Abstract: A hot water heating system having a water tower with a cold water inlet at the upper end and a hot water outlet at the lower end has a large diameter combustion inlet opening in the sidewall. A combustion chamber is secured to the water tower at the combustion inlet so that products of combustion of fuel consumed in the combustion chamber pass into the water tower and are exhausted through a vent. An outer shell surrounds the combustion chamber providing an annular chamber having a water inlet. A port communicates the upper interior of the annular chamber with the interior of the water tower. Water is circulated from a hot water storage through the annular chamber and through the port into the interior of the water tower. The recirculated water thus accomplishing the dual functions of cooling the combustion chamber and salvaging heat extracted from the combustion chamber to augment the heat of the water in the storage tank.