Progress,  page 102
Once the firebox was complete, I began building the rest of the fireplace using standard concrete
blocks.  The entire structure will be faced with granite rocks collected on site (we have
thousands of them), so I placed a large one at the corner for accent.
This is a top view of the fireplace box and the surrounding structure.  I purposely left a few
inches between the firebrick and the concrete block so that I could later pack it with sand as a
thermal expansion buffer.  From this angle you can also see the flapper inside the damper.
The international building code now requires that combustion fireplaces provide a means to
supply outside air directly to the firebox to avoid sucking cold air through the house to replenish
that which goes up the chimney.  Thermodynamically speaking, that reasoning is a bit flawed
since the net result is the same in total ... you either cool the room with air sucked from
inadvertent leaks or you cool the fire with colder air drawn directly from outside.  But the code is
the code, and to comply we had planned ahead and installed two 4 inch diameter tubes that ran
under the concrete floor from an outside wall to spots on either side of the firebox.  The
horizontal hole you see here just to the left of the firebox connects to one of those tubes, and
there is an identical one on the other side.  When I rock face the fireplace I'll leave out the
mortar in those places to allow the outside air to pass.

Here's another fallacy in the building code.  It requires a chimney cross section of roughly 100
square inches for the size firebox I built, yet the outside air source only needs to be about 6
square inches.  Even allowing for differences in air density between hot air and cold air that just
doesn't make any sense.   We've been building this house to code or better all along, though, so I
did it anyway.