Coffee Table RMH

I've been trying to come up with a design for a RMH in my small, 8'x12' Wofati-like structure I use as a guest house and art studio. After being generously helped through a few questions in some other threads, I've realized that my biggest challenge is simply finding an unobtrusive spot to place thing.  It can't be too close to the longer front and back walls because of door openings, combustibles, and vinyl windows. With such a small space, it seems that the barrel will need to sit somewhat awkwardly toward the middle of the room.

So I have this idea of sinking the core down into the ground under the floor about 16 inches, and using the barrel as a kind of coffee table during the off season, and when not hot.  Glenn rightly warned me of the danger this design could pose both from heat, and from the feed hole in the floor.  I drew up an idea I have for deflecting the heat and making it less dangerous by building a kind of tabletop cover for the barrel. Also with this being a very private space, I am not overly concerned about it being mistaken for a typical, "non-scalding" coffee table...

Anyway, I would like some feedback on the possibility of this design. Are there other concerns about going underground?  Would the moisture from the ground be a problem? Would too much heat be lost into the earth? What else am I missing? First time building one of these. Thanks

Hi Jon,   Being the type of structure that it is, doesn't it hold a lot of heat through all the mass in the structure itself and rely on only a moderate amount of heat to keep the flywheel effect going? If so, then a barrel is maybe not what you need. Perhaps if the barrel was replaced with a brick or cob enclosure (which slowly absorbs and releases heat), a lot of the distances that keep you from placing it where you want could be minimized? If you still want the ability to heat some water or provide some instant heat, just leave the top of the barrel exposed.
Going underground may not be the most efficient way to transport the hot gases into the bench, but if you did, you would want to insulate it quite well from below to keep the heat going upwards.  
Also, if you do keep the whole core underground, wouldn't it be nicer to have the feed tube facing the bench so you can watch the fire better?

A 4” batchbox with the mass built into or around the core might be easier and neater but ideally the chimney would go straight up.
One of the riser-less cores only needs to be 30” or less from the floor to top.

Batch boxes are typically more technically demanding to build, and a J-tube is a good first iteration for an RMH as it allows more relaxed construction. However, if you are fairly good at following instructions exactly, a 4" batch box can be quite easy to build. I made a mockup of a 4" batch box using only uncut firebricks a few years ago and it was easy, fast, and worked excellently. EZ No Tools Brick Micro Batch Box Core (The Match Box) Plus Variations, about a dozen posts down from the top.

You could put one of these embedded in the bench with some shielding at the back to protect walls, and have no obstructions at all in the middle of the floor, only about a foot cube of bumpout at the front of the bench and similar on the top.

A permanent shin buster in the middle of the room is a pain. I get it for clearances BUT that includes furniture and carpet and and and.  

I would really try to get it to the edge.

I'm skeptical about the location of the riser/barrel, and of the utility of the tabletop. I see a perfectly conventional RMH layout, with the riser in the spot where you put the cleanout , and an out-and-back pipe in the bench. The bench could be made a bit wider (looks like you have a 16" wide bench with no back, just the walls) to accommodate the two flues, and add a back for more mass (or fill the back with cushions and it could be wide enough to lie down on). This probably would be no net loss of floor space over your coffee table idea. The door area is clearance by necessity... The adjacent post is vulnerable, but could be clad in cob, stone, or brick quite easily.
Another thought, make a traditional coffee table, the same height as the bench, which could be pulled up close to extend the bench into a wider sleeping surface.

Thank you all for the replies. You've given me a lot to think about.

I do want to get this thing to be as small and unobtrusive as possible. Glenn, I like the sounds of the 4" batch box you mentioned. Previously the impression I had gotten from pouring through these forums and videos is that anything smaller than 6" is too small. I don't mind cutting bricks, and I'm good at following directions - if I could only find directions for my specific situation!

If I was to put it closer to the walls, what is the general rule for how far the core has to be from combustibles? I found some things that said 36" if there is just air in between, and that distance can be cut in half with sheet metal and spacers.   But how much distance between the core and the combustible wall if I fill the space with mass such as brick or cob like several of you suggested? In other words, how thick should that mass be to protect the combustibles?

