Using water as the working fluid, air-to-air heat exchangers using thermosyphon heat pipes were designed, constructed and tested under medium temperature (below 300 degrees C) operating conditions. A heat exchanger test rig has been constructed and developed wherein the heated air is recycled to the counterflow heat exchanger. The lengths of both the evaporator section and the condenser section of the heat exchangers were 300 mm and the central adiabatic section was 150 mm. The heat exchangers which were tested used (1) continuous plate finned copper tubes, (2) circular, spirally-finned steel tubes and (3) bare copper tubes for their respective heat pipes. The working fluid was water with a fill ratio of 60% of the evaporator section length. The air face velocity range was from 1.5 to 5 m/s and the heat input into the evaporator section inlet was varied between 4 and 20 kW using electric heating elements. The heat exchangers showed high effectiveness compared with similar heat exchangers using other working fluids, such as Freon 22 (R22). The rectangular plate finned copper thermosyphon heat exchanger had the best performance but there was a limitation on testing this configuration that the adiabatic section temperature operating condition did not exceed 200 degrees C, in order not to exceed the safe working pressure. A steel pipe heat exchanger will be used in the industrial application to which the project is directed. This heat exchanger has been designed, manufactured and tested for heat recovery in industry with medium temperatures (lower than 300 degrees C), for example in bakeries to recover flue gas energy from the oven to heat up the proofing oven or other low temperature heating functions.