Heat Recovery Units are manufactured in a wide range from 500m3/h to 7500m3/h. The basic operational principle is to recover of heat energy from the exhaust air and to transfer it to the incoming fresh air. In the summer operations, when the cooled air inside is exhausted, incoming fresh air is cooled down. During the winter operations while the heated inside air is being exhausted, the incoming new fresh air is heated up. By this way, more than 70% of the energy can be saved. During the process, the air to air heat recovery is obtained without a need for any additional heating or cooling energy source. When utilized with the split air conditioners and the variable refrigerant volume systems which do not have fresh air inlets, in addition to the energy savings, the fresh air requirement of the space could also be met. The units will operate all the year round without any interruption.