Gas/Air mixing device of a gas burner, the gas/air mixing device being configured to mix gas and air to provide a gas/air mixture, wherein the gas/air mixing device has a mixer housing with a mounting flange being configured to mount the gas/air mixing device to a fan housing of a fan of the gas burner in such a way that a sealing surface of the mounting flange is in contact with a sealing surface of the fan housing, the mixer housing provides an inlet opening for the air, the mixer housing provides an inlet opening for the gas, the mixer housing provides an outlet opening for the gas/air mixture, the mounting flange of the mixer housing, namely the sealing surface of the mounting flange, surrounds the outlet opening of the mixer housing, the mounting flange of the mixer housing has protrusions being configured to accommodate mounting screws, characterized in that the sealing surface of the mounting flange has first and second grooves (16, 17) being configured to accommodate sealing elements (18, 19), namely the first groove with a first radius to accommodate a first sealing element, and the second groove with a second radius being different from the first radius to accommodate a second sealing element, the first radius of the first groove is larger than the second radius of the second groove, the protrusions of the mounting flange of the mixer housing have first and second sections (20a, 20b) being configured to accommodate mounting screws, namely the first section being configured to provide first through holes for mounting screws used when one of the first sealing element and second sealing element shall be used to seal the mixer housing versus the fan housing, wherein the first sections of the protrusions having the first through holes are positioned radially outside of the first groove, and the second section being configured to provide second through holes for mounting screws used when the other one of the first sealing element and second sealing element shall be used to seal the mixer housing versus the fan housing, wherein the second sections of the protrusions being configured to provide the second through holes are positioned radially outside of the second groove. the gas/air mixing device has a mixer housing with a mounting flange being configured to mount the gas/air mixing device to a fan housing of a fan of the gas burner in such a way that a sealing surface of the mounting flange is in contact with a sealing surface of the fan housing, the mixer housing provides an inlet opening for the air, the mixer housing provides an inlet opening for the gas, the mixer housing provides an outlet opening for the gas/air mixture, the mounting flange of the mixer housing, namely the sealing surface of the mounting flange, surrounds the outlet opening of the mixer housing, the mounting flange of the mixer housing has protrusions being configured to accommodate mounting screws, characterized in that the sealing surface of the mounting flange has first and second grooves (16, 17) being configured to accommodate sealing elements (18, 19), namely the first groove with a first radius to accommodate a first sealing element, and the second groove with a second radius being different from the first radius to accommodate a second sealing element, the first radius of the first groove is larger than the second radius of the second groove, the protrusions of the mounting flange of the mixer housing have first and second sections (20a, 20b) being configured to accommodate mounting screws, namely the first section being configured to provide first through holes for mounting screws used when one of the first sealing element and second sealing element shall be used to seal the mixer housing versus the fan housing, wherein the first sections of the protrusions having the first through holes are positioned radially outside of the first groove, and the second section being configured to provide second through holes for mounting screws used when the other one of the first sealing element and second sealing element shall be used to seal the mixer housing versus the fan housing, wherein the second sections of the protrusions being configured to provide the second through holes are positioned radially outside of the second groove. the first section being configured to provide first through holes for mounting screws used when one of the first sealing element and second sealing element shall be used to seal the mixer housing versus the fan housing, wherein the first sections of the protrusions having the first through holes are positioned radially outside of the first groove, and the second section being configured to provide second through holes for mounting screws used when the other one of the first sealing element and second sealing element shall be used to seal the mixer housing versus the fan housing, wherein the second sections of the protrusions being configured to provide the second through holes are positioned radially outside of the second groove.