Sauna stove, building the hard way

[findingoutthehardway]

Hi Peter and all, I’ve been digesting the content on this forum and elsewhere and just wanted to say thanks for all the work you and others are doing, it’s a fantastic resource.

I have a quick question that I’ve yet to find the answer for (apologies if I missed it somewhere) - in theory, could the DSR concept work with the secondary combustion chamber to one side, with the exact same dimensions as when it’s on top? So there would be a vertical port off to one side, then the secondary combustion chamber would be the same height as the primary burn chamber, parallel with its side. I’m looking at ways to reduce the overall height and this is one potential solution, although the reduction is fairly small. Also interested from a theoretical perspective whether there will likely be much difference.

I’d like to build a quick prototype core and compare the two - in theory I could use the same panels just in different configurations. But keen to discover if there are any red flags first. I’m guessing making pretty vortex’s might be more of a challenge, but maybe not?

[peterberg]

Looking at the standard batchrocket, you'll see a port at the back. Centered in the firebox and also centered in the riser. The 90 degrees turn up behind the port is essential here. The DSR2 is more or less doing the same thing, but part of the riser is turned 90 degrees again and is resting on top of the firebox.

In order to do something with it so the secondary chamber is at the right or left of the firebox you need to view the port and riser configuration as a whole. So when the afterburner assembly is turned, the port should also do so. Which means the port should end up in a horizontal position. It might be worth to try this, or not at all.

That said, I seem to recall a guy (by the nickname of DCish) on this forum who did something that might be worth to investigate. But if you are trying to make the core lower, the Shorty core might be a better choice since it's extensively tested and works. It's not much lower than the DSR2 design but the exhaust port isn't on top. It's in the front side of this short riser and is venting horizontally. So it doesn't need space above it perse.



When the direction of venting isn't what you are looking for, there's also a sidewinder version. This one is vented to the left or right. It's built into a kitchen range last month. The short riser is, completely with the port, taken away from the back and placed at the side. It's a right handed sidewinder that'll vent to the left. The back of the firebox is flush with the back of the riser, all seen from the front. The cooking plate is right on top of the riser making for a very hot spot. So there's a choice, but the exhaust opening's position is at the same side and directly above the riser port. Hardwired, so to speak.

[findingoutthehardway]

Aug 3, 2024 7:47:38 GMT -8 peterberg said:

Looking at the standard batchrocket, you'll see a port at the back….

Thanks Peter. I think I’d initially dismissed the Shorty due to its depth, but as a sidewinder it’s possible it could work. Although I think the footprint is still a bit larger than the dsrs?

Perhaps I’ll try a dsr with the sidewinder on its side as well as on top to compare, we’ll see. Usinh a well tested model would be preferred, but have to fight the urge not to tinker and try and invent things all the time.

I’m really hoping to cram something in to as small a footprint as I can, ideally max the size of a 210l drum, including the bell. It’s for a sauna, I’m thinking of wrapping the exterior in copper to protect against water and increase conductivity and using pipes or fins to convect as much heat away from it and therefore break the rules relating to bell csa! I will have a short run of horizontal pipe I can try and extract some additional heat from too.

By the way, have you ever come across these stoves? I own one and they’re great! Check out page 14: burley.co.uk/wp-content/uploads/2020/01/Burley-Wood-Burning-Stoves-2020.pdf

They are not a million miles away from the DSR models - narrow port, small secondary combustion chamber above, which expands, then another small port to the heat exchanger. The way they handle air is pretty cool too (although patented) - air comes in through a single inlet at the back, is run around the base of the firebox to preheat and then injected at each corner vertically facing the same direction along the sides, which creates a vortex in the main firebox.

This made me wonder if aiming for a more complete burn in the main chamber, would potentially help with the overrun issue on some of the batch rockets. I mean, I have no idea, but seems logical to try and get as much of the burning done upfront as possible and use the secondary chamber to catch what’s left? I’m still trying to understand what causes thermal runaway and why these models are more prone to it than others, so please forgive my ignorance on the subject.

[peterberg]

Aug 3, 2024 13:16:52 GMT -8 findingoutthehardway said:

I think I’d initially dismissed the Shorty due to its depth, but as a sidewinder it’s possible it could work. Although I think the footprint is still a bit larger than the dsrs?

The DSR2 is marginally shorter but also higher at the same time. The Shorty sidewinder is as deep as its firebox, twice as wide as the firebox and lower than the DSR2 design.

Aug 3, 2024 13:16:52 GMT -8 findingoutthehardway said:

I’m really hoping to cram something in to as small a footprint as I can, ideally max the size of a 210l drum, including the bell. It’s for a sauna, I’m thinking of wrapping the exterior in copper to protect against water and increase conductivity and using pipes or fins to convect as much heat away from it and therefore break the rules relating to bell csa! I will have a short run of horizontal pipe I can try and extract some additional heat from too.

