The performance of MWCNT + Fe₃O₄/water hybrid nanofluids as the coolant of a plate heat exchanger is experimentally investigated for particle concentrations in the range from 0.05 to 0.3 vol % and for coolant flow rate from 3 to 7 lit/min. The hybrid MWCNT + Fe₃O₄ nanoparticles were prepared by the chemical precipitation method and characterized with XRD, SEM, Raman, and VSM techniques. The thermal conductivity, viscosity, density and specific heat of MWCNT + Fe₃O₄ hybrid nanofluid was measured experimentally and compared with the literature values. The results show that the exergy efficiency of the hybrid nanofluid is 10.5% higher than that of de-ionized water at flow rate of 7 lit/min and particles concentration of 0.3 vol %. Thermal performance factor is found to be greater than 1 for the hybrid nanofluids (0.05–0.3 vol %), with its maximal value of 1.12 occurs at 0.3 vol % and flow rate of 7 lit/min. The effectiveness of 0.3 vol % nanofluid is higher than the de-ionized water by 11.65% at the flow rate of 3 lit/min and by 13.21% at 7 lit/min. The number of transfer units of 0.3 vol % hybrid nanofluid is higher than that of de-ionized water by 20.88% and 24.29% at flow rates of 3 and 7 lit/min, respectively. A tradeoff between the first and operating costs suggest that operating at a coolant flow rate of 5 lpm will yield the lowest cost. Therefore, benefits of reducing the size and enhancing heat transfer of the PHE via the use of MWCNT + Fe₃O₄/water hybrid nanofluid outweigh the additional power requirement.