As illustrated in Fig. , there is disagreement between the results of the analytical solutions, for all fluids. A comparison between Figs. 7 and 8 reveals a clear contradiction with the physics of the problem when Zuritz's solution is considered. This contradiction arises since for a given position in the TTHE, the tap water temperature in the outer annular section is higher when the thermal losses to the surroundings (𝑇 𝑎 = 18 ℃) increase. In Appendix C, it is shown that, for parallel flow, the Zuritz solution (Eq. ( ) from ) generally does not satisfy the boundary condition 𝑇 1 (0). Another potential source of discrepancy is the fact mentioned in the Introduction that AS in the present work has three roots, while Zuritz's AS has only two roots. We, therefore, conclude that Zuritz's AS for the TTHE-HL (Eq. ( ) from ) is -in general -incorrect.