Boiler

A boiler comprising an enclosure which forms a heating chamber which contains water to be heated and accommodates a firebox which forms a combustion chamber with a burner associated therewith which comprises a combustion head, arranged in the combustion chamber and connected to a supply duct, and provided with a premixer of combustion air and combustible gas which is interposed along said supply duct, said combustion chamber being connected to a stack for the evacuation of said combustion gases into the external environment, means being provided for generating a flow of a mixture of air and gas toward said combustion head, and further comprising at least one duct for conveying said combustion gases which connects said stack to said supply duct, said flow generation means comprising means which are adapted to place said combustion chamber in partial vacuum with respect to atmospheric pressure.