We investigate the effect of the wall-scalar fluctuations on passive scalar turbulent fields at the moderate Reynolds number R_tau=395 and at several Prandtl numbers ranging from the very low value Pr=0.01 to the high value Pr=10 by means of DNS simulations. The case I is a channel flow heated on both walls with a constant imposed heat flux q_w. We consider then two different boundary conditions. In a first step, the isoscalar boundary condition ?w =0 is imposed at the wall implying that its fluctuation is zero at the wall whereas in a second step, ?w is not prescribed to a fixed value so that it is fluctuating in time at the wall. For illustration purpose, we also consider the case II that is a plane channel heated only from one wall but cooled from the other one at the same rate. For both cases, the distributions of the mean scalar field, root-mean-square fluctuations, turbulent heat flux, correlation coefficient, turbulent Prandtl number, Nusselt number are examined. It is found that the mean scalar field is not affected by the scalar fluctuations at the wall. Significant differences in the evolution of the rms scalar fluctuations are observed in the immediate vicinity of the wall. Surprisingly, the maximum rms intensity remains almost unchanged in the near wall region In addition, the turbulent heat fluxes that play a major role in heat transfer are found to be independent of the wall scalar fluctuations.