Empty space behind firebox in arch...what to do with it?

[MapleME]

The past 2 years I have been running a Mason 2x6 flat pan. Has worked really well for my small hobby operation.

After bricking the entire arch, I had to brick in the back wall of the firebox and I brought this up to about an inch below the pan (when its full with sap!)

Question is....what about that other 4 feet or air space behind the fire box? It seems like its wasted space and since its just a big empty cavity, am I loosing some of the flame after it climbs up over the back wall of the firebox?

Fill it with sand? I dunno. I just know that it seems odd to me.

Thanks for the ideas or input.

Josh
MapleME

[Ed K]

I filled mine with sand.be aware the extra weight may bow your arch.I had to put an extra set of adjusters in the middle,to level up.

[Dennis H.]

Fill it in with vermiculite and Zonilite. They are insulation for chimneys and block walls.
Then cover it with fire brick or ceramic blanket. The stuff is very light and fluffly so if it is not covered it will move around even by natural draft but will end up out side if you have a froced draft.