This note offers a clear view of heat transfer fundamentals in terms of 'heat tubes', which are macroscopic architectures for the flow of energy through complex energy systems. The note focuses on heat tubes with conduction and convection, although heat tubes with radiation through stacks of radiation shields have also been treated in the literature. It is shown how to shape a heat tube and how to cool it along its length so that its thermodynamic performance evolves. Several instances of confusion are clarified along the way. (C) 2019 Elsevier Ltd. All rights reserved.