These images and text illustrate the lining of a standard 18 inch wide fire box.
Previously the Pyromasse contraflow masonry heater construction articles have illustrated a relinable fire box arrangement, as seen below.
The floor of the fire box is made of skew cut shiners the walls of 4 courses of double wythe brick layed as shiners. This gives the option of replacing the inner wythe of shiners should they ever deteriorate.
The weight of the core above the fire box rests evenly upon both wythes of the fire box wall. Inevitably the inner wythe, as it heats up faster than the outer wythe will rise and lift the whole core up of the outer wythe each time the fire is lit.
If ever the inner wythe is removed, the whole weight of the core would be resting upon the outer wythe, which as it is formed from shiners, risks being destabilized during removal of the inner wythe.
The skew cut shiners that form the fire box floor can never be replaced as they are traped in position by the outer wythe.
An alternative arrangement is detailed below:
Note: The images below are taken from 2 different projects.
The fire box walls are formed from brick layed flat onto a flat floor.
The whole fire box is lined with splits layed as shiners. Note that the liner has a one eight of an inch ceramic felt gasket behind the two back corners. The liner is cut one quarter inch below the level of the top of the last course of the firebox wall in order to accommodate a ceramic felt expansion gasket.
The upper surface of the liner is gasketed with two leaves of one quarter inch ceramic felt. This could be done with one leaf of one quarter inch felt or even with only one leaf of one eighth of an inch felt.
Detail of the liner at the point at which it is bridged by the lintel.
The first course of brick above the fire box walls is layed flush with the liner - and filled in behind with strips of brick.
The second course above the fire box walls is layed back flush with the outside face of the core. The row at the back layed as headers starts the corbel. The corbel could also start on the course below if there were an overall hight restriction.
In this example, where oven height, and overall height are an issue, the rear wall starts to corbel on the course immediately above the fire box walls.
Once the core is completed skew cut shiners are layed dry onto the fire box floor each side of the grate. Gravity will hold these in place. If they become pushed out of place they can easily be removed, the fire box floor swept out, and replaced. The function of these skew cut pieces is to shield the fire box floor and to encourage the embers to fall together onto the grate towards the end of the fire.
There is a possibility of the liner peeling at the door jambs if the stove is fired hard from cold. Though in 10 years, two stoves with fire boxes lined in this way have had no problem.