Method for operating a gas burner appliance, wherein during burner-on-phases a defined gas/air mixture having a defined mixing ratio of gas and air is provided to a burner chamber of the gas burner appliance for combusting the defined gas/air mixture within the burner chamber; said defined gas/air mixture is provided by a mixing device mixing an air flow with a gas flow, wherein said mixing device has at least two Venturi nozzles being connected in parallel; said air flow is provided by a fan in such a way that the fan speed of the fan depends on a desired burner load of the gas burner appliance, wherein the fan speed range of the fan defines a modulation range of the gas burner appliance; said defined mixing ratio of gas and air of the gas/air mixture is controlled over the modulation range of the gas burner appliance by a gas regulating valve, wherein said gas regulating valve has a pneumatic controller controlling the mixing ratio of gas and air on basis of a pressure difference between the gas pressure of the gas flow in the gas pipe and a reference pressure, wherein either the air pressure of the air flow or the ambient pressure is used as reference pressure, wherein the pressure difference between the gas pressure and the reference pressure is determined and controlled pneumatically; during burner-on-phases the combustion quality is monitored on basis of a signal provided by a combustion quality sensor like a flame ionization sensor, wherein the defined mixing ratio of gas and air of the defined gas/air mixture can be calibrated on basis of the signal provided by the combustion quality sensor, namely by adjusting during calibration a position of a throttle; the calibration of the gas/air mixture is performed in such a way that for calibration at least one of the Venturi nozzles of the mixing device is closed while at least one of the Venturi nozzles of the mixing device is opened. during burner-on-phases a defined gas/air mixture having a defined mixing ratio of gas and air is provided to a burner chamber of the gas burner appliance for combusting the defined gas/air mixture within the burner chamber; said defined gas/air mixture is provided by a mixing device mixing an air flow with a gas flow, wherein said mixing device has at least two Venturi nozzles being connected in parallel; said air flow is provided by a fan in such a way that the fan speed of the fan depends on a desired burner load of the gas burner appliance, wherein the fan speed range of the fan defines a modulation range of the gas burner appliance; said defined mixing ratio of gas and air of the gas/air mixture is controlled over the modulation range of the gas burner appliance by a gas regulating valve, wherein said gas regulating valve has a pneumatic controller controlling the mixing ratio of gas and air on basis of a pressure difference between the gas pressure of the gas flow in the gas pipe and a reference pressure, wherein either the air pressure of the air flow or the ambient pressure is used as reference pressure, wherein the pressure difference between the gas pressure and the reference pressure is determined and controlled pneumatically; wherein said gas regulating valve has a pneumatic controller controlling the mixing ratio of gas and air on basis of a pressure difference between the gas pressure of the gas flow in the gas pipe and a reference pressure, wherein either the air pressure of the air flow or the ambient pressure is used as reference pressure, wherein the pressure difference between the gas pressure and the reference pressure is determined and controlled pneumatically; during burner-on-phases the combustion quality is monitored on basis of a signal provided by a combustion quality sensor like a flame ionization sensor, wherein the defined mixing ratio of gas and air of the defined gas/air mixture can be calibrated on basis of the signal provided by the combustion quality sensor, namely by adjusting during calibration a position of a throttle; the calibration of the gas/air mixture is performed in such a way that for calibration at least one of the Venturi nozzles of the mixing device is closed while at least one of the Venturi nozzles of the mixing device is opened.