How to compute flow rate?

I noticed that this question has been asked several times, but I still haven’t seen an answer that would help me really understand what is going on, and if I am doing something wrong.
I would like to compute the volume or mass flow rate through a surface (typically, a surface obtained by contour of given variable).
In order to get my ideas straight, I thought I’d do that on a very simple case (the openFOAM “windAroundBuildings” tutorial, to be accurate), taking the boundaries of a steady incompressible CFD computation. Since I have then an enclosed region (a box, basically), I would expect that the net flow rate through all boundaries would be close to zero.
Well, I tried both using the SurfaceFlow filter on these boundaries and using GenerateSurfaceNormals > Calculator with (U dotproduct Normals) > IntegrateVariables. Both operations give more or less the same result, a value of around 9300.

Now, I get that being a quite coarse and simple tutorial case, it might not be converged, but my question is: Is my understanding correct that I should obtain a zero value instead? (if perfectly converged) In other words, are the operations I am performing within Paraview the correct way to obtain the flow rate through a surface?

Thanks a lot,