A gas-fired boiler comprises at least one burner (1) and feeding means (4) for feeding a fluid to such burner (1), said feeding means (4) being connected to a fan (5), a gas duct (7) being provided to feed gas to said fan (5). The burner comprises at least a first main flame-diffuser element (25) and a second auxiliary flame-diffuser element (26), the feeding means being a feeder duct (4) connected to the main diffuser element (25) and to the fan (5), said fluid comprising air or a mix of air and gas which is sent in such feeder duct (4), a tubular body (17) connected to the auxiliary diffuser element (26) opening in said duct (4), a gas carrying pipe (10) opening directly in such tubular body (17).