This study experimentally demonstrated the effect of using a nanofluid obtained from fly ash, which is comprised of various types of metal oxides in varying ratios, on improving the performance of a parallel flow (PFCTHE) or a cross-flow (CFCTHE) concentric tube heat exchanger. Fly-ash nanofluid/water and water/water hot/cold working fluids were used for monitoring the differences in the performance of the heat exchangers. The Triton X-110 dispersant was used in the study to produce 2% (wt) concentration of the fly-ash nanofluid via direct synthesis. The heat exchanger is of the double-pipe type with hot water flowing through the central tube while cooling water flows through the annular space. A double-pipe heat exchanger with co-current or countercurrent flow was utilized along with all ancillary equipment and instrumentation for the determination of overall heat transfer coefficients and heat transfer coefficients during turbulent flow. An improvement of 31.2% and 6.9% was determined in the study for the efficiency of the PFCTHE and CFCTHE, respectively, when the fly-ash nanofluid was used as the working fluid.