  A thermodynamic argument is proposed in order to discuss the most appropriate
form of the local energy balance equation within the Oberbeck-Boussinesq
approximation. The study is devoted to establish the correct thermodynamic
property to be used in order to express the relationship between the change of
internal energy and the temperature change. It is noted that, if the fluid is a
perfect gas, this property must be identified with the specific heat at
constant volume. If the fluid is a liquid, a definitely reliable approximation
identifies this thermodynamic property with the specific heat at constant
pressure. No explicit pressure work term must be present in the energy balance.
The reasoning is extended to the case of fluid saturated porous media.
