Abstract: The invention relates to a heating boiler in which a combined absorption and heat conveyor fluid (22) is heated by the direct contact with the exhaust combustion gas (20) of a fuel. This fluid (22) at the same time cleans the exhaust gas, provides a heat shield between the combustion chamber (4) and the container (1) of the boiler and extracts the heat of condensation of the fuel. In such an arrangement of the boiler it is possible to make the container (1) of the heat conveyor fluid (22) economically of synthetic material while obtaining great efficiency and environmental acceptability.