There is a time-honoured test for vernacular engineering: It's time (and that is cool, because it's a recursive concept that time itself proves the validity of the axiom). The test of time is often downplayed or even dismissed by lots of folks, and I suppose that is a curse of modernity. I sort of had an "Aha!" moment when I first read Nassim Nicholas Taleb's
The Black Swan where he explains it as a rule that applies to ideas, but extends to pretty much any application of human ingenuity. For example, one of his personal rules is to only drink things that people have been drinking for at least 2000 years.
Simply put, if you look at a technology or building method that has been around for a while, you will see all that built examples tend to converge on a set of components, material selections, and designs that fall within close proximity to one another. These get imprinted on people in a cultural and even biological context, so that you can show a kid a drawing of a house and they will say "that's a house." In permaculture, the subject of appropriate technology is totally interwoven with the vernacular and the durability of ideas. We tend to use and extend things that are already known to work, because that saves a lot of time in R&D, and also avoids unnecessary expense and material waste.
I bring this up because the basic J-tube RMH has been around long enough to develop a vernacular, and the reason for that is that enough of them have been built and in service for long enough that the community of builders and users can confidently say that it works and is a sensible, cost-effective, safe, and efficient way to heat spaces. This doesn't mean that there's no room for improvement, but most of that has been in the refinement of batchbox combustion and stratification designs.
For certain use cases, the original J-tube is still a viable choice, and this includes a repurposed steel drum (
The Drum) as the primary radiator. The reasons are simple and obvious: They are everywhere, can be had for free or very cheap, are relatively easy to modify with simple hand tools, and at the end of their service life they can be safely recycled or returned to the earth (be careful if your soils are already high in iron, I guess).
Every now and then, we see someone looking to improve or tweak the J-tube design. More often than not, the improvements involve metalwork, but we also know from experience (the test of time) that the high temperatures in the combustion core don't play nicely with metals, especially not mild steel that is easy to obtain and work with for the average permie. A lot of these earnest inventors are especially keen to get rid of
The Drum. And there are some good reasons for that when you consider aesthetics -- not everyone wants a big old steel cylinder in full view in their living space. But there's a function being served, and anything that replaces
The Drum is probably not going to tick every single one of the boxes, because it's not a drum. This doesn't mean that alternatives aren't out there, but it does mean that you're going to give up at least one of the attributes...most likely the cost and relative ease of getting and fitting one.
As a case study, have a look at this cleaning and inspection video featuring an RMH at Wheaton Labs.
The Drum in this application has been in service for 13 years and appears to be more than ready to do at least double that, if not more. I think it's been coated with linseed oil, and it's still shiny:
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