Heat recovery units are used for pre-heating the fresh air in waste heat plants. The nanofluid used as working fluid in the heat pipe will allow the heat recovery unit to benefit from the waste heat at lower temperatures, since it can evaporate at a temperature lower than the temperature of the base fluid. Therefore, the range of the operating temperature of the heat recovery unit will be increased. Different temperatures and flow rates of waste heat were used to find the optimum conditions for the evaporation of spinel nanofluid in the evaporator region of the heat pipe. Similar conditions were investigated for both of the cold fluid and optimal conditions on the condensation of nanofluid in the condenser region. In this study, an experimental setup was designed to improve the thermal performance of the air-to-air containing heat pipe heat recovery unit. The experiments were carried out using pure water-based nanofluids containing nano-sized ZnOAl2O3, MgOAl2O3, FeOAl(2)O(3)particles, and the efficiencies of the heat transfer were investigated in this experiment. A thermosiphon mechanism that consists of five heat pipes, the length of each one is equal to 1 m, and the inner and the outer diameter of them is equal to 23.4 mm and 25.4 mm, respectively, those pipes are vacuumed, without wick and copper material. Heat pipes in the experimental setup; the evaporation zone is positioned to form 450 mm, the condensation zone 400 mm and the middle 150 mm parts to form the adiabatic region. At the beginning of the experiments, the working fluid charged to fill the 1/3 of the evaporation zone volumes of the heat pipes. Two different cooling air flow rates, 30 g center dot s(-1)and 60 g center dot s(-1), and three different heating air flow rates, 50 g center dot s(-1), 70 g center dot s(-1), 90 g center dot s(-1)have been used, to calculate the heat obtained from the condensation zone. All the experiments were carried out by applying two different heaters with a power of, 1000 W and 2000 W, in the evaporation zone. Thus, the optimum temperatures and flow rates were found for all Reynolds numbers in the hot and cold fluid region, and its helps to determining the ranges of the operating temperature in the heat recovery unit. The experimental results that obtained from the two different working fluids (pure water and nanofluid) are compared. The best thermal performance of the system was found in the hot and cold air duct, when the Reynolds number was equal to 12 000. The thermal performance improvement rate was found as 92.8 % when the FeOAl2O3/water nanofluid used as working fluid, and 1000 W heater power and 60 g center dot s(-1)cooling air flow applied.