See Any Problem Adding a Layer of Fire Brick

i bought a hitzer 82 ul . with the intention of making my own cabinet . still in progress . I was told by a few dealers the only differance was the cabinet . they are not quite the same I have seen pictures on the forum of the inside of the 82 fa furnace model it has a row of fire brick standing then 1 row on its side would make for a deeper coal bed . any reason why I can't add a layer on its side in the 82 ul ?

I'm not familiar with your stove at all...but... I see no harm in adding the firebrick & seeing how it goes. Lot's of stoves could use some geometry changes.

Make sure your ash pan won't overfill with the extra-large load of coal.

I'd maybe move this to the hand-fired forum, there are several of us there that have these. I think this would be an interesting discussion, markvii I think posted some pics of an 82 fa. I have the 82 u.l. and built my own cabinet. A friend of mine has an fa on order and I am interested to see his in person.

There are three versions of this stove, the radiant model, the radiant model with a cabinet, and the furance model. The firebox in the furnace model does have extra brick, and looks like this:
The burn times on one of these stoves are pretty generous, you may want to try a season with the existing setup before you tear into it. I considered the same modification on my stove, but after using it for a while I was more than happy with the performance. To add the extra brick you would need to drill new holes for all the brackets that hold the firebrick, plug the old holes, and modify the brackets for clearance and to hold both rows of bricks.

Something else to consider is the impact of these modifications on the UL approval of your stove.