Fox James

The main issue I can see from your picture and description, is the amount of unburnt fuel exiting the riser?

I have developed a small vortex stove, it is built from insulating material but uses large viewing windows and a large ceramic glass hot plate to disperse the heat into a space. (Space heater)
The placement of the glass allows huge amounts of heat to be released which in turn heats a space very efficiently while keeping the chimney temperatures very low, resulting in a super efficient hot plate and space heating stove but, nor a mass heater as such.

So drawing the flame path away from the glass (like most stoves do) might work but if the flame is going straight up without
any turbulent hot, gas, air mixing, then it wont be very efficient at all?

However we are here to help, so if you want to offer more details then perhaps we can give you more food for thought…..

You can try here https://permies.com/t/206700/Sand-battery-thermal-mass-proto

I think Rico gives a very good answer,
Personally I have inquired about installing a DIY rocket stove in my house but, there is just no chance, as any installation has to have a certified makers plate.
I am in the building trade myself but, the inspectors can not pass a stove that has not been tested and certified by the manufacture.
Also a rocket mass heater, simply does not exist on the building inspectors list so he can’t pass it either!
So if I decided to just go ahead anyway, should there ever be any form of a house insurance claim, I would not be carrying valid insurance and even be liable for fraud!
Having said that I do live in a highly populated area of Europe where we are smothered in red tape!

I know you have tried hard to show us the issues you think you might have it is still not very clear to me what your problem is?

If you have built a 6” or 150mm stove then the riser is fine with the same diameter, you do not need to match the fire box square area to the riser square area.

There are hundred maybe a thousand J tubes running just fine with a six inch square fire box, six inch square burn tunnel and a six inch round riser with. No problem at all.

I ready don’t think fitting everything inside a barrel will give you the best performance possible but if you don’t see any smoke out of the chimney and your barrel top is reaching 250c  then you stove is probably working as expected.

If you want more performance then you need to make a more free flowing design using insulating material and a taller barrel.
Because you have used dense fire brick, the stove will take longer to reach peak operating temperature, maybe 30-45 minutes to saturate the bricks.
However if you don’t add insulation then the heat might not rise high enough inside the fire box to satisfy your needs.

Can you buy  vermiculite board where you live? It is very easy to work with and very quick to build with.

Hi Kees, where do you live, what is the air temperature?
I have had a quick run through your post but I am not sure exactly what your problem is?
Do you see white or black smoke coming out of the chimney?
What temperature do you see on top the barrel above the riser after the fire has been burning for an hour?

Sounds good apart from putting super wool in your oven, that is not a good idea due to the risk of microscopic fibres being ingested!

I have this short video from this mornings experiment …

I think you have a point however I am not so sure, it depends on how you intend to use your stove.
If it is one load in the morning and one in the evening then the slab wont take in enough heat.
If your stove is more dedicated to cooking then you could burn it for longer and saturate the slab, then it might be a good option.
There are millions of pizza ovens in use that use thick mass for prolonged cooking times but they do take several hours to saturate with heat. You can expect the face side of a 3” thick pizza oven to reach around 500c and the reverse side around 300c after 2-3 hours of heating. The good news is it takes two days to return to ambient temp.

I really dont know how effective your idea will be as you will have the opposite issue with a thin kiln shelf as it will only work while the fire is running, maybe it will still stay warm once the fire dies down to coals but not for long?

White ovens are one of those things we see regularly on this forum but rarely find out how or if they work!
It can certainly be done with a J tube that is used for continuous radiant heat but a batch box does offer some unknown challenges.

I think a thick solid mass is a good choice for the oven floor as it will hold and stabilize the heat inside the oven.
Kiln shelves are amazing and certainly have there uses although for me they are an expense option for me.
A lot of refractory materials seem to be very regionally priced, for me 25kg of casting cement would be around £25 a 12” x 18” kiln shelf would be around £75.

I think we are talking about Molar clay, it is readily availible throughout Europe and the UK.
It is a form of expanded clay and used extensively in the growing industry as it hold water well but aids drainage.
It is used as cat litter due to the high absorption rate and also for growing bonsai trees.
It is also  key ingredient for lightweight, insulating building blocks.
Because the clay is baked at over 1000c it also has refractory uses.