In this study, the thermal efficiency of the rotary (regenerative) heat exchanger has been experimentally investigated in a heat recovery mechanical ventilation device. Beside impact of the same variable on pressure drop of the heat exchanger has been researched. Experiments have been carried out in a laboratory which belongs to a company manufactures heat recovery ventilation devices. The rotor has been tested within a commercial ventilation device rather than being tested alone in order to realize the actual conditions. Test results have also been compared with mathematical model results calculated with respect to the equations according to the literature research. NTU (number of transfer units) method has been used to calculate the thermal efficiency by using the various relevant equations which had been previously proposed by various researchers. The efficiency equation, which had a good agreement with the experimental results, has been determined by evaluating this comparison as a conclusion. Keywords and Phrases: heat transfer, heat recovery, energy recovery, ventilation, mechanical ventilation, regenerative heat exchanger, rotary heat exchanger (rotary wheel), thermal efficiency, thermal effectiveness, pressure drop.