Reducing primary energy consumption is an essential issue for the sector of building construction. This paper refers to building ventilation systems and focuses on low pressure flat plate heat exchangers, designed for low pressure drops and low air velocity, minimizing the electrical consumption of fans. The device is conceived for working within passive ventilation systems, as a ventilation heat recovery stage during winter and sensible heat dissipation during summer. CFD simulations are used for testing and optimizing some flow-mixing device (or turbulators) inside the heat exchanger, in order to minimize the need for fans, still assuring an acceptable efficiency. Experimental testing of the prototype is in good agreements with numerical simulations.