Waste heat recovery system is a system using for preheating fresh air needed in industrial and waste heat plants. The aim of this study is to improve the performance of a heat recovery unit by using a heat exchanger consisting of a heat pipe which uses a (CuO + ZnO)/water hybrid nanofluid as a working fluid. As is known, the nanofluid in the heat pipe is able to evaporate at a temperature lower than the temperature of the base fluid, so the heat recovery unit will also be provided to benefit from the waste heat at lower temperatures. Thus, the temperature range of the heat recovery unit will be increased. The optimum conditions for the evaporation of hybrid nanofluid in the evaporator region of the heat pipe were investigated by performing experiments at different temperatures and flow rates of the waste heat. Similar conditions were made in a cold fluid, and the optimum conditions for condensing the nanofluid in the condenser region were investigated. Experiments were performed at 2 different cold air flows (30 g/s and 60 g/s), 3 different hot air flows (50 g/s, 70 g/s, and 90 g/s) and 2 different heating powers (1000 W and 2000 W). Thus, optimum values of temperature and flow were found at all Re numbers on the hot- and cold-fluid sides, and it was helped to determine the operating temperature ranges of the heat recovery unit. The efficiency improvement rates in the heat pipe were between 14% and 73%. The best result was achieved when the cold-air duct Reynolds number was 6700 and the hot-air duct Reynolds number was 11,250. The use of the hybrid nanofluid at all Re numbers reduced thermal resistance in the heat pipe. The maximum reduction rate in thermal resistance was achieved to be 40.4% compared to pure water when the cold-air duct Reynolds number was 12,400. As a result, the efficiency of the heat tension recovery unit increased.