Abstract: A boiler having a first pass furnace and a set of second pass tubes, a reversing recess proximate to the furnace and tubes for reversing the flow of gases between the furnace and tubes, and a rear access door providing access to the recess, in which there is an integral, self-sustaining reversing chamber sized and configured to fit within and be removably held within recess. The chamber is enclosed by walls on all sides thereof except the side facing the recess which is open and concave. The chamber is formed of loosely-packed ceramic fibers with air entrapped between said fibers to provide greater insulation.