Abstract: A watertube boiler for all types of boilers including natural circulation, forced circulation and one-through types. The boiler includes a plurality of heat absorption water tubes forming a heat absorption watertube unit in the furnace. The heat absorption watertube unit comprises a relatively dense arrangement of heat absorption water tubes disposed in the combustion path of the burner, such that the flame of the burner impinges on the unit. The heat absorption watertube unit can be arranged in the first stage of a watertube boiler having at least one additional furnace stage receiving exhaust gas from the first stage. The additional stage can be disposed either perpendicularly and horizontally, and the air/fuel mixture in each stage is maintained at a specificied ratio to optimize combustion of the fuel. A method of operating a watertube boiler having three stages is also provided.