Cork Sauna with a Batch Rocket Heater

[findingoutthehardway]

Hi all, noob rocketeer here. Thanks for all the invaluable information on this forum and also on Peters site batchrocket.eu/en/.

I’m building a sauna, mostly out of expanded cork and have decided a batch box rocket could be a great way to heat up the space and sauna rocks. The plan is to extract most of the heat via the bell, with a short run of floor level pipe before exiting the side of the building and up. This could be covered in some kind of mass to recover more heat, but the primary aim is to heat air+rocks via the bell and throw water over it for steam.

There are still some gaps in my knowledge I’d like to fill before designing it, so thought I’d tap into the collective intelligence here first. If you can help or point me in the right direction I’d be forever grateful ….here goes…

1. What size system would be ideal for a well insulated 12m3 sauna space in order to reach temps of around 90c? Smaller will be better for space reasons, but I don’t want to risk having an underpowered heater. Will a 4 or 5 inch suffice? This leads to…

2. I’m short on space, particularly height (ceiling height is 2m) so have a conundrum of wanting to extract a lot of heat from the bell whilst also not making it too big. Can fins be welded to the bell on the inside and/or outside to increase its surface area, so the height and width can be kept smaller than would otherwise be the case. Chat gtp tells me that 2mm thick, 15cm wide fins every 10cm of circumference on the outside would increase heat transmission by 2.91 times, even though ISA remains unchanged - does this sound believable?! It provided convincing calculations but I didn’t bother checking them.

3. Could a smaller diameter stainless steel cylinder be used for the top section of the bell and a standard oil drum for the bottom, as long as there’s enough space around the heat riser? The stainless steel section would then be used to hold the rocks/have water thrown on it and protect the mild steel below. It’s the best solution I could come up with, given the price of stainless and its lower conductivity, vs the perishable nature of mild steel when in contact with water. Other option is to just use black stove paint on a normal barrel and try and get really good coverage with the rocks, so most of the water evaporates before it reaches the bell, but this scares me a bit.

Thanks again for all the knowledge, looking forward to gaining more!

- Dave

[martyn]

Have you considered a j tube and a barrel?

[findingoutthehardway]

Jul 30, 2024 23:30:52 GMT -8 martyn said:

Have you considered a j tube and a barrel?

Hi Martyn, thanks for responding! Since posting this I’ve gone off in many different directions and came across your videos on YouTube, which are excellent, as well as Trevs builds and Peters double shoebox rocket - what a treasure trove of information! Hats off to all of you.

I’m particularly interested in what you have been working on with the 4 inch system in a barrel as this could suit my needs very well due to the low height and overall footprint. I like the idea of a batch style rather than J tube as it’s nice to see the flames and it would be good to be able to stock it just once ideally, get it up to temperature and not have to reload, or at least reload less often. Aesthetics is also a thing as I plan to take videos and pictures of the sauna to market and hopefully get commissioned to build another. I won’t build the stove for other people any time soon, but needs to look good and I don’t want to buy one off the shelf!

My main concern about this system is the potential for overfuelling and carbon monoxide from the coals. From what I can see you’ve made good progress in overcoming these issues so far, but given the confined space I’d need to be sure I don’t a) burn it down and b) poison anyone!

This mornings coffee sketches have been toying with the idea of using 3 metal tubes - a big one for the fire box, smaller one for the secondary burn chamber and a barrel for the bell - the firebox and the secondary burn chamber would be metal on the outside, with a cast insulating core, then protrude out of the barrel. I like the idea because it could be easier to build and also look cool, but I’m not sure about how this would affect things in the fire chamber (I’ve seen some of peters videos using a cylinder for the secondary burn chamber)

Anyhow, again for all the great content, any pointers are welcome : )

[martyn]

I can see all your logical thinking but, the humble J tube has a lot to offer, my designs does have a visible fire, very easy to operate and clean, easy to build, super hot performance and about 45 minute re load time.
Perhaps the main advantage is the fact you can continuously feed the fire without over fuelling or without any drop off in temperature.
It is possible to control the temperature as well, not so easy with a batch design!
To be honest I have not perfected my version of the vortex stove as yet, each year I get closer but it still misbehaves on re loading, unless the fire has really turned to coals and you only add small logs.
If you can source vermiculite board, then you can build a J tube in an afternoon, if you plan to burn every day then vermiculite might not be the best choice but my own J tube has been running for several years now.
You have to bear in mind that my 6” J tube can heat an uninsulated outdoor room measuring 8x8m to over 25c. In a small sauna room you will need some form of temperature control.
If you just build a 6” vermiculite J tube without any mass but a good chimney then it will get really hot, really fast, expect the barrel sides to reach 300c or more within 10-15 minutes and the top can get over 500c!
If you build a 6” batch design then there will be a lot more wood burning at one time, at full chat the whole load will be on fire and pouring out uncontrollable heat!
You would have to consider a way to control the amount of heat retained inside the sauna, maybe just opening the door or a bypass or a fan to suck in cool air.
I agree that a batch design can be made to look  nice but the J can also look ok…. youtu.be/LbZTkuiMBRU?si=Mev7BZhmvn1seGAJ

[findingoutthehardway]

Aug 1, 2024 3:51:42 GMT -8 martyn said:

I can see all your logical thinking but, … the J can also look ok…. youtu.be/LbZTkuiMBRU?si=Mev7BZhmvn1seGAJ

Thanks, these are all very logical considerations. I think with either option I’m going to have to be meticulous about heat shielding. By the sounds of it 4” will be plenty.

That is indeed a nice looking J tube and impressive outdoor heating stats!

The main issue with a J tube would be the overall footprint, if I can get it roughly the size of a standard oil drum plus a few bits sticking out here and thereit would be ok, but the J does tend to protrude quite a bit where the flame chamber is. Maybe I could get around this by offsetting the barrel so it’s closer to the riser at the back?

With the vortex, an ideal scenario would be being able to heat the thing to the right temperature with one load and not have to open the door again. But a door in the sauna can be opened if it’s too hot and it’s going to have a pretty big ventilation hole somewhere behind the stove.

I have to admit I’ve become seduced by the idea of the vortex batch, but should probably get my logical hat on. Am I right in thinking that currently, remedies for the over fuelling issue make it harder to get started and finding a delicate balance between the two is proving tricky? Also, could you share what you think is the cause of over fuelling as haven’t worked this out yet, but I’m guessing it gets too hot too quickly?

Any thoughts on the round fire chamber? Would this likely cause the wrong conditions or could it work? If it could, do you have any thoughts on the internal diameter in relation to the system size (4”)? If there’s a way to share images here I could upload a sketch, albeit a poorly drawn one of what I’m thinking