Adam Stjohn

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since Sep 12, 2012
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Recent posts by Adam Stjohn

Yes, I like Konkle's addition here.

Furthermore, rather than running it through a window, why not make (cut) a new exhaust-exit in a wall? Doing so wouldn't be very difficult, would it?

Secondly, CAUTION: Consider the location of the exhaust-exit (say, through a window) in relation to the location of any input-entrance (say, the old chimney)... What I mean is: be careful not to place the exhaust-exit too close to a place where the exhaust could draft back into the house or system. Make sense?

I mean, all indoor woodstoves (rocket-heater or not) pull air into the house, through windows, under doors, cracks, etc-- If the house were perfectly sealed, then the stove would not be able to send any exhaust up and out the chimney! (Exhaust is going out, fresh air needs to be coming in.) So anyway... Remember to consider the location of the exhaust in relation to the location of any input(s).

10 years ago
NOT everything can be scaled down.

What can't be? The surface-friction in the exhaust pipes.

R Scott wrote:The problem is surface tension and friction are constant so they won't scale with the rest of the system.

You can find that quote within the first few posts at this topic:
10 years ago
This question has come up a lot. Look around on the forums for more info.

Check out this page on the forums: " "

where some people talk about this:

There are definitely more posts which address this idea.

Disclaimer, I'm not an expert; but anyway: this is what I seem to remember about the issue:

Truly this would be a 'massive rocket heater,' and not a 'rocket-mass-heater' --- why? I'll explain

At the heart of the general design of rocket-stoves, there is a 'heat-riser' (an internal chimney, inside the barrel). The hot gases burn up through this and then hit the barrel. The barrel acts like a radiator and cools the gases, which then drop down and out the exit. All of this creates a strong draft.

It is that strong draft which makes possible the horizontal exhaust system, around which an earthen-'mass' is constructed to reap heat. By the time the gases are exiting the system, they have 1) gone through a bunch of piping, the friction of which has already put quite a drag on the draft and 2) the gases have cooled down a lot (the heat from them has gone into warming the bench).

Now, for many rocket-mass-heaters it is recommended that the exhaust system finishes with some sort of a vertical chimney. The warm gases love to flow up the vertical chimney; This helps with the draft, as long as 1) there hasn't already been too much pipe with too much friction and more importantly 2) as long as the gases aren't already to cold. In other words, if the gases have already gone through the mass (which has reaped a lot of their heat), then they may be too cool to properly move up the chimney. So this all depends on, 1) what temperature the exhaust-gases are, relative to the outdoor air and 2) how long the chimney is.

If your chimney is too long, then the gases will cool too much before exiting, and this will screw up the draft.

In any case, you should be able to run a rocket-stove through a conventional chimney; that is, you might have to subtract the 'mass' from the equation. This way, you may not have a mass, but at least you'd have a super-efficient rocket-stove.
10 years ago
Well, I consulted Ianto and Leslie's book. The sections called "Burning Questions" and "Troubleshooting" we're particularly helpful.

More or less, with any new masonry stove, the stove will need some time to dry out. Until the stove drys out, there may be some problems with smoke-back.

I also noticed that I still had many loose fragments of excess mortar in the burn tunnel (from the original construction). So I cleaned that stuff out.

I'll let you know how it goes. Also, Larry, I'll take a closer look at the specifications of your stove, that you mentioned, and consider your situation in more detail. Good luck!

10 years ago

Aj French wrote:Oh I thought I'd just mention I would cover it in Cob. I don't actually want to look at a huge hunk of metal in my house.

1) DONT COVER THE BARREL or whatever you're using to serve the function of the heat-exchange-barrel. It's called the heat-exchange-barrel for a reason: As the insulated heat-riser gets super hot, gases flow up it. Then they hit the barrel. This is where shit goes down! Literally! The hot gases are cooled as the heat-exchange-barrel acts as a radiator. This is what causes the downward draft that makes the whole system work. So yeah: find as many ways as you want to decorate the barrel, but do not cover it in cob -- or much of anything for that matter.

2) Do cover the exhaust in cob. Otherwise it wouldn't be a rocket-MASS-heater, but merely a rocket-stove (unless you're mass is made of metal too?) So long story short: Yes do cover it in a big cobby mass mess. Beyond just the usage of the mass, I thiiinnk maybe that such mass is also important for the thermo/aero-dynamics of the whole thang.
10 years ago
Yeah. A lot of forums have topics that are permanently posted to top of the forum. Such topics are usually: "Beginners and Newbies"; "FAQs"; or "Tips on how to use this forum."

Anyone may post in these topics, but the first/original post of each topic is maintained, edited, and updated by a moderator and may include helpful links, etc.

"Rocket stove debugging facts" reminds me of "FAQs," or etc. The topic could have an ongoing conversation (like any other topic), but the original/first post (at the top of the topic) could be almost like a contents list, or main-page, which is edited, updated, and maintained as new "debugging facts" or "FAQs" are questioned, debated, and resolved within the body of the topic.

Make sense?

I feel like this would help the forum a lot. There wouldn't be so many posts that ask the same ol' questions. Also, a better 'search' interface would help. But I dont know how that works..
10 years ago
I have a similar question. Having recently completed an RMH, it is experiencing similar problems.

First off, I'll let you know what my question is:

Do RMH's usually require some time to dry-out or 'cure' before they start working really well??

