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.