Air-exchange system for the ventilation of at least one room of a building with the following features: a first air exchange device (1, 28) for the ventilation of at least one room (24- 27) of a building, which comprises - a first casing with an electric supply air fan that is arranged within the first casing for conveying a supply air current from an outer side of the building into a room of the building, and - a heat exchanger that is arranged within the casing, through which the supply air current and through which an exhaust air current flows, and wherein at least one further air exchange device (1, 30) comprises the following features: - a further casing with an electric exhaust air fan arranged in the further casing for conveying an exhaust air current out of the room of the building to the outer side of the building, and - a further heat exchanger that is arranged within the further casing, through which an exhaust air current flows and through which a supply air current flows, and wherein a control unit has been provided, which is connected to the first and to the further air exchange device in such a way that the rotational frequency of the fan blades of the supply air fan and the rotational frequency of the exhaust air fan are adjustable such that the supply air current at the first air exchange device (1, 28) is designed to be greater than the exhaust air current, and the exhaust air current at the further air exchange device (1, 30) is designed to be greater than the supply air current, wherein an air movement is generated from the first air exchange device to the further air exchange device, and that this air movement is modifiable by changing the rotational frequencies of the supply air fan and the exhaust air fan. a first air exchange device (1, 28) for the ventilation of at least one room (24- 27) of a building, which comprises - a first casing with an electric supply air fan that is arranged within the first casing for conveying a supply air current from an outer side of the building into a room of the building, and - a heat exchanger that is arranged within the casing, through which the supply air current and through which an exhaust air current flows, - a first casing with an electric supply air fan that is arranged within the first casing for conveying a supply air current from an outer side of the building into a room of the building, and - a heat exchanger that is arranged within the casing, through which the supply air current and through which an exhaust air current flows, and wherein at least one further air exchange device (1, 30) comprises the following features: - a further casing with an electric exhaust air fan arranged in the further casing for conveying an exhaust air current out of the room of the building to the outer side of the building, and - a further heat exchanger that is arranged within the further casing, through which an exhaust air current flows and through which a supply air current flows, - a further casing with an electric exhaust air fan arranged in the further casing for conveying an exhaust air current out of the room of the building to the outer side of the building, and - a further heat exchanger that is arranged within the further casing, through which an exhaust air current flows and through which a supply air current flows, and wherein a control unit has been provided, which is connected to the first and to the further air exchange device in such a way that the rotational frequency of the fan blades of the supply air fan and the rotational frequency of the exhaust air fan are adjustable such that the supply air current at the first air exchange device (1, 28) is designed to be greater than the exhaust air current, and the exhaust air current at the further air exchange device (1, 30) is designed to be greater than the supply air current, wherein an air movement is generated from the first air exchange device to the further air exchange device, and that this air movement is modifiable by changing the rotational frequencies of the supply air fan and the exhaust air fan.