Gerry Parent

Hi Jon,   Being the type of structure that it is, doesn't it hold a lot of heat through all the mass in the structure itself and rely on only a moderate amount of heat to keep the flywheel effect going? If so, then a barrel is maybe not what you need. Perhaps if the barrel was replaced with a brick or cob enclosure (which slowly absorbs and releases heat), a lot of the distances that keep you from placing it where you want could be minimized? If you still want the ability to heat some water or provide some instant heat, just leave the top of the barrel exposed.
Going underground may not be the most efficient way to transport the hot gases into the bench, but if you did, you would want to insulate it quite well from below to keep the heat going upwards.  
Also, if you do keep the whole core underground, wouldn't it be nicer to have the feed tube facing the bench so you can watch the fire better?

Hi Jon,    You could also consider Matt Walkers Riserless Core

Hi Chris,

Your dream picture showed one of the Wisners J tube builds but there is also batch boxes to consider as well.
A great resource to learn about them while you wait for your book is here: batchrocket.eu

Matt Walker has a lot of great info on his youtube channel as well: broaudio
and plans on his website: walkerstoves.com

and finally, our local Permies rocket scientist Thomas Rubino has a store selling RMH parts at: dragontech.com

Hi Jonas! Welcome to the RMH Forum,

I am going to be bias towards encouraging you to build one as I have loved building and operating several of my own.

Even though there have been innovations since Iantos book, I think that in keeping with your wanting to have it be an inexpensive build, I would follow most of your plans the way you described them. On thing I would change though is to eliminate the potentially expensive pipe for the bench and just make a bell instead - which is just a hollow chamber where the hot gasses can stratify more efficiently and with much less impact on draft. The shape is much more forgiving (especially for a small space) as it doesn't have to follow the straight lines set by the pipes.

Also, the 10.5" feed would mean that in order to keep the burning wood below the height of the feed for safety and potential smoke-back issues, your wood will need to be cut rather short. J-tubes tend to burn hot and fast so you'll be feeding it more often than with a taller feed tube and longer wood.
Not a deal breaker, just saying it upfront.

Pictures of your place where you are thinking of installing it and perhaps a drawing would also be helpful.

Elyes Kallel wrote:HI,
I have a steel bell with such dimensions (view attachment 1). Height is, obviously, too long compared to diameter.
I have been inspired by the "Liberator rocket heater" model (attachment 2). Unfortunately I don't have the dimensions of this model
Concerning the burning chamber, I am planning to insulate it from inside with a 2 cm refractory cement layer cast
So my questions are:
- What is the best height for a 40 cm bell ?
- What about the dimensions of the J tube ?
- Is castable refractory cement a good insulation material? and is 2 cm thinkness OK ?

Hi Elyes.... Welcome to the Permies RMH forum.
What size of building are you trying to heat? Dimensions will vary according to whether you choose a typical 6" or 8" J tube.
Do you have experience with casting? I know several seasoned builders here have and it seems a lot harder way to start a first build.
Have you instead considered fire brick?
What is the shape of your steel bell? Round or square?

3 things that come to mind :

- Building a brick heat riser then wrapping it with superwool can be replaced with a 5 minute riser.

- Using pipe in the mass can be replaced with a bell or stratification chamber

- Many people have switched from a J tube to a batch box design for its longer burn time and being able to see the fire

A Wisner design will still work just fine, its just a matter of what your goals are and sometimes what you have available to use.

Some very useful information has been given by our top rocket scientists.

To add: A small scale model won't behave the same way as a full scale model.
The smallest reliable size for a J tube is a 6" system. Any smaller and gas flow friction becomes much harder to overcome leading to smoke back.

A "small fire burning many hours" is not the way a RMH works.
Hot and fast is the way to a clean efficient burn.

As labeled in your diagram, all of "J" piping could be eliminated and instead just be made into an open chamber. This is known as a bell or stratification chamber. (A search on permies will bring up much information about them). Your vertical exit pipe then could run straight up through the roof. This pipe should have its opening very close to the bottom of the bell.

Hi Mike,
I don't see that the bolts are needed. If you sandwich some superwool between the bricks and the barrel (front and back), the friction should be enough to help hold the core in place. It also provides an expansion joint so that the dissimilar materials can shift independent of each other.

I think a door made of a part barrel as you suggested would shed the heat and not be a problem. You could also line it with superwool to help insulate the fire if desired. Some stainless bolts with a washer and nut could hold each corner of the wool in place as a simple way to secure it firmly to the inside of the door.

Are you going to insulate the core or just the heat riser?

Will your primary and secondary air be separate or combined?

To be fair, Peter uses a Testo gas analyzer to calibrate and modify his stoves, making his recommendations based on repeated unbiased collected data rather than just ideas or theories.

Innovation is however a wonderful thing to have fun with as it helps stretch our rules out to see what is possible.

Hi Diane,
If your stove was built to be super efficient, leaving no wiggle room for any spare heat to escape with just enough draft to make it run, I doubt the crack theory as being your problem... unless its a big one.
If so, a small amount of cob should easily fill it in.

As for the potential creosote buildup, I'm assuming your referring to the final vertical chimney where you saw it? If thats the case, your stove is not performing as it should or your not burning wood in it correctly. Perhaps you can get us a few pictures or link to your build to help us see what your working with.
Every RMH I've ever run does produce a powdery dull black fuzz, especially in areas where the gas flow slows or in corners. It easily can be removed and is not shiny or hard like creosote is.
Perhaps this is what your seeing? A simple brush cleaning and vacuuming should easily take care of that.

If you've ruled out a critter nest, when was the last time you cleaned it?
Ash buildup on top of the riser or manifold can easily pinch off these areas and cause draft issues.