Abstract: An improved multi-fueled hot water or steam boiler comprising a plurality of compartments, one compartment for the combustion of a solid fuel, such as wood or coal, and the other for the combustion of a liquid fuel and separated from each other and the exterior of the boiler by water-conducting walls; a water supply tank positioned above one of the compartments and connected to the water-conductive walls; and a plurality of horizontal fire tubes passing through the water supply tank and connecting one compartment to an exhaust flue. The compartment not positioned below the water supply tank has a water-conductive ceiling slanted upwardly toward and connected to the water supply tank. One compartment has an adjustable air-flow secondary damper in addition to a primary air damper. The water-conductive walls, fire tubes, adjustable secondary damper, and upwardly slanting water-conductive ceiling reduce boiler heat losses and improve boiler efficiency.