In many applications, such as power plants, thermal energy may be lost to the ambient environment so, using devices such as heat pipe heat exchangers is necessary to recover this energy. In this study, a thermosiphon heat pipe heat exchanger is designed and manufactured for this purpose. The thermal performance of this exchanger is experimentally and numerically investigated at different filling ratios (35%, 50%, and 75%) of a working fluid (DOWTHERM™ A) and condenser inlet air velocities (0.5, 1, 1.5, 2, and 2.5 m/s). The study is carried out at a wide range of evaporator inlet air temperatures (160 to 320°C), constant evaporator inlet air velocity (1 m/s), condenser inlet air temperature (24°C) and heat flux (8000 W). The exchanger model used in this study includes 60 copper heat pipes with square aluminium fins. The heat pipe (100 cm total length) consists of three sections namely: the evaporation section (40 cm length), the adiabatic section (20 cm length), and the condensing section (40 cm length). The results show that the filling ratio, air velocity, and evaporator air temperature have an important effect on the thermal performance of the heat exchanger. In overall, the optimum values of the filling ratio, air velocity, and evaporator air temperature to obtain the best thermal performance of the exchanger are 50%, 0.5 m/s, and 320°C, respectively. Where at these values, the highest condenser air temperature and maximum exchanger effectiveness (240°C and 74% numerically and 250°C and 77% experimentally) are obtained. Through the comparison between the numerical and experimental results, an excellent agreement is found between them with a maximum difference percentage that does not exceed 5%.