calculating mass flow rate from exported velocity values
 

Hi.
I made some calcs on a simple incompressible flow in enclosure (laminar Rayleigh, Boussinesq approximation, two vertical walls at different temperatures). First order upwinding, default settings. After the residuals have fallen to e-20!! I've stopped. Marked some rectangular region in the middle of the domain. Splitted the fluid interior (it was only one in the original mesh) into two parts - inside the marked rectangular and outside it. The result was two additional interiors - one the "inside" of the marked region and the other is its boundary. Splitted the boundary into four lines. Making report on mass fluxes through the closed rectangular boundary (sum of the fluxes through each one of the 4 lines) gives flow rate very close to zero. Exported the cell values of the velocities of the whole domain. Tried to calculate the flux through the same 4 lines making use of the "upwind" cell scalar value to obtain the face value of the velocity vector perpendicular to the faces (lines) caculated - up to 15% difference in the flux through each one of the lines????
So, finally, how should I, or, how exactly Fluent takes care of continuity? How can I check it aside from the program, i.e. using the exported values of the field variables? What is the algo?
Thank a lot for the patience (to those who had one).