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Water resources are diminishing in many (semi) and regions, thus becoming a concern in the (near) future. Desalination of brackish or salt water can be a good solution to provide water for agriculture and human consumption. The Watergy project proposes an integrated system in which plants and fresh water are produced. The system is closed and air is being cooled during the day with a central heat exchanger in a chimney. Since this heat exchanger is a vital part for the functioning of the system, it was decided to test its characteristics such as heat transfer and air velocities in a controlled environment. A chimney was built with a heat exchanger installed inside with a surface of 11 m(2). Several experiments were conducted with varying layout of the heat exchanger, varying conditions and excitation signals. This paper describes the results of steady state situation with five baffles during heating and cooling. The measured air velocities are in the range of 0.72 - 0.6 m/s. The measured heat exchange coefficient is in the range of 25 - 28 W/m(2)K. The efficiencies of all the presented experiments are in the range of 70 - 80%. The results of the experiments are compared to a simple physics based, steady state model that describes the convection, conduction and condensation processes. The model results are quite close to the experimental results; the maximum temperature deviation between model and observations on the water-side is 0.5 degrees C (at 40 degrees C) and on the air side 2.5 degrees C. The accuracy of the model is sufficient to use it for design purposes and, later on, as a starting point for model based control of the greenhouse. Finally, the results of the experiments show that the heat transfer in the proposed design can be sufficient to cool and heat the Watergy greenhouse.