I did the development on a 130 mm Shorty system, using two barrels on top of each other. The core was housed in the lower barrel, the top barrel was used only as a bell. A DSR2 is employing a top exhaust at the front end. The Shorty has the advantage of an exhaust at the front side of the short riser. Which means it doesn't need a lot of headroom, 50 mm is adequate and 100 mm is plenty. So in conclusion: a 130 mm straight S(horty) R(ocket) can be housed in a single barrel. It will stick out the front but not overly much so.

Regarding thermal runaway or overfuelling: at long last, this issue has been solved in the Shorty design (and the DSR3 before that). The 150 mm sidewinder SR version (which won't fit inside the barrel by the way) has been run in two occasions for more than 6 hours continuely and it flatly refused to switch to thermal runaway. Even reloading couldn't get the thing to smoke, which is very good!

There's a saying in my country: "Better well copied than badly designed yourself." I think there's a lot of wisdom in that.

[findingoutthehardway]

Aug 5, 2024 7:50:16 GMT -8 peterberg said:

…"Better well copied than badly designed yourself." I think there's a lot of wisdom in that.

That’s a wise saying indeed! You make a compelling case for the Shorty and the DSR3. I think I’ll mock up both options in sketchup and get a better feel for how they will fit within the available space. I’ve also been wondering about whether something similar to the DSR3 could work in a vertical cylinder shape, mainly as it could then all be cast inside a big pipe or barrel. I’m particularly interested what effect the shape would have in the secondary burn chamber, as it naturally would create an expanding and then contracting space.

I’ve also been thinking about whether ceramic glass could handle the temperatures in the secondary combustion chamber, to be used as the roof of this chamber, with more glass on top of the bell, so it would create a ‘glowing’ effect under the sauna rocks placed on top. Unless enough light reaches the secondary burn chamber into the bell, then I’d only need glass on the top. First things first though!

[findingoutthehardway]

It's looking like the DSR3 or Vortex designs are going to work best in this situation due to their size and shape. I'm not ruling out a J tube as suggested by martyn, but its not ideal due to its shaped and would make things around the fire a little cramped.

I have a few questions, if anyone can help...

1. Could a cylindrical design work, if the CSA's and volumes of the firebox/secondary burn chamber were kept the same or similar? I would cast the parts and they'd sit inside a layer of ceramic wool, which would line the inside of a 500mm diameter x1000mm height x 10mm thick steel pipe, which I've seen for sale and would be an ideal size for the space. The bell would all be above the core in this case and I'd look to compensate for the smaller bell surface area by welding fins and/or pipes to the inside and potentially outside too to increase surface area and convection.  
   
    
2. For the secondary burn chamber, what would be the effect of splitting the path in two as shown in the image below, in order for the gasses to exit in the middle, into a mini riser within the bell? This would mean they take a longer path than in the current DSR3 design. I'm not thinking of making this part visible, so losing the cool rams horns wouldn't be so much of an issue in this instance.
   
   
3. Could a mini riser, located within the bell, after the secondary burn chamber, help increase the draw and the amount of time the gases have to burn off before they enter the bell (see other image)? Part of the motivation for this is also aesthetic as I want to create an orange glow at the top of the stove via a little ceramic glass window, which would have sauna rocks arranged around it, giving the impression of glowing rocks. But I'm also thinking that with the smaller bell size, it may be an advantage to ensure the flue gasses exit nearer the top of the chamber, so they can't get drawn straight out of the exit at the bottom. Not sure about this though. 
   
   
4. For the air intake, I was thinking of having the air enter a chamber below the firebox floor. The floor itself would be made from a kiln shelf and I'd insulate the bottom of the air intake chamber, so the air would be preheated by the floor of the burn chamber. I'd then use kiln shelf props (little ceramic cylinders, easy to obtain) with little holes drilled in all facing the same direction, in order to try and create a swirling vortex inside the main chamber. One of these cylinders would sit next to the port and inject air horizontally across it. This is inspired by a little wood stove I own, which does something similar (see page 14 burley.co.uk/wp-content/uploads/2020/01/Burley-Wood-Burning-Stoves-2020.pdf) it claims 90% efficiency and is great for what it is, the flue doesn't go much beyond 100c and it leaves little ash or coals after the burn. It uses a stainless steel baffle to create a secondary burn chamber (which needs beating back into shape now and then), but the design isn't a million miles away from the DSR3. The air comes in through one port, then is sent round some steel pipes that surround the base of the firebox and also rise up in each corner, with little directional holes as described and also lots of little holes around the bottom, in order to burn off the charcoal. I like the idea of one input port and lots of places to inject the air, feels like it might be self limiting to a degree whilst letting the fire draw air where it's needed 🤷
   
    

I'm guessing it might be difficult to answer any of these questions without running actual tests, but if anyone has any informed opinions or can see any potential red flags, please let me know. There's a high chance I'm straying way too far off course and I'll eventually come full circle and just build someones existing design, as recommended! But the urge to invent things, foolish or not, is strong! 

