Are you interested in building a rocket mass heater? What is holding you back?

Just saw this thread. I think I'm the target group for a RMH and am very intrigued by the concept. I am listening to the Paul/Matt Walker podcast right now.

My main reasons why it's not an "obvious yes!" thing:

- It's all custom. A wood stove you just pick the one you like on the internet or in the store, it shows up on your door, you put in the chimney pipe, done. There is no designing or custom making you have to do. I know some people like Matt sell "verified good" RMH designs but you're still sort of an "owner-engineer" not just "slap money on the table and done" kinda deal.

- It's heavy and bulky, even compared to regular box wood stoves. Maybe it's not as bulky if you consider the burn zone and clearances, but in the summer months I can still put stuff in that space next to the box stove. Maybe this is a climate thing, but a big bulky heavy appliance that's only useful a few months out of the year is not an obvious win.

- Conventional heating fuel is cheap enough that cost is not really my #1 concern. Be it gas, oil, or wood, even if I cut my heating bill by 100% that wouldn't do that much. Housing and food and vehicle related expenses are all higher.

- It's difficult to "just try it out." If I don't like a wood stove, I can pack it up and sell it so someone else. For RMHs, not only is there not really a market, they're also more difficult to uninstall and to ship, because they're heavy, bulky and often custom. So if I built one and it was bad, or I didn't like it, now I'm stuck with it. Can't just sell it to someone else who might make better use of it.

- The "heat your home with sticks from deadfall all winter" thing is cool, but only really applies to people with land (that has trees on it). So you're already talking to a small minority of landowners. Nobody in an apartment and almost nobody in a suburban home will have as easy a time doing this.

That said I'd still love to try one, but I think there are a lot of potential downsides/blockers/unknowns for the average person.

Glenn Herbert wrote:Steel barrels over the riser do not wear out in less than decades of normal use. Thomas Rubino is in an extreme climate and building situation where the RMHs had to burn almost all day every day, and his barrels did not wear out; one was retired because the RMH design was changed, and the other is still going strong as far as I know.

I think the bit about running continuously is an important factor. I just replaced the piped mass on my 4" J-tube with a half barrel (cut lengthwise) covered in cob. It needed to happen, because the flue in the mass was getting clogged with ash that I couldn't reach from the cleanout points, and as I took it apart I discovered that the thin galvanised steel flue pipe itself was deteriorating badly and the rust flakes were contributing to the blockage. The postmortem suggests that the way I run this RMH is probably part of the issue: I only burn on cold nights in winter for the most part, and sometimes there will be a week or more between fires. Also, the fires tend to be short duration, 3-4 hours typically, so the mass never got fully warm and that led to lots of condensation in the pipe. That's a recipe for corrosion and I saw the results...but I did get eight or nine years of service before failure and teardown.

Now, I'm hoping that the stratification chamber as a replacement for piped mass will alleviate this problem, but the underlying factors won't change. I will still be burning sporadically through the cold season, so there will still be plenty of opportunity for moisture to condense on the inside of the bell and start rusting it out. I'll keep an eye on this, using a USB endoscope and the inspection/cleanout hatch I put in the side of the bench. Maybe i will get twenty years out of this setup before the drum collapses.

So the rule of thumb for RMH construction is still valid: Mild steel anywhere but the initial drum location over the heat riser is at risk, either from spalling and destruction by high temperatures, or downstream from condensation and rust.

Is the stratification chamber design just the new go-to now, or is the answer "it depends?" Seems a lot of newer builds are using that w/ the "juice box" design.

With the juice box design I've wondered if physically attaching the exhaust to the drum (with metal of some sort to transfer heat) would help heat up the exhaust faster..

I would say 95% of new builds use a shaped stratification chamber.
Of course, there are some folks who are building on a budget; they have pipe, and they have clay mud, and they do not have bricks or barrels.
Pipes covered in cob are expected to fail; your thick cob surround of the metal pipe becomes your new pipe once the metal rots out.
Your strategically placed cleanouts are there for a good reason. That old metal will fall into the gas flow and partially block it. The cleanouts make clearing that possible.
A barrel over your riser is too hot for moisture to stick around long enough to cause rust. Those barrels can last for decades.
With half barrels as a bench, if you do not run your RMH hot enough, moisture will eventually compromise the barrel's structural strength.

From your riser barrel (if used) on, moisture is not your friend.
If building a bench, then I prefer to construct a brick transition area rather than a cut-down barrel, where moisture can do no harm.
If asked, I will always recommend skipping the bench and going with a full-size bell.