Abstract: A furnace having a primary heat exchange unit also providing a combustion chamber, a secondary heat exchange unit connected by an upper crossover conduit to the primary heat exchange unit, and a tertiary heat exchange unit connected by a lower V-shaped crossover conduit to the secondary heat exchange unit. A third crossover conduit connects the V-shaped crossover conduit with the primary heat exchange unit. A fly ash removal screw is located in the V-shaped crossover conduit to move fly ash therefrom through the third crossover conduit back into the primary heat exchange unit to an ash depository for removal therefrom. Vibrating means are provided between the secondary and tertiary heat exchange units to vibrate the walls thereof and dislodge clinging fly ash so that it falls into the V-shaped crossover conduit for removal by the screw conveyor. A burner assembly of the furnace includes a combustion air housing carrying a circular, stationary grate with an annular valley for carrying fuel during combustion.