interior surfaces are calculated based on the averaged adjacent cell center values. For an inlet surface the difference between cell center values in the zone and out of the zone are different and taking average of them for the inlet surface causes to have higher enthalpy value (in case of adding heat) that ends up to under estimate the heat mdot(h_out - h_in)=Heat source
To transfer the inlet to some place a bit further of the zone such that the adjacent cell which is outside of the zone is used to calculate the values on the surface.
This can be done in FLUENT under Surface/Transform
a) Save the boundary conditions in FLUENT (file/write-bc bc.dat)
b) Open the case file in TGrid and flip the normal vectors of the inlet and outlet such that they show outward directions of the zone. Save the *.msh file.
c) Read the new *.msh file and read the bc file and use facet average in the surface integral to report the enthalpy.