An air-gas mixing unit for an air-gas mixture burning appliance, comprising: at least one air-gas mixer that forms a point of mixing for mixing of air and gas to form a combustible air-gas mixture, an air supply with an air way that is connected to the point of mixing for supply of air to the point of mixing, and a gas supply with a gas channel that is connected to the point of mixing and a gas governor that is adapted to control supply of gas to the point of mixing via the gas channel, wherein the gas governor comprises: a gas valve that is adapted to control gas pressure in the gas channel dependant on an air pressure signal that is indicative of a static air pressure in the air way, and a gas restrictor that restricts flow of gas from the gas valve to the point of mixing; characterised in that the gas restrictor is interchangeable to permit adaptation of the gas governor to operate with a hydrocarbon gas, in particular methane, by means of at least one first gas restrictor or with hydrogen gas by means of at least one second gas restrictor, wherein the at least one first gas restrictor is embodied to form a gas flow with a first Reynolds number and the at least one second gas restrictor is embodied to form a gas flow with a second Reynolds number, the first Reynolds number being at least three times greater than the second Reynolds number, wherein a flow velocity, taken as the area-averaged flow velocity in the smallest cross-sectional flow opening in the gas restrictors, is evaluated at a maximum heat input rate common to both gases the air-gas mixing unit is operated with, wherein the at least one first gas restrictor forms a first reduced cross-sectional flow opening that is embodied with a first isoperimetric ratio, and wherein the at least one second gas restrictor forms a second reduced cross-sectional flow opening that is embodied with a second isoperimetric ratio, the first isoperimetric ratio being at least two times smaller than the second isoperimetric ratio. at least one air-gas mixer that forms a point of mixing for mixing of air and gas to form a combustible air-gas mixture, an air supply with an air way that is connected to the point of mixing for supply of air to the point of mixing, and a gas supply with a gas channel that is connected to the point of mixing and a gas governor that is adapted to control supply of gas to the point of mixing via the gas channel, wherein the gas governor comprises: a gas valve that is adapted to control gas pressure in the gas channel dependant on an air pressure signal that is indicative of a static air pressure in the air way, and a gas restrictor that restricts flow of gas from the gas valve to the point of mixing; a gas valve that is adapted to control gas pressure in the gas channel dependant on an air pressure signal that is indicative of a static air pressure in the air way, and a gas restrictor that restricts flow of gas from the gas valve to the point of mixing; characterised in that the gas restrictor is interchangeable to permit adaptation of the gas governor to operate with a hydrocarbon gas, in particular methane, by means of at least one first gas restrictor or with hydrogen gas by means of at least one second gas restrictor, wherein the at least one first gas restrictor is embodied to form a gas flow with a first Reynolds number and the at least one second gas restrictor is embodied to form a gas flow with a second Reynolds number, the first Reynolds number being at least three times greater than the second Reynolds number, wherein a flow velocity, taken as the area-averaged flow velocity in the smallest cross-sectional flow opening in the gas restrictors, is evaluated at a maximum heat input rate common to both gases the air-gas mixing unit is operated with, wherein the at least one first gas restrictor forms a first reduced cross-sectional flow opening that is embodied with a first isoperimetric ratio, and wherein the at least one second gas restrictor forms a second reduced cross-sectional flow opening that is embodied with a second isoperimetric ratio, the first isoperimetric ratio being at least two times smaller than the second isoperimetric ratio.