A bypass controlled mechanical ventilation installation of a building, comprising: - an air blowing network, allowing supplying fresh air to at least one room called reception room (1, 2); - an air extraction network, allowing extracting foul air from at least one room called technical room (3, 4); - the air blowing network and the air extraction network being equipped respectively with an air blowing fan and an air extraction fan, each provided with means allowing adjusting respectively the air blowing flow rate and the air extraction flow rate supplied by the fans (12,13); - means for determining the flow rate in the air extraction network; - at least one of the air extraction or blowing networks (8, 5) being equipped with a modification member of the air passage section, automatically reacting, that is to say without external control, to external parameters such as the hygrometry or the occupancy rate of a room, this installation further including: - means (16, 17) for measuring the pressure in the network (5, 8) equipped with the modification member of the air passage section; characterized in that it further comprises: - means for determining the flow rate in the air blowing network; - control means designed for: a) after variation of the air passage section in the network equipped with the corresponding member and b) depending on the pressure measured in said network, controlling the fan (12, 13) of this network, called master fan, in order to modify the flow rate in this network, and c) returning the pressure of this network to a reference pressure, d) after determination of the air flow rates in the blowing network and in the extraction network, e) controlling the fan (13, 12) of the other network, called the slave fan, in order to servo-control the flow rate supplied by the slave fan depending on the flow rate supplied by the master fan. - an air blowing network, allowing supplying fresh air to at least one room called reception room (1, 2); - an air extraction network, allowing extracting foul air from at least one room called technical room (3, 4); - the air blowing network and the air extraction network being equipped respectively with an air blowing fan and an air extraction fan, each provided with means allowing adjusting respectively the air blowing flow rate and the air extraction flow rate supplied by the fans (12,13); - means for determining the flow rate in the air extraction network; - at least one of the air extraction or blowing networks (8, 5) being equipped with a modification member of the air passage section, automatically reacting, that is to say without external control, to external parameters such as the hygrometry or the occupancy rate of a room, this installation further including: - means (16, 17) for measuring the pressure in the network (5, 8) equipped with the modification member of the air passage section; - means for determining the flow rate in the air blowing network; - control means designed for: a) after variation of the air passage section in the network equipped with the corresponding member and b) depending on the pressure measured in said network, controlling the fan (12, 13) of this network, called master fan, in order to modify the flow rate in this network, and c) returning the pressure of this network to a reference pressure, d) after determination of the air flow rates in the blowing network and in the extraction network, e) controlling the fan (13, 12) of the other network, called the slave fan, in order to servo-control the flow rate supplied by the slave fan depending on the flow rate supplied by the master fan. a) after variation of the air passage section in the network equipped with the corresponding member and b) depending on the pressure measured in said network, controlling the fan (12, 13) of this network, called master fan, in order to modify the flow rate in this network, and c) returning the pressure of this network to a reference pressure, d) after determination of the air flow rates in the blowing network and in the extraction network, e) controlling the fan (13, 12) of the other network, called the slave fan, in order to servo-control the flow rate supplied by the slave fan depending on the flow rate supplied by the master fan.