Design assistance required

Hello everyone.
I'm new to this forum and to the world of knowledge related to Rocket stoves & mass heaters.

I would be sincerely grateful if all you brilliant minds would take the time to look over a design of a rocket i'm in the process of building. (Admin, apologies if i'm posting this in the wrong place, if so please feel free to sort me out)

A bit of background, i'm from Johannesburg, South Africa.  Space inside the house is too limited for a nice big RMH...  
Winter doesn't get too cold over here with temperatures outside rarely dropping below 8 deg Celsius.
There are three main reasons why i'd like to build the attached rocket, one: just to take the chill out of the air inside the house during the very cold nights.
two: to have a cooking option other than gas and electricity(issues with supply in the country currently). three: i'd like to use this to warm up water for bathing etc.

I have started to build this design but am experiencing a lot of smoke back, my tests did not include the riser being insulated just yet and i suspect that may be why but would like to get experienced input...

Hope you can assist.
Regards Ruhan.

Hello Mr. Venter,

Have you built any working Rocket Mass Heater in the past?  

If this is your very first rocket stove, well, while I respect your creativity and guts to try your own design right off the bat, here are somethings I learned about rocket stoves. One must follow the design constraints exactly and run it for a year before one attempts to design a new type of rocket stove.

A rocket stove is deceptively simple looking however, a rocket stove is a complicated device which creates and then burns gases that can power vehicles (google truck that run on woodgas).

A rocket stove design is sorta like rocket science. If you ask other rocket scientist to help you with your first basic rocket model, they'll all try to help you. But if, on your very first attempt, you display a brand new rocket design that no one has ever seen actually work and ask for their help, you may get silence.

A little variation can cause great issues: I thought that as  long as I had the same cross sectional area as a 6" stove pipe for my feed tube and burn tunnel, I would get the same results -oh how wrong that thought was- and I built a 6" system with a rectangular cross section equal to pi *3^2 -----ooooooops-------  the burn tunnel and feed tube should be square or round not axially asymmetrical---- I don't get nice clean burns like I did with my first 8" rocket stove.

Your design doesn't just have a little variation; the whole design is, in my opinion, brand new.

You had other choices if you didn't want the 8" Rocket Mass Heater because it won't fit in your space. You have this choice: Cyclone Style rocket stove. It's an awesome solution for small areas and follows a tried and true design.

Good luck with your experimentation, I'm glad to see you actually tried!

Posting a copy of your drawing for easier discussion.

First, it appears you are building this from metal. That will corrode quickly under the extreme heat stresses and atmosphere of a properly functioning rocket core; you need to use refractory materials (certain kinds of ceramic) if you want it to last.

You show the feed tube flaring dramatically to fit lots of wood. This may be a factor in your smokeback, as the feed needs to stay the same size approximately as the burn tunnel, to keep the inlet air flowing fast through the fuel and pull the flames down instead of allowing them to rise as they would on their own.

From a logistical standpoint, what parts do you have assembled when you experience smokeback? If it happens without anything over/around the riser, you have some major problem with the build and probably need to start from scratch. If you have the barrel over the riser, but no chimney connected, you are just seeing the effect of limited total draft. You need some amount of vertical chimney to positively establish draft in the right direction. If something else, we need more details to advise.

Insulating the riser is essential to getting the extreme heat in the riser that allows complete combustion and good draft. Be aware that as you insulate the core properly, the metal will experience heat stress and may soften and warp in the hottest parts, and will start to corrode from the atmosphere and heat combination.

You have a 75mm square core passage (about 3"), which is very small. The smallest RMH core that tends to work reliably is 6" (150mm); anything smaller needs to be tweaked just right, and have perfect conditions and a bit of luck, to work well. The J-tube design does not scale infinitely down, but as it gets smaller, the heat concentration inside the core drops as the square of the width while the surface area drops linearly, which means you lose heat faster than build it up, and cannot get the temperatures that make an RMH function.

I would advise starting over with a minimum 100mm square cross section, and firebrick if you can get it. There are plenty of other options in ceramic materials, but even old red brick will probably work well enough for a small system. A J-tube at 4" or less needs to be fed frequently, and for small sizes you would get better results with a batch box core feeding your rocket. This is distinctly more complex and exacting, though, and will not work decently unless you follow exactly the dimensions and setup shown at batchrocket.eu.

You say your space is limited; how limited? If you can fit a 600mm square brick mass a meter or so tall in the room, you can have a good small RMH with cooktop, reasonable for taking the chill off of cold nights and minor cooking. The brick mass, unlike metal, does not need large clearances to furnishings or combustibles, only 100-200mm at the size you are looking at.
What financial constraints do you have? Can you afford the best materials for this, or do you need to scrounge?

Hi Ruhan; Welcome to Permies!
Congraulations on your first RMH attempt!  Even though it is currently not working properly.
Many thanks to Glen for adding the drawing.
It looks like you have changed all the recommended dimensions.
As Orin mentioned , building your first rmh by the book and running it for a season, is a solid backround... before attempting modification's.
We need more details from you about your construction methods and about the smoke back.
Due to your warmer location a small stove is all thats needed. There are proven designs that will work as expected.