Sauna & rocket stove - j6 or j8 ?

I’ve been lurking for over 5(?) years on this forum ...and the time is finally come to post. I’m building an 8sided sauna in my backyard. I was planning originally on having a rocket mass heater inside (with the feeder box outside  , to keep smoke out), running pipes under the benches on the inside. But... I don’t know what size would be optimal. I want a stove that heats quickly and  absorbs  heat well So the heat from the exhaust pipe is cool...but the actual area of my sauna is so small… Could I get away with a j6 inch system?  I don’t want to build a system that burns too hot if I don’t need to.What are your thoughts ?👍🏼🙏🏼😀

Hi Vince; Welcome to the wonderful world of rocket science!
A 6" J tube will be plenty for your sauna. You may want to cob 50% of your barrel to slow down radiant heat.
If I read your design correctly it appears you are right around the 30' of pipe recommended for a 6" system.
Have you considered "bell" benches in place of piping ?
Take lots of photo's as you build, our happy crew of rocket scientists is eagerly awaiting guiding you on your first build.

Thank you Thomas! Actually I was wondering about purchasing (or finding) plans to do a small bell style rocket stove or a 6 inch J tube style rocket stove… I’ve heard of Ernie and Erica’s j-6 plans somewhere on these forums, But I have not heard of another stove plan that is thrifty on wood but also warms up fast. Maybe the cyclone bell rocket is in the running— a stove that was covered in a permies video that I saw a recently.  Is that what you were referring to?  Is there a tried and true trusted plan for a stove like that by chance ?
Lastly, is there a rocket stove version that heats up very quickly?  My sauna is small, will be thoroughly insulated via clay slip or lite straw clay , With 4 to 6 inch wide insulated  roof/walls... should be able to  hold the heat for a couple hours of a sauna session. 150-180° tops target temp in room.

Given the fact the size of the sauna is so small, would a 4inch tunnel design cyclone bell style work? How small could I go ? I feel a 25 gallon barrel would be over kill. Thoughts and opinions welcome 🙏🏼 Thank you all !

You could ask Matt Walker/Remine how his rocket sauna worked out?

I have been a part of two wood heater saunas. The second was a small sauna with a 4.5" batch rocket. The thing cranks to 130c with that heater. We find that between 65c and 80c to be a good balance so it is more than enough power.
Yours is a lot larger but I would think even a 5" would do the job.
Adding mass will cause it to take a very long time to heat so I would advise just using a barrel with some rocks on top. That way you can still use water on the heater but get instant heat when you light the fire.

Any progress or updates on this? I am new to the forums and have already built my 8 person barrel sauna. The stove that came with it took up too much room so I am looking to alternate ideas right now. I started building a 4" square tubing based rocket stove and was just thinking about surrounding the heat riser with sauna stones for thermal mass, but also considered an idea much like yours for a RMH. My sauna is a barrel sauna so it is not insulated, but built from 1.5" thick cedar staves.

I would love to see photos and hear about the progress. Happy to share progress I am making as well. Currently I have part of it welded up and have a pellet feeder working pretty well so far. Here are some photos. I could easily convert this to RMH if you are having any luck.

Matt

Thank you for sharing your 'floating-sauna-in-the-trees' Matthew. A real piece of artwork indeed.

I don't see a chimney stack in the photo?

I think your build should have its own thread as I'm sure there is lots you could share about yours alone. I know I'm interested in seeing more!