In this study, the effect of using graphene oxide-water-based nanofluids in plate heat exchangers on thermal performance was investigated. A combined hot water production system with an integrated plate heat exchanger was used as the test set. Graphene oxide was used as nanofluid. The effect of nanofluid on thermal efficiency compared to pure water was investigated. The size of the graphene oxide nanoparticles used in the nanofluid preparation process is 7.5 nm and the purity is 99.8%. In order to obtain nanofluid, graphene oxide particles were added to pure water in the amount from 0.02% to 0.04% by volume. The volumetric flow rate of distilled water used as the cold fluid was adjusted at five different values, namely, 4, 5, 6, 7, and 8 L/min. The temperature of the graphen oxide-water nanofluid used as hot fluid was set at 40, 45, 50, 55, and 60°C. Experiments were carried out in the form of reverse flow. Heat transfer, efficiency, Re number, and improvement rate were analyzed in all experiments. According to the test results, the best improvement rate was determined when the volume concentration was 0.04% and the cold fluid volumetric flow rate was 4 L/min. This rate was calculated as 8.8%. As the volumetric flow rate of the cold fluid increased, the efficiency decreased. When the analyses were evaluated, it was seen that the use of nanofluids significantly increased the heat transfer compared to pure water.