Abstract: A spiral heat exchanger for producing heating and/or sanitary use hot water, specifically designed for condensation applications, characterized in that said spiral heat exchanger comprises a duct, preferably though not exclusively made of a thermally conductive material and having a contoured cross-section, called outer duct, inside which can be either arranged or not a second duct, preferably though not exclusive made of a thermally conductive material, called inner duct; the geometrical construction of said outer duct being defined as a resultant obtained by joining two different diameter circumferences, said circumferences being spaced from one another by a constant or variable radius junction pattern; the thus obtained single or double duct being coiled or spiral wound on a diameter substantially larger than the size of said duct thereby providing a hollow cylinder.

Abstract: A boiler furnace avoids NOx and increases thermal efficiency with a small boiler. The boiler furnace has a outer water walls and inner water walls. All of the water walls contain water which absorb heat from the flame and cool the water tube which act as a container, passage, evaporator of water and water walls with membrane that connect water tubes to each other. The spaces between outer water walls and inner water walls makes a cylindrical space which contain fires and avoids making a fire that produces nitrogen oxides due to high temperature induced from a concentrated flame.

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.