Sauna stove, building the hard way

[martyn]

What size is your sauna room?
I made a 4” vortex stove that is tiny and I have made a 4” J tube vortex (all on my youtube channel) that is very small but would probably need feed ing to often.

[findingoutthehardway]

The room is approximately 2m (width) by 3m (length) by 2m (height) - but the benches and walking area take up quite a bit of that space, the floor space left for the fire and room to get in front of it and feed it is 800mm x 1800mm or thereabouts.

Here’s a pic with a barrel for reference, the stove will be parallel with the window and there will be a slightly sunken stone floor that runs the width of the sauna in front of the window, then there will be a little stool in front of the window we’re you can sit and lay the fire



Ideally I’d oversize rather than undersize the system as would be a pain if I just couldn’t get it hot enough. Aware I may have to compromise on something though!

[martyn]

You could sink the whole J tube so you only have the top of the feed showing at floor level.

[findingoutthehardway]

I like your thinking, but can’t drop it more than 130mm as the building is sitting on railway sleepers on top of aggregate, so might end up with a wonky sauna if I go too deep.

My other slight reservation with a j tube is the slightly increased risk of fumes going into the sauna - I gather this shouldn’t be an issue if it’s drawing properly, but given the small enclosed space I’m a little wary. Given your experience, maybe you think otherwise?

[martyn]

I dont want to be trying to convince you to use something you are not keen on and I think we have now covered the J tube as a no.
So lets look at your other options you have mentioned.
The vortex stove is something I have spent many hours and lots of money on trying to adapt from Trevs design to meet my own needs.
Although I think it offers the most attractive display and it certainly pours out heat from the front viewing glass, I just can not get it to re load with fresh wood until the stove cools down to ashes.
So if you want to keep the heat consistent for a few hours I can not recommend the vortex stove, however if you think you could manage the stove then a 5” model will be enough to melt your face off if you look to close!
Then we have Peters latest designs, he has said the shorty can be reloaded without issues so that sound like the one you should pursue ?
I am not sure how you intend to produce the required steam, will you just pour water onto of the barrel?
If you just want radiant heat and be able to boil some water then the simple method would be a commercial box stove, the modern designs are fantastic!

[findingoutthehardway]

Aug 14, 2024 10:06:33 GMT -8 martyn said:

I dont want to be trying to convince you to use something you are not keen on and I think we have now covered the J tube as a no.
So lets look at your other options you have mentioned.
The vortex stove is something I have spent many hours and lots of money on trying to adapt from Trevs design to meet my own needs.
Although I think it offers the most attractive display and it certainly pours out heat from the front viewing glass, I just can not get it to re load with fresh wood until the stove cools down to ashes.
So if you want to keep the heat consistent for a few hours I can not recommend the vortex stove, however if you think you could manage the stove then a 5” model will be enough to melt your face off if you look to close!
Then we have Peters latest designs, he has said the shorty can be reloaded without issues so that sound like the one you should pursue ?
I am not sure how you intend to produce the required steam, will you just pour water onto of the barrel?
If you just want radiant heat and be able to boil some water then the simple method would be a commercial box stove, the modern designs are fantastic!

I won’t rule it out completely - simple design and reliability are big positives.

Not being able to refuel would certainly be an issue. I think this was overcome in the DSR3, but sounds like things are very finely balanced still perhaps? The shorty does look great, but has similar dimensional issues for me as the j tube - again, not ruling it out though, it’s just a bit long, or a bit wide if done as a sidewinder.

Modern stoves are great, see my previous post on the Burley, I love it! It heats a little cabin I built opposite the sauna with a couple of logs and kindling. What I find interesting is that many seem to have similar designs to the DSR3 and Vortex - fire chamber with narrow exit at the rear top, smaller secondary combustion chamber above created by stainless steel baffle or refractory board, mini ‘bell’ where heat is extracted, pre heated air supply.

Specialist sauna stoves are pretty good too. I have to admit, a big thing driving wanting to do this myself is the desire to tinker. You’re also talking £1.5-2k for a specialist sauna stove, so saving money would be a bonus…but I’m prepared for that not to happen!

Steam is produced simply by throwing water on to hot rocks whenever desired, so the rocks would sit on top of and around the bell. This is different to a steam room you might find in a gym, that uses a dedicated steam generator and has lower temperatures and much higher humidity. Target temperature in this sauna will be around 90c. Radiant heat isn’t really the goal, hot convected air is, so I’m planning to fix various pipes around the bell and between the stones, maybe even on the flue, in order to draw heat away and get the air moving. Also planning to run the air supply for the room up behind the flue so you get nice preheated fresh air.

I’m sort of working backwards from the desired external dimensions - fitting most or all of the core and bell inside a 100x500mm steel pipe. That would be the most ideal scenario I think, it would look great, fit well in the space…but it does need to work and ideally be quite efficient, which is asking a lot. The DSR3 is really the only design that could potentially do this out of the box, but its seemingly sensitive/unpredictable nature is somewhat of a concern.

I’ve been thinking about how best to prototype this. Vermiculite board is really pricey, so it’s either firebricks or pre cast sections for the vertical bits and probably kiln shelves for the horizontal. I think I’ve seen you use all of these methods in your videos - any thoughts?

[findingoutthehardway]

Ok, here’s my plan, I’m going to build a cheapish prototype, based on dsr3 dimensions as much as possible, but in a cylindrical shape and with a mini riser in the centre after the secondary combustion chamber as shown in the design.

