We have studied the extent to which buoyancy effects in horizontal pipe flows of drag-reducing viscoelastic fluids cause distortions to both laminar and turbulent temperature profiles. In the case of laminar flows, these distortions may lead to variations in Nusselt numbers that are larger than those seen for Newtonian pipe flows under similar conditions. In the case of turbulent drag-reducing flows, the effects of buoyancy can also be large and may in turn result in large errors in estimated Nusselt numbers if not properly accounted for. These errors are quantified and recommendations are made on how to reduce them.