The present ventilating system is used for homes and buildings located in cold climate areas. The system includes a heat exchanger for extracting heat from the inside air discharged to the exterior and transferring said heat to the outside air admitted into the building. Blowers circulate air through the two paths of the heat exchanger and a motorized damper on the outside air path closes or opens the same. An integrated circuit and the following sensors serve to operate the system as follows: sensors detect the concentration of air contaminants, the humidity and the negative pressure of the air inside the building, while an outside air temperature sensor is also provided. When the level of air contaminants is above an adjustable preset level, the system operates in an inside air upgrading mode, namely air is circulated in both paths of the air exchanger; the same mode is used where humidity level is found to be excessive. When negative air pressure is detected, the system is operated to admit only outside air within the building, and if the outside air temperature is below a preset level, the system is operated in a heat exchanger de-icing mode for an adjusted time after each adjusted total time of system operation in either humidity or air-upgrading operating mode.