Abstract: A hot-water boiler of the direct-heating type basically comprising a combustion chamber and a heat-exchange chamber which encircles the lower half part of the combustion chamber and which has a number of reticulate plates therein. Combustion gas generated in the combustion chamber effuses into the heat-exchange chamber where water to be boiled is stored. The combustion gas goes through the water in the heat-exchange chamber and is broken up into smaller bubbles by the reticulate plates; thereby facilitating a prompt and efficient heat-exchange between the gas and water.