I also got the impression that a tall riser is desirable. Anything above table height seems like it would disrupt the space and feel like a wall jutting out blocking lines of sight when people are seated. That's why I thought it would be nice to keep it low if it is going to take up space. Fox, you mentioned riser-less cores. Do you know of any good examples? I did look at the Walker Stoves Tiny cook stove and even that is 40"x40" which is still too big for this little space.

Thanks again everybody for sharing your knowledge!

Well yes I do, I have a whole series of videos about developing a tiny vortex stove.
The first one even looks like a coffee table.

The 6"" limit is for J-tubes. Batch boxes scale down at least to 4" size; smaller than that is so low-powered that it would not be worth the effort for most cases. My experiment with a 4" batch box indicated that you might need to reload every 15-30 minutes, but a few loads for a small space would probably keep you warm through the night. This page at batchrocket.eu gives proper dimensions for a batch box of any size.

I would advise making a composite bell for a system in your conditions, masonry at least 6" thick for the back and sides and metal for the front to give some immediate radiation to the space. The 4" cyclone batch box built at Wheaton Labs heated the space overnight, but took so long to get the space warm from a cold start that the mass got too hot for comfort later. It didn't have any instant radiator surface in its bell. A system of this size would power a fairly small bench, probably not the 12-14 foot duct you show. I think I would site the core in the back where the chimney rises, to allow for an easy startup bypass if you find that helpful, with a hollow bell bench running along the back and maybe widening or turning the corner a bit. This page gives bell sizing guidance. A 4" system would want somewhere around 2.6 m2 or 25 square feet of internal surface area, not including floor. A bench with cavity about 6' x 1' x 1' high would have 20 ft2 ISA, plus a tall section for the riser adding say 1' square x 2' tall above the bench for another 8 ft2, 28 ft2 total.

The masonry part of the bell could safely be as little as 4" from combustible framing, though I would keep it farther from things like fabric just in case. Facing the metal surface into the room above the firebox door would be best for space heating as well as safest. If you have a metal top for cooking on, I would give that woodstove clearances: maybe 18" from combustible walls with a metal heat shield spaced 1" from the wall surface.

Glenn, thank you. All of those measurement references are very helpful. Just to be clear, the masonry bell/bench should have 6" thick walls, and then 4" of air gap around the outside before combustible materials? It seems a bit awkward to have a 4" gap between a bench and the wall it is next to. Plus isn't there a risk of something falling back into that gap and not being seen by the person starting the fire?

Fox, the vortex stove is incredible! And it's nice and small. Can the exhaust gases from one of these run thru a masonry bench to create a RMH? Or would that end up slowing down and ruining the vortex effect?

Hi Jon, thete are quite a few 6” vortex stoves running through mass but most people choose a bell rather than a piped bench.

There is a huge long thread on the Pro Boards forum all about the vortex stove.

In my case I have been developing the tiny 4” version, the core measures 10” wide 20” high and 12” deep but it would be slightly bigger if made from brick.

I have one video where I connect the stove to a bell …….
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Code for masonry heaters allows a wing wall 4" thick to close the gap where the heater wall meets a combustible wall. The corner of a bell enclosure will be significantly cooler than a straight section of wall, because the heat from the inside of that corner has to spread out over 90 degrees and travel farther to reach the exterior. The top corner of a bell bench will have a similar condition. That is not explicitly addressed in code, but I believe you could close off the top gap with a grille of some sort; metal would certainly be safe. If you make channels under the bench connecting front and back, it will be easy for warm air to rise up from the rear gap and be replaced by cool floor air. Something as simple as a layer of bricks spaced a few inches apart with the bell floor sitting on them will give the circulation you want.

With a 4" batch box system, I don't think you need to make the walls and top more than 4 or 5 inches thick, just enough to be structurally stable. The cavity for free airflow purposes needs to be a minimum of 8" wide and about a foot tall, more is better. 2 1/2" bricks + 1/2" cement board + 12" air space + 4" bench top = 19" minimum height bench. You could probably sink the bench cavity a few inches into the floor, forgoing the channels below, and have a bigger flow volume and lower bench height. The floor of a bell does not get very hot, so you would not lose much to the earth. Peter van den Berg does not insulate the floors of his bells sitting on the ground.