My hypothesis goes like this:

As the soils around my place do not have enough clay content to make good cob, my mass is composed of dirt and gravel (partially encased in a brick exterior, and then partially encased in a wood frame similar to Paul Wheaton's portable RMH). So anyway, the mass is composed of dirt and gravel...

To compact the mass, I added probably FIFTEEN BUCKETS of water to it. (It soaked it right up.) And then I manually squished and squashed and compacted the fairly wet mass. (This was to get the air out of it, to help the mass act like a dense conductor, rather than an aerated insulator.)

When the RMH is running/burning, the exhaust is obviously heavy with lots of steam and water vapor. The exhaust smells like wet bricks, clay, dirt... wet earth. I know that some of the water (from the mass) is leaking in to the ducts through their seams. Hell, the bricks and mortar around the stove-core aren't even totally dry yet.

SO the stove burns alright. The draft can work okay. But I'm wondering...

Do RMH's usually require some time to dry-out or 'cure' before they start working really well??
10 years ago


That sounds like a good length. Any shorter than that and the system would not reap enough of the heat from the exhaust before it exits the building. In other words, it would be less efficient and the exhaust's exit-temperature would be hotter than usual.


By heaviness do you mean thickness? What does length-of-pipe have to do with heaviness-of-pipe, anyway? Don't most rocket mass heaters use the same gauge pipe, anyway? I suppose that with a larger diameter system (for example, 8" rather than 6") you may get some pretty hot temperatures. Such temperatures have been known to 'eat' the steel pipe (that is, the high heat causes the metal to deteriorate and flake-off over time); however, if the rocket mass heater is built appropriately, the clay content in the insulation of the heat-riser --as well as the clay in the thermal battery-- should maintain the smooth shape of the system, regardless of whether the steel frays away or not; so, more or less, the ducting acts as a mold for the first portion of the rocket-mass-heater's life-- after which, the clay holds the shape. Anyway, also, high temperatures can cause the zinc on galvanized duct to vaporize (resulting in heavy-metal-poisoning or something like that); make sure to use black stove pipe or stainless steel stove pipe (not galvanized duct) during the first portions of the rocket mass heater (in the heat-riser or in the first 8 feet of the bench)... farther down the system, where temperatures are cooler, galvanized pipe should be okay.


Please remember that
1) this stove may be dangerous if built improperly (heavy-metal-poisoning; carbon monoxide poisoning; the heater's 5-ton-weight causing the floor of your living-room to collapse; etc)
2) it may last for a very very long time
3) it will save you a lot of money on heating.
So all I'm saying is, a little bit of time, money, patience... can pay off in the long run.

Having just built one myself, I found out how complex some aspects of the project can be-- especially since I was doing it mostly by myself.

Now, I'm not trying to sell anything to you, because I have nothing to gain from doing that. But anyway, I seriously recommend making a small investment in Ianto and Leslie's book, which can be found here:

or here

The book is great; it is simple, straight-forward, and it covers a lot of crucial details that may otherwise be less obvious.

Or, if you want, order a copy of the Erica and Ernie Wisner's blueprints, which can be found here:

Also, for free, you can check out these websites:

Also, those websites include more links to more helpful websites.


That depends on how much wood you shove in there. During a podcast with Erica Wisner and Paul Wheaton, Ernie Wisner mentioned how some people say that it is impossible for wood-heat to get above a certain temperature; furthermore, he doesn't know how it works, but the temperature gets way hotter than those people think. (Hear the podcast, here ) I seem to remember hearing that rocket-mass-heaters can produce internal-temperatures of somewhere around 1500-2000 degrees farenheit. Also, somebody built a system to automatically feed wood-chips into their rocket-mass-heater, but the thing got out of control and they had a red-hot barrel melt-down on their hands!

Good luck!
10 years ago
Is that clay? Or raw steak? hah
10 years ago
Hey, beautiful location by the way. Nice lugging-prius. And that looks like quite a burly stove-core. I mean it looks HUGE. Maybe it looks larger, in the picture, than it is, in actuality-- but I wanted to share this with you, just in case: this was a recent post from a different thread:

Larger diameters can work too, but there is a limit: if ducting is too large, then the system will have the potential of getting REALLY HOT. This means that the design will require different materials (with higher-heat-resistance) than the standard rocket-mass-heater stuff. There is a podcast by Paul Wheaton, Ernie and Erica Wisner, which talks about exactly this. The podcast is long, and they talk about a lot of stuff, but eventually they mention the maximum size rocket mass heater duct. You can get the podcast here:

Looks really awesome though. A pile of fire bricks is a great way to get started!

Oh and another thing: I recently was testing my stove-core, which is made of bricks. I did a full mock-up, with a barrel and pipes and all. I applied some mud as temporary mortar to seal some of the main cracks and holes, but the darn thing just wouldn't draft correctly. It was getting 100% smoke-back. Not drawing any air in the proper direction. The next day I rebuilt the whole thing, again using that mud as temporary mortar, but this time I mortared between nearly every brick-- especially throughout the heat-riser. It produced a lot of steam (during the first hour or two), but otherwise it drew and drafted a lot better. So.. moral of the story is, when building semi-full-scale mock-up, don't underestimate the importance of mortaring all the cracks in the bricks.

be well!
10 years ago