A pre-mix gas-fired boiler for heating a fluid for sanitary use and/or for heating a room comprising: at least one burner operating with a variable modulation ratio, a fan provided with an on/off valve member and a mixer, a feeder duct for feeding a fluid to the burner, said feeder duct being connected to a the fan and said fluid consisting of air or a mix of air and gas, a gas duct for feeding gas to said fan, a first main flame diffuser element and at least one second auxiliary flame diffuser element, wherein the feeder duct is connected to the main diffuser element and to the fan, a tubular body connected to the auxiliary diffuser element opening in said duct and, a gas pipe connected to the tubular body for feeding it directly with gas, characterized in that: the flame diffuser elements (25, 26) are separated from each one another and are capable of generating different calorific powers, wherein the primary diffuser element comprises a surface area on which flame is generated which is greater than the corresponding surface area of the secondary diffuser element, wherein an on/off member is provided within the feeder duct and placed between said tubular body and said main flame diffuser element, wherein said on/off member is configurable to allow flame generation only to on either of said diffuser elements (25, 26) or to on both so as to modify the calorific power generated by the burner, wherein said main diffuser element and said auxiliary diffuser element are being able to be activated either independently or together. at least one burner operating with a variable modulation ratio, a fan provided with an on/off valve member and a mixer, a feeder duct for feeding a fluid to the burner, said feeder duct being connected to a the fan and said fluid consisting of air or a mix of air and gas, a gas duct for feeding gas to said fan, a first main flame diffuser element and at least one second auxiliary flame diffuser element, wherein the feeder duct is connected to the main diffuser element and to the fan, a tubular body connected to the auxiliary diffuser element opening in said duct and, a gas pipe connected to the tubular body for feeding it directly with gas, characterized in that: the flame diffuser elements (25, 26) are separated from each one another and are capable of generating different calorific powers, wherein the primary diffuser element comprises a surface area on which flame is generated which is greater than the corresponding surface area of the secondary diffuser element, wherein an on/off member is provided within the feeder duct and placed between said tubular body and said main flame diffuser element, wherein said on/off member is configurable to allow flame generation only to on either of said diffuser elements (25, 26) or to on both so as to modify the calorific power generated by the burner, wherein said main diffuser element and said auxiliary diffuser element are being able to be activated either independently or together. the flame diffuser elements (25, 26) are separated from each one another and are capable of generating different calorific powers, wherein the primary diffuser element comprises a surface area on which flame is generated which is greater than the corresponding surface area of the secondary diffuser element, wherein an on/off member is provided within the feeder duct and placed between said tubular body and said main flame diffuser element, wherein said on/off member is configurable to allow flame generation only to on either of said diffuser elements (25, 26) or to on both so as to modify the calorific power generated by the burner, wherein said main diffuser element and said auxiliary diffuser element are being able to be activated either independently or together.