Would this work if I build a 4" vortex core according to Fox's measurements, put it inside a metal file cabinet box, and then pipe it thru some kind of mass bench?

It might very well work but the 4” stove is very small so you cant expect to much from it, on the other hand the 6” version is a very powerful stove and is well supported on the other forum.
However Peters riser less  Batch box designs have been more reliably tested.

And remember that the batch box has been tested and found to scale well, so you could make a 5" version if you want a bit more oomph without building a full 6" firebox. It would make a few inches difference in length, so in tight conditions that could help.

Thanks guys. Your help is invaluable.

Is your concern that the 4" stove wouldn't put out enough heat to keep the space comfortable, or that it wouldn't create a powerful enough force to drive the system?

Does a standard file cabinet have thick enough steel to withstand the heat that comes out of the core?

I would think both are reasonable concerns. A 4" batch box would probably give enough heat for a well-insulated small space, while a 5" system would definitely be sufficient. I know that the 4" cyclone batch box at Wheaton Labs was able to get the uninsulated tiny cabin toasty through the next morning, but had to be fired for several hours the evening before.

A 4" system may not have enough power to establish 8-10 feet of horizontal run before rising up in the chimney. I would strongly advise locating a 4" batch box right next to the vertical chimney, even though the running airflow goes horizontal out and back, so that you can have a bypass to make startup quick and reliable.

The problem is not many people build 4” systems so it is difficult to know exactly what they are capable of.
In my case I am experimenting with the 4” vortex stove and I have just started on feeding a barrel and mass.
The stove can supply huge amounts of radiant heat for its size but I fear it will have limited use for a piped mass system!
So as a space heater it will work very well, as a mass heater, I am about to find out…..
However there are youtube videos showing a 5” vortex stove heating a large house?

So it sounds like one of these two options might be best:

1. Save space by sticking with the 4" system (same box measurements as in previous post), but do away with the bench. Have a bypass chimney which vents directly out the top, or one could close that off and use the "juicebox" chimney pulling cooler gases from below. I could stack a bunch more bricks around this file cabinet box to add more mass if desired.

2. Scale everything up 25% to a 5" system and run it thru a mass bench. Still keep the bypass for easier starts.

What do you guys think of these options?  Would the top of the file cabinet get hot enough to boil water?

This is an awfully long thread but the first few pages might help you decide https://donkey32.proboards.com/thread/703/vortex-stove

My version based on the 4” vortex, does deviate quite a bit from Trevs design and I am still developing it at the moment, there will be a new video on my channel later today showing my first results from running the stove through a barrel and mass….

You can see how I am getting on with the testing here….. https://permies.com/t/181437/vortex-rocket-cook-stove#1775571

Very cool, thanks Fox. I'm very inspired by you and am planning to try something very similar. I think I will put it in a steel drum like you do. I worry the file cabinet thing would be too thin, and also that the metal is often galvanized.

I'm thinking I'll try lining the barrel with bricks like in the "Minnie Mouse" design on these forums so I can have less clearance to the walls and to retain heat longer. Also a bypass to make the starts easier.

This should work right? Is there any problem with having a horizontal run in the chimney like this? probably about 4ft horizontal.

Well my own experiments are still ongoing so I cant really recommend what will or wont  work but as a rule horizontal chimney sections are ‘not’ a good idea.
Anything that changes a straight run will slow things down, elbows, bends or  especially horizontal sections should be avoided if at all possible. .

This is a wofati-like structure with soil on the roof, so I think I have to go horizontal with the exhaust at some point.  If I need to let more heat out of the chimney to speed it along I guess I can do that with the bypass.

Vermiculite is harder to find around here so maybe I'll go back to building the core from firebrick and I can use kiln shelves for the flat surfaces. I think I'll build a core like this, and then put it inside a 55 gallon drum.  What should I use to hold the core together so it doesn't just topple over?