Master thesis deals with heat recovery in ventilation systems. Thes is an experimental measurement of the plate heat exchanger for a period of one year. Thes is Compaq es the efficiency of the heat exchanger manufacturer with achal field measurements. The first part describes the theoretical heat recovery systems, thein advantages and disadvantages, the basic formula for calculating efficiency. In the second part I come to the experimental measurements of the plate heat exchanger. There sult of this thesis is to Compaq the efficacy, showing behavior exchanger seasons and energy evaluation.