Abstract: A fire an inner chamber wall circumscribing and defining an inner chamber. A plurality of primary air apertures defined through the inner chamber wall at a first, lower level and a plurality of secondary air apertures defined through the inner chamber wall at a second, upper level. A fuel grate is supported within the inner chamber at a level between the lower level and the upper level. Solid fuel supported by the fuel grate, when combusted, is provided primary combustion air from below the fuel grate by the primary air apertures and provided secondary combustion air from above the solid fuel by the secondary air apertures, the secondary combustion air promoting combustion of unburned gasified combustibles rising within the inner chamber.

Abstract: A wood-type solid fuel fired water heater of modular construction includes a radiation heat transfer section or module in the form of a first annular or cylindrical side wall of refractory material. A water circulating first coil is embedded in the inner surface of the first annular side wall or cylinder to provide a partial water wall or water jacket in the refractory material. The first annular side wall defines the combustion chamber and locus of combustion for direct radiant heating of water in the first coil. The convection heat transfer section or module is formed by a second annular side wall or cylinder of refractory material positioned above the first annular side wall. A second coil for water circulation is mounted inside the second annular side wall but spaced from the inner surface for circulation of hot flue gas around the second coil for convection heating of water in the second coil.