An iterative numerical method to determine g-fwictions of borehole fields with the boundary condition of uniform temperature and time-constant surface-averaged heat flux is proposed. The method employs boundary conditions of uniform time-dependent temperature that converge to time-constant surface-averaged heat flux. Two simulations are sufficient for technical purposes, while additional simulations yield very precise g-functions with the desired boundary condition. The method is applied to analyze the overestimation of the g-function yielded by the condition of uniform and constant heat flux and the effects of the buried depth and of the spacing between boreholes on the g-function, for a 3 x 2 borehole field.