Silly Questions From Someone Wanting To Learn About RMH

I see many posts that say that you cannot use steel in your J-Tube as it will get too hot and not hold up. Why then, can you use a metal barrel over the combustion chamber. Also, I see a stainless barrel for sale online. Would it benefit me to use stainless rather than a cheap metal barrel?  Thank you for your input.

Hi Tamara;
In the feed tube, burn tunnel, and riser, temperatures can be up to 1800F. metal will quickly spall and flake apart.

As soon as the hot air leaves the riser it is rapidly cooling.
The thin metal of a barrel releases heat almost as fast as it gains it.
My 8" J-Tube commonly ran at 800F on top of the barrel but I could push it and get a glowing orange circle that was at 1100F
That same barrel is still in service today, on top of a 6" Batchbox.
It was purchased in 2013 and could use a bit of paint, but it is as solid as the day I got it.  

Stainless barrels are desirable, they patina very well and look great, but they tend to cost quite a bit more.
Having a removable lid is more important than looks.

Oh, there are no silly questions, ask until you understand.

Tamara,

Thomas is correct, but also important to mention is the fact that by the time the air/fuel mixture has exited the riser and enters the radiator-like steel barrel, most, if not all of the atmospheric oxygen has already been used to oxidize the wood gas fuel and is no longer available to oxidize the steel in your drum.

Many say that a Stainless Steel drum is far superior to a steel drum as it will outlast a steel drum and is far more aesthetically pleasing.

Tamara Carroll wrote:I see many posts that say that you cannot use steel in your J-Tube as it will get too hot and not hold up. Why then, can you use a metal barrel over the combustion chamber. Also, I see a stainless barrel for sale online. Would it benefit me to use stainless rather than a cheap metal barrel?  Thank you for your input.

Tamara Carroll- I used a broken water heater for my barrel. It works beautifully and is still going strong after many years. Unlike many other mediums I've used, the radiant heat it produces far exceeds my expectations.
As with most things, I got the idea online.
I’ve also decided to revamp the existing seating to incorporate something I saw on permies using half barrels to capture more heat exhaust.

Another consideration is that stainless steel will radiate a little less heat. That isn't good or bad - it's just something to consider. If you're building in a spot where the emphasis is on quick heat, a regular old barrel might serve a little better. But if you want to move a little more of the heat on down the line to the mass, then that's perfect. Also, I can change the amount of radiant heat on my manky old barrel by choosing to coat it with oil, or not.  Putting a layer of some kind of drying oil on the metal makes it radiate more heat - upping the radiant temperature about 10 degrees. So I only painted the side toward my living space, and the side facing the wall remains dry. (I used slightly rancid walnut oil that I wasn't going to cook with, but probably any drying oil will do.)  Most people seem to like the look of the stainless steel better, so there's that. If you're going to live with a big old barrel for years and years, you might as well like the look.

(BTW, I made a 4" metal J-tube RMH work as my primary heat source for 3 years. It burned through the burn tunnel during the second year and I had to patch it daily with clay. It worked, but fire brick works sooo much better!)

Two reasons. 1) The J-tube is intentionally insulated to stay hot, while the barrel isn’t. The barrel is radiating heat to a cooler environment. 2) The barrel has a larger surface area to distribute the heat.  

So even though the same amount of heat is flowing through both, the barrel is cooler.  

Oh, another reason is that the heat transfer by conduction across the thin metal is likely to be relatively high compared to the heat transfer by convection and radiation on the barrel surfaces. This means the barrel is close to being the same temperature throughout the metal. This means that in some sense, roughly speaking, the barrel temperature is sort of an average between the j-tube temperature and the room temperature.