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.

Oh interesting that you reply, your name comes up in the Paul/Matt podcast as someone who has "very extreme climate & use conditions" and thus had to replace a barrel. Could you briefly explain what is so extreme about your climate or use case that necessitated this? Thank you.

Hi M
I didn't get to watch Paul and Matt's chat (I'm waiting for the Netflix version), so I'm not sure what was said.
Two of my RMHs are in uninsulated buildings; one is an art studio, and it needs to stay above freezing throughout a northern Montana winter.
My wife works out there every day (almost), so it needs to be warm as well.
The other is in my uninsulated auto shop, and I work out there every day.
I consider them to be Extreme Burning RMHs.  Lit from morning till night, especially the art studio dragon.
Until my recent change in the studio to a full bell double skin first-generation Batchbox, I was using the original removable lid 55-gal mild steel barrel that I bought in 2013 for my first 8" J-Tube.  That barrel is now sitting out behind the art studio, waiting for a new home. Free to the first Permie who stops by and asks for it. I even have an 8" 5-minute riser waiting for a new home as well.
The shop stove is a hybrid using two stacked barrels and a brick bell; it burns all day every winter day.
No issues at all with those barrels either.
In my home, I have a 6" Shorty Core with a single-skin bell; she is only lit once or perhaps twice a day.

The Purpose of a Rocket Mass Stove in my opinion.

A simple review:

Just like everything in life, it (A Rocket Mass Stove)   is not and cannot be everything to everyone.  What it is, when built to the numbers, a extremely viaable wood burning-enviromentaly sound-and quite simply a very stable and consistent heat source.

If you want to go out and look for all the reasons that it won't work you will find them. If you convince yourself you do not have the ability to build it, you will not beable to.  Such is life for some.  Mr. Can't, never did anything, and speaking to your self, "I can't do this, and I can't do that, will simply enforce your ability to not get it done, you won't"   The desire to build is something you either have or don't have.  It can be developed and nurtured with sound advice. Much of that advice is here on the forum.

Can a person develop the skills, learn a trade good enough, find new friends along the way, and end up with a soul warming, house warming endevor?  Absolutely I have friends on this forum site, I will probably never meet, but enjoy them imensely.  .

Can you drop everything, and build over the next 30 days, sure, but you don't have to. There have been many winter projects, started at the beginning of fall and finnished before spring. Nothing wrong with that.

I am off my stump, carry on good folks!

Scott

I had similar questions about durability, but running a cleaner fuel helped a lot. I use kiln-dried wood from https://onestopfuel.ie/ and haven’t had cracking issues since switching. Consistent moisture content seems to make the burn easier on the bricks and metal parts.

Mike Oswaldrosnick wrote:I had similar questions about durability, but running a cleaner fuel helped a lot. I use kiln-dried wood from https://onestopfuel.ie/ and haven’t had cracking issues since switching. Consistent moisture content seems to make the burn easier on the bricks and metal parts.

Mike, I was wondering how you determined this boughten fuel has worked better for you.   What were you burning before? And if you had cracking issues, did you replace those particular bricks?

In general, wetter wood, would have a cooler burn with the moisture converted to steam. Or were you thinking this "steam" was the problem?

Just curious.

Scott

I am very interested in building a rocket heater; given my cooling biased climate (Texas) and current housing (an RV), a mass heater doesn't make sense to me, what I'd be inclined to do would be a rocket water heater with a large insulated heat storage tank and an air coil (or radiant floor/ceiling if/when I build). In my current location I don't have much outside space for a rocket water heater and storage tank, but still considering building something here since pumps and a fan would be a lot easier to run off a battery backup and/or generator than the electric heat I'm currently stuck using.

Thinking of doing similar (with a super large insulated storage tank also fed by solar and heat pump) if/when I ever get to build.

Three things have held me back. The first is just making the time because of the other problems.  The second is Insurance.  Even if I was to get the Rocket mass heater certified as a wood stove, it would cost at least an extra $150 dollars a year, and severely limit  insurance companies that I could use. The third is the main reason. I have a split level home with water heat.  The normal air heating, pebble or cob stoves that just radiate heat would not work well. I might even be able to remodel my fireplace  into a rocket stove, but it would only heat one of the four floors. What I need is a plan Rocket mass heater that heats water, (yea, probably pressurized as we don't want to to "Boom Squish" I would probably do a batch rocket heater ask I would not want to constantly feel the heater as I believe you have to do with the J tube heaters.  What I would want to do is heat the water for the day or two usage  when the weather is mild and more often when it gets colder.

Eric Rothoff wrote:Three things have held me back. The first is just making the time because of the other problems.

Yes, they do take commitment to get 'er done.
 

Eric Rothoff wrote:
The second is Insurance.  Even if I was to get the Rocket mass heater certified as a wood stove, it would cost at least an extra $150 dollars a year, and severely limit  insurance companies that I could use.

While this has been discussed in the past, this is something to be considered, but I presume you have a house with NO pre exhisting chimney? NO one has said this is for everyone, I think of this forum (and subject matter) as more of a "if you want to build, this is how it has worked, and please toss out your planned ideas" to others that have successfully made it work.

Eric Rothoff wrote: The third is the main reason. I have a split level home with water heat.  The normal air heating, pebble or cob stoves that just radiate heat would not work well. I might even be able to remodel my fireplace  into a rocket stove, but it would only heat one of the four floors.

Yep, sometimes the  layout is simply not right for the stove. Again it may not be for everyone.

Eric Rothoff wrote: What I need is a plan Rocket mass heater that heats water, (yea, probably pressurized as we don't want to to "Boom Squish" I would probably do a batch rocket heater ask I would not want to constantly feel the heater as I believe you have to do with the J tube heaters.  What I would want to do is heat the water for the day or two usage  when the weather is mild and more often when it gets colder.

I read this to mean you think a pressurised system is safer and more likely to NOT have "boom squish" effects?  Please explain?  As far as heating water and making use of that warm water as a mass storage. It would seem that on this forum, if there is a will, there is a way.  It works both ways. to be sure.