Sauna stove in a gas bottle

Hey all,

I'm working on a "stove in a bottle" design to put into a sauna. The idea is to seal the bottle such that the operations of the stove would be completely contained except for fuel and oxygen intake from an exterior room and exhaust into the chimney. Of course,  in an enclosed space, like a sauna, it will be very important to ensure that gasses cannot leak into the sauna room itself. The gas bottle seems to be a good option because of the quality of the metal, and its ability to shed any water that might splash onto its surface during sauna sessions.

Initially, I planned a welded metal tube internal design, but abandoned that after Iearning about the high heat spawling that was being reported. The problem is that the ballon doesn't allow much space for a firebrick heat riser.

My question:
Can I use firebrick chimney cylinder elements for my heat riser? Here in Estonia, we have 20cm and 16cm cylinders. These are tested for withstanding chimney fires but not for sustained, repeated high temperatures.  My plan is the sand a few mm off of the 16cm cylinder to allow for expansion when heated and to slide it into the 20cm cylinder. That would give an overall riser thickness of 2cm + 1.80cm with a 2mm gap between the nested tubes. Would this setup theoretically withstand the high heat and account for repeated expansion and contraction?

https://www.bauhaus.ee/fibo-moodulkorstna-samott-toru-o-200-mm.html

Hello Matt,   Welcome to permies!

Not sure what your weather is like right now but for me, a rocket sauna sounds really nice in the middle of winter.

As far as being concerned about exhaust gasses leaking into the room, your safe since the draw from the fire will always be sucking air inwards under negative pressure and out the building.
Even small leaks in the system would only leak very briefly if you don't already have an established draft.
Of course, leaks are best filled in ASAP as they will rob the draft from the feed tube which is where you want it.

I'm not sure what you mean by "balloon". Do you mean the gas cylinder acts like an enclosed area?

I am pretty certain that your chimney liner will not work as it just won't be able to withstand the high temperatures that rocket stoves get. Others have tried ceramic chimney liners in the past and they have cracked badly due to the immense thermal shock inside the combustion zone - particularly the heat riser. May I suggest instead you try using a product called superwool.
To make a 5 minute riser out of it would fit your bill perfectly. See here to learn about how to make one: Working with Morgan Superwool Plus ceramic blanket

You may find that the river rock shield all the way around your heat exchanger may rob too much quick heat from getting into the sauna. You may want to limit it just to the top or sides and have a certain percentage of the heat exchanger exposed.

One other place of concern is your angled feed tube. Unless your going to be there all the time to monitor how the wood is being delivered to the fire, the angle itself prevents the wood from reliably dropping into the fire below as its being consumed. A J tube is a much more reliable way to ensure the wood drops as intended.

Thanks for these tips Gerry. This is really helpful.

I've read about the J-tube vs 45* tube issue. Makes sense. I just have to find a way to set this up so that the burn channel (right term?) can extend through a wall.
I suppose I could just make a brick facade of some kind. Might look kinda cool.

The superwool sounds like a miracle cure. The texture of the wool doesn't create problems within the riser tube? Also, do I need to worry about metal deterioration in the bell, if - for example - I use chicken wire to hold the wool tube together?

Weather here is getting chilly but we'll be without a sauna for a while yet I'm afraid. BTW, shout out to all my peeps in your backyard. I'm a Spokane boy, college in Helena, seminary in Vancouver, BC. Small world.

All the best,

Matt

Bricks, rock, cob, tiles etc can all be wonderful additions to make the front of your stove look really attractive. They are all very forgiving and can be put together very quickly, usually very inexpensive (if not free) and with little experience.

Superwool is definitely a miracle product indeed!
When you line the metal cylinder with it, there will be wrinkles on the inside but that won't affect the flow of gasses through the length of the riser.

I have found that any metal past the core will hold up just fine without any modifications.

Small world indeed. Us rocket scientists have to stick close!