Abstract: A boiler has an exterior housing, and a water-cooled fire chamber within the exterior housing having a rear face wall and a front face wall. A door is provided on an end of the exterior housing, and forms the front face wall of the fire chamber. The door is cooled by water circulating therein and has an opening therethrough for a burner producing a flame. The boiler includes an insert having an opening therethrough. The insert surrounds the door opening and is secured to an inside of the door to project into the fire chamber so that the insert opening is essentially aligned with the door opening. The insert is cooled by the water circulating through the door and reduces a cross-sectional area of the fire chamber available for the flame over a portion of the length of the fire chamber. Also provided is a water-cooled partition located in a rear portion of the fire chamber in front of the rear face wall for dividing the flame.