Fox James

Ok thanks Glenn, so I guess what I am trying to get at, is this something we should be recommending?

I ask because there is an awful lot of info out there, like books, videos and hundreds of forum post that state the opposite to your findings!
Not necessarily in an exact sense but as a general term …. a 8” system requires a 8” chimney.

As a group of enthusiasts I am sure that we are all keen to progress and learn by others experiences but, we also have a responsibility to offer the correct advice.

So Just to be clear…. With this new information, we can safely say a 8” J tube connected to a large bell ‘can’ employ a 6” chimney?
Or are we now saying, in fact a 8” J tube connected to a large ball ‘should’ employ a 6” chimney?

So the conclusion is ‘yes’ you can use a 6” chimney on an 8” J tube core but … only while using a large free flowing bell or similar open system.

So the answer is ‘yes’ but the question remains… would a 8” chimney work any better?

This is an interesting question, I have read many hundreds of post on this and other rocket stove sources that openly state the chimney must match the system size.
I can only assume (as I dont know) that the people (builders designers) have tested their conclusive advice but Glenn seems to think otherwise and has put his case forward with confidence and stated his reasoning behind his thoughts.

Without testing one alongside the other I dont see how one could really know?
I can obviously  see the benefits of using a smaller chimney for ease of use and cost.


Funny enough, in my case, my J tube is a 6” system with a 7” chimney but, that has also been openly discussed and using a larger chimney is said to be a good method….. however  in turn, I have not tested my stove with a system size 6” chimney!

My 6” J tube is an awesome space heater, it just pours out radiant heat ……

Well there are rocket stoves and there are ROCKET stoves!
That one is not insulated, not designed to run for several hours every day and is pretty small … so yes the stainless should last quite well.
However, stainless will deteriorate, the process  is  known as intergranular corrosion, this corrosion results from heating stainless between 550C and 850C.
This causes chromium to precipitate from the stainless steel and reduces the ability for the passivation layer to regenerate. (I dont really know what that means but that is what is says on Google)
Also stainless is very prone to expand and contract when heated and cooled and that is one of the main reasons it is not used very often for stoves.
I have tried using it for pizza oven doors but it warps really badly.

Well my own experiments are still ongoing so I cant really recommend what will or wont  work but as a rule horizontal chimney sections are ‘not’ a good idea.
Anything that changes a straight run will slow things down, elbows, bends or  especially horizontal sections should be avoided if at all possible. .

Hi Jon, vermiculite is very porous so most high temp glues will work but to be honest, I used to glue every joint but now I try not to use any glue!
I worked out it is far better to allow movement of the individual components, so I pre drill a clearance hole in one piece and a smaller hole for the screw threads in the other piece. Then I dip the screw threads in the glue before screwing the parts together. Vermiculite is quite soft and as a rule you can only use the screw fixing once or in other words you cant take the parts apart and re screw in the  same hole.
Best to use stainless screws but as they will be in the middle of the board any hard metal screw should survive.

Also bare in mind that although vermiculite is fantastic for experimenting it will not be as durable or long lasting as some other refractory products!
There seems to be different grades as well, the one I use is much better than some others I have tried!

Where I live vermiculite is a very common product and sold in many stores, as is the glue and fixings.
In my part of the world,  most if not all new metal box stoves are lined with vermiculite!

You can see how I am getting on with the testing here….. https://permies.com/t/181437/vortex-rocket-cook-stove#1775571

There are two new videos on my channel, here is todays offering…..working a lot better than yesterday!

This is an awfully long thread but the first few pages might help you decide https://donkey32.proboards.com/thread/703/vortex-stove

My version based on the 4” vortex, does deviate quite a bit from Trevs design and I am still developing it at the moment, there will be a new video on my channel later today showing my first results from running the stove through a barrel and mass….