There are currently no compact decentralized ventilation systems combined with indirect adiabatic cooling, ideal for renovation tasks and new buildings. The project is to develop and long-term demonstrate/test a smaller decentralized ventilation system with capacity up to 1200 m³/h, where a coated aluminum counter flow heat exchanger is combined with indirect adiabatic cooling. The size of the air handling-/cooling unit will be able to ventilate/cool a classroom or equivalent office environment. Combining the counter flow exchanger with indirect adiabatic cooling provides cooling without increasing indoor humidity, eliminating the operation and installation costs of traditional compressor cooling with refrigerants and high power consumption. This makes the unit ideal for comfort cooling and legal use in public buildings, where traditional compressor cooling is banned. The entire unit is designed for low energy operation. The average dry basis temperature recovery reaches 82-85% efficiency during the heating period, thereby meeting the requirements of future building regulations. During the cooling period, a cooling of 6 - 8 ° C relative to ambient conditions is achieved by moisturizing filtered water on the return air. By using a compact water filter before humidifying the return air, ordinary tap water can be used, even combined with rainwater.