***UPDATE***I have researched passive fireplace air heaters.enter image description here 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.

  • You're asking the wrong hackers. Ask steam engine guys. Either in the old tractor clubs, heritage railways, or scale live steam railroads. Commented Jan 16, 2016 at 1:06
  • What's in the pipes - water? Air? Is it pressurized? What pressure? What is the temperature of the fluid in the pipes? Are the pipes directly exposed to flames, or just hot gases? Are you burning wood, or something nasty like garbage or plastic? If it's water in the pipes, is there a pressure relief system, and is the water demineralized?
    – Mark
    Commented Jan 16, 2016 at 2:50

4 Answers 4


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)? ;)

  • I agree cast develops a rust layer then it dosent really degrade for many years from some of the old demos I have done when upgrading to modern systems.
    – Ed Beal
    Commented Jan 15, 2016 at 14:15
  • Just updated my question. Cast was my second consideration. How does it weld with 220 Arc or Braze with a torch? Thanks.
    – ojait
    Commented Jan 16, 2016 at 3:20
  • @ojait - From what I understand it doesn't. You'd pipe-fit to whatever the casting is. Apologies for what may be an offhand answer, cast iron would be the 'best' to stand up to heat, but that doesn't mean you can make what you want with it easily (unless you have a foundry ;). But this is for hot air? Use cast iron stack pipe. And like I said, keep the fittings out of the chamber; maybe you'd get away with HVAC elbows on the outside? ... I'm still not sure what you're building. Supplemental heat ducted to what?
    – Mazura
    Commented Jan 16, 2016 at 5:08
  • @Mazura-added a photo of what I'm building.
    – ojait
    Commented Jan 16, 2016 at 16:37
  • 1
    I actually made a fireplace heat exchanger like what you are trying to accomplish out of plain black pipe. Worked great for a couple of years, but these don't come close to the efficency of a fire place insert. So once we had enough money to upgrade to an insert we did. After two seasons of heavy use, the black pipe had no more rust than if it had been outside in the weather (IMHO).
    – bigbull15
    Commented Jan 16, 2016 at 16:57

If this is the average fireplace, some thick walled corrosion resistant alloy like 304 stainless sch 40 pipe would work great.

Exotic alloys like inconel would be needed if it was something like a forced air wood furnace with the flame directed at the metal.

  • I heard Stainless was either hard or impossible to arc or MIG weld? SS was my first consideration, but was told to reconsider.
    – ojait
    Commented Jan 16, 2016 at 3:03
  • Stainless tubing is commonly welded when making exhaust headers for cars, which by the way is also a high temperature corrosive environment. Though usually TIG welded. For the MIG process you need the right gas to weld stainless. For stick/arc welding... as long as it is thick wall tubing/pipe it will be easy enough.
    – Netduke
    Commented Jan 18, 2016 at 19:00
  • @Netduke- so at the right duty cycle setting and burning a schedule 40(?) SS pipe it's possible to get good penetration with a stick welder?
    – ojait
    Commented Jan 18, 2016 at 23:27
  • As long as your arc welder can output 100+ amps, weld penetration should not be an issue with the sch40 ss pipe you are likely using. You could always just bevel the pipe ends for better penetration.
    – Netduke
    Commented Jan 19, 2016 at 15:28
  • @Netduke- Agreed, bevel mitered or fish mouth cuts for all connections.
    – ojait
    Commented Jan 20, 2016 at 2:54

Just go with car or truck exhaust pipe from the auto parts store. It'll be the cheapest & weld great. I'd even go with top & bottom C's as your back, top & bottom cross-member mountings & stands to really optimize efficiency.

  • 1
    +1 Readily available and easy to weld. I think it will last quite awhile - certainly long enough to decide if this relly works.
    – Mark
    Commented Jan 16, 2016 at 16:52
  • 1
    @ojait - If I had to build what's in your picture; this.
    – Mazura
    Commented Jan 17, 2016 at 17:48
  • Yeah, the pic that came after my answer is pretty close to what I was thinking. But, I'd probably go with another large C for it to sit on that wraps under the back & then a front C as the log holder...ends 90'd out to the front. I'd also do the same bottom big C on the top, though smaller diameter pipe. I understand the natural convection updraft, but heat will still be directed away from the chimney draw...hopefully enough to actually get blown into the room instead of just radiating.
    – Iggy
    Commented Jan 17, 2016 at 18:48
  • @Manzura- what does your last comment above supposed to mean? Whats "this" ?
    – ojait
    Commented Jan 18, 2016 at 23:30
  • Sorry. I received & replied to him & now you, since the @'s don't do anything, on what I thought he was asking. What I might weld it up if I were you.
    – Iggy
    Commented Jan 18, 2016 at 23:37

I decided to use 1 inch steel black pipe (commonly used for gas lines). The reasons are that: I don't need any special equipment, the pipe is inexpensive and it will make a good first test run to see if the design will work.

Thanks to all for the good suggestions. I up voted them all.

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