An automatic adjustment system for automatically adjusting fan pressure comprising: a controlled mechanical ventilation unit for extracting air from enclosures of a single-family dwelling, and ducts through which the controlled mechanical ventilation unit is communicated with the enclosures, the ducts are provided with adjustable opening and closing mouths, the mouths being configured to open and close depending on the flow rate of air needed to be extracted from the enclosures; the mechanical ventilation unit comprising: a fan configured for rotational speed control; means measuring the pressure at the inlet of the fan for detecting pressure variations in the ducts; and a control logic box comprising an internal memory, the internal memory storing a reference flow rate/pressure curve, the reference flow rate/pressure curve is a curve defining minimum output, represented by minimum flow rate and corresponding minimum pressure, and maximum output, represented by maximum flow rate and corresponding maximum pressure, according to ventilation needs; characterized in that the control logic box is configured to define above the reference flow rate/pressure curve a pre-established band in which the working area of the fan is established by means of data of the number of revolutions per minute of the fan and the corresponding pressures at the inlet of the fan, the control logic box is configured to receive information of the revolutions of the fan and of the pressures at the inlet of the fan, and the control logic box is configured to adapt the speed of the fan in response to a variation of pressure in the ducts such that the fan remains in the defined working area, thereby achieving automatic adjusting of the pressure of the fan depending on the required flow rate. a controlled mechanical ventilation unit for extracting air from enclosures of a single-family dwelling, and ducts through which the controlled mechanical ventilation unit is communicated with the enclosures, the ducts are provided with adjustable opening and closing mouths, the mouths being configured to open and close depending on the flow rate of air needed to be extracted from the enclosures; the mechanical ventilation unit comprising: a fan configured for rotational speed control; means measuring the pressure at the inlet of the fan for detecting pressure variations in the ducts; and a control logic box comprising an internal memory, the internal memory storing a reference flow rate/pressure curve, the reference flow rate/pressure curve is a curve defining minimum output, represented by minimum flow rate and corresponding minimum pressure, and maximum output, represented by maximum flow rate and corresponding maximum pressure, according to ventilation needs; characterized in that the control logic box is configured to define above the reference flow rate/pressure curve a pre-established band in which the working area of the fan is established by means of data of the number of revolutions per minute of the fan and the corresponding pressures at the inlet of the fan, the control logic box is configured to receive information of the revolutions of the fan and of the pressures at the inlet of the fan, and the control logic box is configured to adapt the speed of the fan in response to a variation of pressure in the ducts such that the fan remains in the defined working area, thereby achieving automatic adjusting of the pressure of the fan depending on the required flow rate. a fan configured for rotational speed control; means measuring the pressure at the inlet of the fan for detecting pressure variations in the ducts; and a control logic box comprising an internal memory, the internal memory storing a reference flow rate/pressure curve, the reference flow rate/pressure curve is a curve defining minimum output, represented by minimum flow rate and corresponding minimum pressure, and maximum output, represented by maximum flow rate and corresponding maximum pressure, according to ventilation needs; characterized in that the control logic box is configured to define above the reference flow rate/pressure curve a pre-established band in which the working area of the fan is established by means of data of the number of revolutions per minute of the fan and the corresponding pressures at the inlet of the fan, the control logic box is configured to receive information of the revolutions of the fan and of the pressures at the inlet of the fan, and the control logic box is configured to adapt the speed of the fan in response to a variation of pressure in the ducts such that the fan remains in the defined working area, thereby achieving automatic adjusting of the pressure of the fan depending on the required flow rate.