***UPDATE***I have researched passive fireplace air heaters. The pipes are configured in a "C" shape. There are between 4-6 pipes joined side-by-side at the top and bottom with plates. The wood and than fire is built in the crook of the pipes. When placed correctly the lower openings pull in the rooms cool air were it is heated from the fire. It re-enters the room from the top pipe opening.
If you notice the thin pipe walls in the photo I don't see this product lasting more than a few fires before the fire degrades the metal. To me, the pipes in the photo appear to be similar to fence posts from a chain link fence or automotive muffler pipe.
I'm planning a build that would follow this design and would like to know what weld-able (arc or MIG) or braze-able pipe material would be the most durable? Durable as far as being able to withstand being heated for several hours without the pipe walls decaying?
The pipes would not be under any internal pressure nor carry anything but air from the room.