Shape of rocket mass heater

Hi, I'm going to build a rmh for the first time in my new house in Spain, I want it to look pretty sleek and modern, like those lovely zaug stoves. Rather than using an oil drum, I want to use thick plate steel, which I have no way to roll into shape. Would a square sided body work? Our octagonal maybe? Anyone tried this? The inner riser will be thick insulated steel pipe and the general functioning I will base on the zaug model, with a pedestal etc. Would really appreciate any input!!! Thanks.

Hi Robby;  Welcome to Permies!
Good news, you can use any shape or material you like as a barrel replacement.
As far as your plan for a riser... the only insulated metal riser that will last, is known as a five minute riser. It utilizes ceramic fiber blanket and the only metal needed is a piece of regular stove pipe, that the blanket is inserted in.
Do you have a copy of the rocket mass heater builders guide ?  Highly recommended by all builders.
As long as it is built to spec. your stove can be as modern looking as you desire. Be aware that metal must be completely insulated from the heat or it will warp and crack quickly.
I am not familiar with how the zaug is built, although I will be going to check them out shortly.
There are certain parameter's that a rmh must be built to, after that it can be as cool looking as you can create.  

Ok;  I went and checked out Zaug stoves. They are vey modern looking. None are currently being built do to health issues.
When hooked into a horizontal mass it should provide the same level of comfort as a home built rmh.
When used as a stand alone stove they use masonry brick as a mass , but without going horizontal they would be little different from a standard brick lined wood burner.

This is a new home, so you can install a proper chimney? Is your floor being built strong enough to support a rmh mass?
Do you have a plan in mind or is this still conceptual?  Designing a central location for your mass and chimney is best done during the planning stage.

Thanks for the interest and quick response!
The house is new to me, but is 20 years old, it is a concrete frame that is sturdy enough for the mass, the floor slab is thick anti-seismic concrete. I don't have any real data on its strength, but I'm confident it can hold up.

It's still conceptual, I would like to finish by next winter, so I plan to really refine and define the design before I get building. The mineral blanket you mention, is that glass wool insulation commonly bought in rolls?

And the stove pipe inside the blanket is just thin-walled steel? I had imagined using a 120mm inside a 150mm pipe filling the space between them with the glass wool, but that's only 30mm difference, which would be 15mm thick insulation (stuffed in tight). Maybe not enough. Also, I imagined that it needs thicker walls than normal (.06mm in Spain) stovepipe to not get holes in it at those high temperatures I read about!

I realise that this is going to work only if I get the dimensions correct, I guess I will get the book you mention as I keep hearing about it!

I plan to build it and test it outdoors to see how long the horizontal part can be, and yes the the actual stove part will be central in the house and I'll pass the exhaust through a masonry bench and over to the existing brick chimney which I will line with the 120mm tube.

But the great piece of info you have given me is that the outside of the stove doesn't need to be round, so I can be free to build it myself!

Thanks again!

Hi Robbie;  Your floor sounds strong enough.  If that concrete is insulated from the ground then your good to go. If it's not you might consider putting some down before building your RMH.
No I think your referring to fiberglass or rock wool insulation, ceramic blanket is a specialty insulator that can easily withstand the super hot temps a rocket stove produces.  
A 10" thin wall pipe with a 1" thick ceramic blanket inside produce's an 8" riser.  Check out the attached posts.  


permies.com/t/93198/ceramic-fiber-blanket

permies.com/t/95849/Working-Morgan-Superwool-ceramic-blanket

Ok thanks, I'll make sure I find the right blanket, it sounds like a key component.

Once I get started I'll share some photos and get some expert opinions!

Robbie; I just realized you said 120mm chimney!  
That is only 4.7"  Only a batch box design rmh will function with that small a chimney. 5" is really the recommended smallest chimney to use.

A 150mm chimney would give you enough to construct a 6" rmh of J tube design or batch box design.

Ah ok, I was just guessing at those dimensions based on some pictures I'd seen; when it comes to building I'll make sure I nail down the design checking that my tubes square with tried and tested designs!

This is my first experience with permies, it's great to know you people are out there caring about how my heater is going to work!!

Thanks again!