Abstract: A fluid heating system specifically designed for the use of solid bulk fuel, such as wood or the like. The burner unit includes a continuous coil of fluid carrying conduit which acts both as a grate for supporting the fuels of combustion and as the primary implement for circulating fluids in close proximity to the combustion material so as to maximize the heat absorption by the fluid. Primary and secondary combustion air inlets combine with a baffling system that maximizes the efficiency of the combustion process within the heat transfer coil. Room heating and hot water storage apparatus are also disclosed as desirable end uses for the heated fluid.