[martyn]

The problem is you are wanting to try new things with proven designs and nobody really knows if they will work.
There are things like chimney draw that can make or break an idea, a straight, tall insulated chimney can make a poor design work or a short chimney with bends and elbows might make the same stove fail. The high humidity inside a sauna wont help the case either.
Just bare in mind a well built stove will absolutely pour out heat, with outside barrel temps at 300 + c you cant even sit within two feet or more without boiling your blood!
Also these things are not cheap to build so trying out new ideas, although great fun, is expensive too….

[findingoutthehardway]

Aug 13, 2024 0:02:22 GMT -8 martyn said:

The problem is you are wanting to try new things with proven designs and nobody really knows if they will work.….

I suspected that might be the response and a reality check is probably in order!

The main idea in a sauna is to heat the air rather than bake people with radiative heat, so I’m planning to cover the outside with pipes and rocks to convect heat off the bell and store a little within the rocks on top for making steam, so hopefully reduce the baking factor a bit. The relative humidity will likely be around 50%, but there’s more moisture in the air due to the higher air temp and its capacity to hold more. I’m guessing the issue is that more moisture ends up inside the fire as it’s drawn in, which makes sense.

I suppose what I’d like to discover is where small deviations are less or more likely to cause problems . For example, if overall dimensions are kept similar, but the shape is a bit different eg cylinder vs rectangle, is that more or less likely to cause issues? Would extending the length and shape of the secondary burn chamber likely be a problem, if CSA was kept the same? Is there any mileage in adding a short riser after the secondary burn chamber?

My chimney will be fairly short by the way, around 3m and include a bend to exit out of the wall, although straight out the roof with no bends is feasible.

Aware I may be trying to find answers where they can’t be given, but I’ll settle for a hunch at this stage.

You’re right about costs too, that was part of the reason for casting the parts, although that still costs - I watched your video on this by the way, very helpful. If I ditch the cylinder idea I could potentially use firebricks plastered in clay for testing, so they can be dismantled. The biggest singles cost will be the kiln shelves, which for the sizes I’m looking at are around £40 each and I’ll need at least two, probably 3.

[martyn]

I dont mean to be negative, I love experimenting, from my experience most rocket stoves are pretty fickle and are effected by many factors so moving things around might just work or even work better, or equally not work at all!
The exception seems to be the J tube as that is far more tolerant probably due to the simple design and direct path up a tall riser.
If I built a sauna, I would use a J tube because I like the idea of feeding the fire as an activity while in the sauna, the simple build, the cost of the building one and the proven performance.
I believe that Peter has even built one already, was it the ‘Mini Mouse’? It fitted inside a single barrel.

[findingoutthehardway]

Aug 13, 2024 3:58:26 GMT -8 martyn said:

I dont mean to be negative, I love experimenting….

Not at all, hearing from experienced people like yourself is exactly what I need! I’ll check out Peters little J tube and certainly won’t rule it out. The thing that appeals to me about a batch is being able to gauge a fuel load for a given temperature, load it up and maybe take a shower or do something else nearby whilst it’s getting up to temperature. But would be easy enough to do something similar with a j tube, set the wood aside and pop back now and then to top it up. If I can make it work within the desired space and with the desired aesthetic, then it may well be just what I’m after.

The horizontal burn tunnel was the main thing putting me off J tubes. An ideal scenario would be to have the riser directly above the main burn chamber, which is kind of how I’ve ended up with this hybrid dsr3 with a mini riser on top. I wonder if you know of an my J tube-ish designs that have a centred riser like this?

[martyn]

Here you are, it is on the other forum permies.com/t/62190/Peter-van-den-Berg-inch

[findingoutthehardway]

Thanks very much, that was an entertaining read! I’ll do a bit more digging on different j tube designs and see where I end up

[martyn]

My own vortex J tube design only has a short burn tunnel.
I think the first Js were designed to have the riser in the centre on a barrel and were made from 3” thick fire brick making them rather larger than necessary.
The main issue is the hight of the riser, I can get away with a pretty short one of around 31”

[findingoutthehardway]

That’s a really nice build. Height is actually less of an issue, I can probably get away with up to 1.5m, but the offset firebox is more of an issue - it might be workable, but makes things a bit more cramped and eats into a big window. An ideal scenario would be to fit the whole thing, or most of it, in a 500x1000mm steel pipe, which is why the DSR3 or something similar is appealing. It would mean sacrificing quite a bit of bell surface area though, I’d have to try and recover more heat in a smaller space using fine and pipes and rocks.

[findingoutthehardway]

I wonder what the effect would be, if you took your design, but had the gasses exit the firebox at the top and the burn tunnel sit on top of the firebox, so the riser can sit vertically above the firebox? I suppose that’s what the shoebox and vortex designs are doing, but with different proportional dimensions to the j tube