For the prototype, I’ll try and keep things cheap and easy to change - I’ll use 3 round kiln shelves, 400mm diameter, at a cost of around £30 each from a pottery supplies company. One will form the floor of the main fire chamber, the middle one will need the port cutting out, which I’ll base on dsr3 sizes for a 5 inch or 6 inch system (tbc), the top on will need a hole in the middle cutting out for the mini riser, equal to system csa.

The shelves will be propped up either using kiln shelf props, costing £5-10 each, or potentially steel scaffolding tube offcuts, as I have one of these already and they’d hopefully last long enough for the prototype.

I’ll wrap the kiln shelves in ceramic blanket (£120ish for 7m, less if I can find smaller size), then in a thin aluminium sheet which I already have and use hose clips to pull this tight together and form a temporary outer wall. I’ll need to cut out a door too and probably just use wire to attach ceramic blanket to the inside of this.

The mini riser could be made in a similar way, with either just ceramic blanket, or with aluminium too.

Air will come from a little chamber under the fire box and up through holes in the kiln props/steel poles.

In theory, it should then be pretty straight forward to experiment by changing the heights of the first and second combustion chambers, any obstructions in the second chamber that divert gases to the central mini riser and the number of/direction of holes in the air input tubes.

Changing anything to do with the kiln shelves is more difficult/expensive so I’ll keep these variables fixed at first and keep fingers crossed I can get something decent working without changing them.

One issue with this is I’ll have no visibility inside the core so have to judge everything by what comes out the top, or remove the door to see inside the main chamber. I could potentially add some ceramic glass to the top of the secondary combustion chamber, but this adds quite a bit of cost and also means I can’t insulate above it, so reckon I’ll just have to go blind at first.

Hopefully, if all goes terribly, which is probable, a good amount of materials could be re used for something else.

Thoughts/ideas welcome!

[martyn]

Ok good luck and keep the post coming…..

[findingoutthehardway]

Finally got round to prototyping this. This first effort was a tale of two halves - lots of smoke for 45 mins, then a nice clean burn after that.

Part of the reason for the slow start was I forgot to insulate the top of the secondary chamber, so about 40 minutes in I chucked on some (wet) foamed glass insulation I had lying around. 5 minutes later it was burning clean l, although difficult to say whether was close to getting hot enough anyway. Side note - don’t use wet foamed glass insulation for anything hot! It explodes, like every loud rice crispiies. Luckily my neighbours are away, but next time I’ll use superwool.

I also think the secondary chamber is much too high, so I’ll reduce this next time and see how that changes things. Pics to follow…

[findingoutthehardway]

[findingoutthehardway]

Here's where I'm at with the design...

• Square steel tube to act as the doorway and preheated air input. The frame has little holes drilled all facing the same direction, in order to swirl gasses around the main chamber and also around the door to keep any glass there clean. They are all fed through the same air input port, which in this design is a bit less than 25mmx50mm. 
• Kiln shelves for floors/ceilings
• Cast in place insulating refractory concrete between the frame, which could be replaced if needs be
• Horseshoe shape to direct gasses above the firebox and into a small central riser
  
• Riser will probably be a 5 minute riser style, using superwool inside hvac tube

Dimensions are in mm:

• Firebox: Hieght 250, Diameter 400
• Port: Diameter 38
• Secondary Chamber: Height 25 
• System size: 150mm
• Riser: Diameter = system size, height = 350

I'm planning to test something as close to this as I can using kind shelves wrapped in superwool and aluminium. If I can get something fairly reliable, I'll fabricate the metal tubes and have a go at casting it in place in a steel pipe I have, which I intend to use - its 10mm thick, 500mm diameter, 900mm tall or thereabouts. The top half of the pipe will act as the bell and I'll weld steel fins internally to try and compensate for the small size. 

Any feedback, ideas or insights you might have - please do share, particularly on the dimensions, or if you can see any potential red flags with this design. 

[martyn]

Wow you really are trying something new!
Unfortunately this forum does not seem to have many active members anymore, you could move your post to the other forum where you may get more help.

[findingoutthehardway]

Thanks, I'll do that and keep this tread updated if anything interesting happens!

[peterberg]

Sorry to say, but I'm inclined to think you aren't on the right path. The round opening between the firebox and the secondary chamber is far too small in my opinion. None of my designs has a port that's smaller than 50% of chimney cross section area, 70% is much more common. I did some calculation: your 38 mm diameter hole is 6.4% of system csa. In order to rise that to at least 50%, it should be somewhere around 106 mm diameter. As it is now, gas speed is far too slow, turbulence is lazy and the really hot afterburner flame won't occur.

The volume of the secondary chamber should be enough to accommodate rapid expansion and lots of turbulence. To my eyes, it looks far too cramped now. What you could do is rising the secondary chamber's height to 200 mm again. The horseshoe height to 100 mm or thereabouts and have the riser resting on the horseshoe. Or, with a full height horseshoe, the top half as a complete square. By doing that, the hot gases have plenty of space and need to go down in order to get into the riser.

The firebox could be defined as high as wide, say, 250 mm, being not critical.
Signals to keep in mind:
There should be an audible roar, then you'll know there's lots of turbulence.
Assuming the whole of the thing is completely dry, smoke should disappear completely within 10 minutes, preferably in 5.
When the above points are met with an open system, all air inlet should be in the top half of the fiebox, 30% of system csa in total is enough.
If and when the thing is running like it should, the afterburner flames could rise out of the riser because there's no back pressure of a bell at present.

Of course lots of different configurations are possible, although in my experience just one or two will turn out being the best performers.

All the above recommendations are based on educated guessing. Whether or not you are implementing those is entirely up to you.

[peterberg]

The discussion is continued on Permies Rocket Mass Heater forum. Please see permies.com/t/264650/Experimental-batch-rocket-ish-sauna.