The concepts of entransy and thermomass have been developed for heat transfer analyses and optimizations. Thermomass is the equivalent mass of heat based on the Einstein's mass-energy relation. The concept of entransy reflects the energy of thermomass. The entransy balance equations of heat conduction and heat convection, which are the basis of the heat transfer optimization principles, are derived from the energy equation of thermomass in this paper. The relationship between the entransy and the thermomass is discussed. (C) 2013 Elsevier Ltd. All rights reserved.