Heat recovery ventilators are frequently used to save heating/cooling loads of buildings for ventilation. There are several types of HRV's, including a parallel plate type, a rotary type, a capillary type, and a heat pipe type. The breathing HRV is a heat recovery ventilator of a new kind using the concept of alternating-current ventilation. The AC ventilation is the ventilation with the airflow directions reversed periodically. It has an advantage of using a single duct system, for both supply and exhaust purposes. In order to develop a breathing HRV system, the thermal recovery performance should be investigated depending on many parameters, such as frequency of flow reversal, airflow rate, thermal properties and dimensions of storage medium. It is also necessary to investigate the effect of the parameters on the ventilation performance of the space with the AC ventilation employed, in comparison with the conventional DC ventilation. Other research topics currently being conducted at Kookmin University would be introduced briefly if time remains.