Abstract: A hot fluid generator is disclosed for exchanging heat between a first fluid having a first temperature and a second fluid having a second temperature. The generator includes at least two spaced parallel metal sheets spirally wound with spaced turns about a vertical central axis and joined at their top and bottom edges to define a hollow spiral wall. Upper and lower closure members arranged normal to the central axis are connected with opposite ends of the spiral wall to define a first spiral passage between successive spaced turns through which a first fluid is circulated and a second spiral passage between the spaced sheets through which a second fluid is circulated, and a central combustion chamber. A burner is centrally arranged within the combustion chamber to heat the chamber and the first fluid so that as the first and second fluids are circulated, heat is exchanged between the first and second fluids.