SUMMARYIt is shown that codes employing boundary-fitted grids (BFG) in heat conduction problems involving heat sources must be implemented in strictly numerically conservative form if accurate results are to be obtained. It is demonstrated that, for one-dimensional problems, non-conservative form imply errors originated in grid non-uniformity that cause a spurious increase in the heat source. This in turn leads to significant errors in the computed soution. Therefore the implementation of BFG codes using non-conservative forms should be avoided. An application to an unsteady, axisymmetric benchmark problem involving a spherical, time-decaying heat source is presented.