A thorough analytical study is performed to identify the range of influence of each term of the general equations for flow within a fluid saturated porous medium. The first part of the analysis results into a set of algebraic equations useful in identifying the most appropriate flow regime for a given configuration (e.g. physical properties, geometry, etc.). In the second part, a unique heat transfer correlation for the general time dependent flow is obtained. This new correlation is successfully compared against theoretical and empirical results from the literature for the asymptotic flow regimes of Darcy and clean fluid, and for the Forchheimer regime. Predictions from the theoretical correlation for other (intermediate) flow regimes are also compared against numerical results available in the literature.