Different types of thermal boundary conditions are conceivable in numerical simulations of convective heat transfer problems. Isoflux, isothermal and a mixed-type boundary condition are compared by means of direct numerical simulations (for the lowest Reynolds number) and well-resolved large-eddy simulations of a turbulent forced convection pipe flow over a range of bulk Reynolds numbers from Re-b = 5300 to Re-b = 37700, at two Prandtl numbers, i.e. Pr = 0.71 and Pr = 0.025. It is found that, while for Pr = 0.71 the Nusselt number is hardly affected by the type of thermal boundary condition, for Pr = 0.025 the isothermal boundary condition yields approximate to 20% lower Nusselt numbers compared to isoflux and mixedtype over the whole range of Reynolds numbers. A decomposition of the Nusselt number is derived. In particular, we decompose it into four contributions: laminar, radial and streamwise turbulent heat flux as well as a contribution due to the turbulent velocity field. For Pr = 0.71 the contribution due to the radial turbulent heat flux is dominant, whereas for Pr = 0.025 the contribution due to the turbulent velocity field is dominant. Only at a moderately high Reynolds number, such as Re-b = 37700, both turbulent contributions are of similar magnitude. A comparison of first- and second-order thermal statistics between the different types of thermal boundary conditions shows that the statistics are not only influenced in the near-wall region but also in the core region of the flow. Power spectral densities illustrate large thermal structures in low-Prandtl-number fluids as well as thermal structures located right at the wall, only present for the isoflux boundary condition. A database including the first- and second-order statistics together with individual contributions to the budget equations of the temperature variance and turbulent heat fluxes is hosted in the open access repository KITopen (DOI : https: //doi.org/10.5445/IR/1000096346).