Abstract: A combustion assembly housing contains primary and secondary combustion chambers and an ash receptacle. The primary combustion chamber has primary and exhaust grates. Fuel pellets fall in downward onto the primary grate; intake air flows in upward through the primary grate; exhaust gas flows out rearward through the exhaust grate. The primary grate supports unburned fuel pellets and permits partially burned pellets to fall through. Multiple vertical passages through the primary grate are each deeper than they are wide. The secondary combustion chamber has a secondary grate. Partially burned pellets fall in downward onto the secondary grate; intake air flows in rearward, through the secondary grate, and out upward into the primary combustion chamber. The secondary grate supports partially burned pellets and permits ash to fall through downward into the ash receptacle. Intake air flows rearward into the ash receptacle and upward into the secondary combustion chamber.