Boiler for producing hot water or steam

A boiler for producing hot water or steam, comprising a containment enclosure, which forms a heating chamber that contains water to be heated, and accommodating a firebox body, which forms internally a combustion chamber with a burner associated therewith which has a combustion head, accommodated in said combustion chamber, and a device for premixing air and gas which is connected to said combustion head, wherein said premixing device comprises an air supply duct, which is connected to said combustion head, and a mixing tubular body, which is extended coaxially to said supply duct in order to create, between an internal lateral wall of said supply duct and an external surface of a sidewall of said mixing tubular body, an intake region which has a transverse air passage section that is narrower than the air passage section formed by said supply duct upstream of said mixing tubular body, along the air flow direction, said mixing tubular body having an internal cavity and being closed, at a first end that is directed against the air flow that arrives from said supply duct, by an ogive-shaped head and connected, at a second end, which lies opposite said first end, to a duct for supplying combustible gas, in the sidewall of said mixing tubular body there being at least one dispensing opening that is connected to said internal cavity of said mixing tubular body and to said intake region.