***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.
Best Answer
Cast iron. Which is conveniently already in the configuration you need at your local scrap yard: old radiators. That is, assuming this doesn't have to be potable.
Other than that I'd guess we're talking about some expensive exotic steel you probably can't wield very easily anyway, like lab equipment.
I'd like to think that regular old black pipe would stand up to this, especially if you could keep the fittings out of the flames and you mount it allowing for expansion.
However and whatever you build it out of, I'd test a mock-up before you start making endless loops only to find it pops your wields or the fittings. It's not the pipe walls, it's the joints. As long as they aren't subject to physical damage, your no where near hot enough to damage the metal.
Also... if you ever do get the pipes hot enough to melt with water inside them, something has blown up. What are we building here and where's the safety valve(s)? ;)