Abstract: A monobloc boiler particularly suitable for hydrocleaning machines, comprising a combustion chamber (6), the outer surface of which is wrapped by a double-coil pipe (8), wherein water to be heated flows; a cylinder wrapping up said chamber (6); a fuel mixing and ignition unit (4); flanges (5) for air distribution and braking in boiler body; a fan system (16) for air inlet in boiler body and an external box (12 wrapping and enclosing all these components. A fan system (16) is fixed on a first rear wall of the external box (12) and generates an air flow in an air gap (18) formed between the external box (12) and the cylinder (10). A mixing and ignition unit (4) is also located within boiler body and is fixed in the center on the other wall of the external box (12), opposite to the first rear wall.