Analogy between heat and momentum transfer-the concepts as developed by O. Reynolds and L. Prandtl-are based on certain hypotheses about the mechanism of turbulent transport phenomena. Therefore, heat transfer equations based on these concepts are semi-empirical. In addition, a new heat transfer equation is presented which applies to non-developed turbulent flow e.g. in plate heat exchangers. This equation can be derived from the classical differential equations for viscous flow and heat conduction which are not bound to any modeling concept of turbulent transport phenomena whatsoever, and may be taken as rigorous.