Abstract: A hot water boiler having a dual combustion chamber arrangement capable of burning either solid or fluid fuels, and arranged for acting directly on a heat transfer system generally in direct contact with firebox walls of the combustion chamber arrangement. Disposed atop the combustion chamber arrangement is a heat conserving flue assembly which absorbs heat from smoke and flue gases exhausted from the combustion chamber arrangement and heats a fluid medium which in turn is circulated to the heat transfer means for eventual release for circulation or storage as desired in a residence or other building structure.