Ventilation system for a building, comprising: ▪ vertical ventilation or discharge ducts for combustion products providing a natural thermal draft, ▪ air injection nozzles located at the upper part of the ends of the ventilation or discharge ducts and, by injecting a primary air flow at high speed, enabling a negative pressure to be generated at the upper part, thereby causing a secondary air flow to move upwards through the ventilation or discharge duct, ▪ a primary air fan supplying a network having injection nozzles at its ends, the impeller of the fan not being in direct contact with the air flow to be discharged, the fan being controlled by a control system, the system being such that: ▪ the control system of the fan comprises two sensors (3, 4), namely a temperature sensor and an anemometer configured to measure two outdoor parameters, namely the outdoor temperature and speed of the wind, ▪ the two sensors (3,4) being connected by first transmission means to two inputs of a control module which receives from the two sensors (3, 4) a received signal corresponding to the two measured outdoor parameters, either continuously or at selected intervals, ▪ the control module comprises output channels, an output signal having one of several possible values being transmitted to a frequency converter by second transmission means, ▪ the programme enables a calculation to be run to establish what the driving frequency of the fan should be from a number of possible values based on a certain number of outdoor temperature and wind speed combinations, these results, in the form of computer data, being input to the control module and stored by storage means incorporated in said control module, ▪ the frequency converter is connected to the fan via third transmission means and a frequency of the three-phase current circulating between the frequency converter and fan corresponds to the current received by the frequency converter, the driving speed of the fan being proportional to the frequency of the three-phase current, ▪ a time delay being provided between the measurement of the wind speed and driving of the fan ▪ in order to adjust by proportion the amount of primary air injected by the induction nozzle relative to the climatic factors acting on the natural thermal draft or to switch the fan from an operating mode controlled by the control module to a mode using the natural thermal draft. ▪ vertical ventilation or discharge ducts for combustion products providing a natural thermal draft, ▪ air injection nozzles located at the upper part of the ends of the ventilation or discharge ducts and, by injecting a primary air flow at high speed, enabling a negative pressure to be generated at the upper part, thereby causing a secondary air flow to move upwards through the ventilation or discharge duct, ▪ a primary air fan supplying a network having injection nozzles at its ends, the impeller of the fan not being in direct contact with the air flow to be discharged, the fan being controlled by a control system, ▪ the control system of the fan comprises two sensors (3, 4), namely a temperature sensor and an anemometer configured to measure two outdoor parameters, namely the outdoor temperature and speed of the wind, ▪ the two sensors (3,4) being connected by first transmission means to two inputs of a control module which receives from the two sensors (3, 4) a received signal corresponding to the two measured outdoor parameters, either continuously or at selected intervals, ▪ the control module comprises output channels, an output signal having one of several possible values being transmitted to a frequency converter by second transmission means, ▪ the programme enables a calculation to be run to establish what the driving frequency of the fan should be from a number of possible values based on a certain number of outdoor temperature and wind speed combinations, these results, in the form of computer data, being input to the control module and stored by storage means incorporated in said control module, ▪ the frequency converter is connected to the fan via third transmission means and a frequency of the three-phase current circulating between the frequency converter and fan corresponds to the current received by the frequency converter, the driving speed of the fan being proportional to the frequency of the three-phase current, ▪ a time delay being provided between the measurement of the wind speed and driving of the fan ▪ in order to adjust by proportion the amount of primary air injected by the induction nozzle relative to the climatic factors acting on the natural thermal draft or to switch the fan from an operating mode controlled by the control module to a mode using the natural thermal draft.