Lightweight stove for a small mountain hut

[bombadyl]

Hi there ! And happy/healthy new year to you all !

I have a small hut in the French Pyrenees which I would like to equip with an efficient wood burner, mostly for heating but also occasionally for cooking.

The main challenge here is the transportation of building materials : there is a 45min climb on a tricky mountain trail to reach the place (no wheeled vehicle of any kind, only by foot or possibly horse).
So i need to keep the total weight of building materials as low as possible (Let's say max 100kg or 150Kg which can be divided in 15Kg loads for multiple trips).

So I think I can forget about the "mass" heating part and focus on radiating as much heat as possible using barels.

For the core, my idea was to use a 4" (100mm) batchbox made of refractory concrete cast core.
I started from the plans for a straight cast core in 5 parts from Peter's website, and scaled it down to a 4" version. I have not done the exact maths yet but each part will probably weight less than 10Kg.



I made 2 dimensional changes to better suit my purpose:
- reduce the heat riser from 10 x base (as in spreadsheet) to 9 x base (Peter states it should be between 8 and 10 times the base). => This is to minimize overall height to 1.20 meters to give me a chance to cook on the top of the barrel

- increase the box length from 4 x base to 5.5 x base (Peter states in should be between 4 and 5.5 times the base). => This is to increase the length of wood pieces I can burn.

I have also made some changes to try and simplify the casting. If you see big mistakes please shout ! Maybe the riser was cut in 2 parts in the other direction for a good reason...

Then my plan is simply to fit a full barrel on top of the core as per the picture below :



The heat exchange surface of this bell is smaller (1.72m²) than the max recommended (2.36m²) but I guess the only consequence would be "wasting" a bit of energy because the exhaust gasses will be hotter than necessary ? Hence reducing the overall efficiency but I guess it would still be much more efficient than a standard fireplace.

The top of the barrel would be at approx 1.2m from the ground which is a bit too high but still manageable for cooking occasionally.

But before I go further into building this, I would really appreciate getting feedback from you on this very basic setup !

My main concern would be: is the distance between the top of the riser and the top of the bell sufficient ?

It would be approx 52cm which is much bigger than the recommended 2 x riser diameter, but I fear this recommendation is only based on air flow considerations, assuming the top of the bell is made of refractory material. What will happen to my metallic barrel ? Will it melt ? Corrode hell fast ? In Peter's workshop setup (the tower made of 3 barrels) It seems there is approx 1m between the top of the riser and the top of the tower. And he says the temperature at the top is over 200°C. But it was for a more powerful batchbox (6"). Picture and scheme from Peter's stove below, which I am basically trying to make a smaller (less tall) version of:

The heater will be placed in a room with a ceiling made of wood at approx 2.2m high (so 1m above the top of the bell). Clearly I do not want the dragon to melt the barrel and burn the ceiling  .

Also, the existing chimney is made of a 125mm diameter pipe. So I guess it is fine for a 100mm batchbox ? It should work as long as the exhaust pipe is equal or larger than the riser diameter, correct ?

For the exit at the bottom of the barrel, I plan on using a "T" shape piece of 150mm diameter, then a adapter to match the existing 125mm pipe. The other side of the "T" would be closed and used only before starting a fire to pre-heat the chimney (with a candle or something else...).

Any idea or past experience to share with me ?  Many thanks in advance for your comments, advice, warnings, anything ...

[fuegos]

Hello Bombadyl nice to see another member in the Pyrenees , i'm on the other side . It seems like you have done a lot of research & pretty much know all the answers to you own questions. As for casting i think that the conventional opinion on here is that it has no real advantage and a ceramic fibre board or firebrick build is "better" . In your case i guess the idea of transporting a pre cast to site is attractive but you have to take into consideration the time invested in making moulds & the added complication/expense of a refractory mix , building a vibrating table etc.As for the other questions "Also, the existing chimney is made of a 125mm diameter pipe. So I guess it is fine for a 100mm batchbox ? It should work as long as the exhaust pipe is equal or larger than the riser diameter, correct ? " going bigger on the exhaust is fine , and i'm sure others will be along soon to comment.Good luck and keep us posted.

[bombadyl]

Hello fuegos and many thanks for your comments !
About ceramic fiber boards, do you know where to buy (shops or online) in this part of europe ? I have seen many sellers in the U.S but not much around here...
Then what do you use to assemble the boards ? some refractory cement or just "dry" assembly ?

Do you have any advice about my main concern : will the top of the barrel survive so close to the top of the riser ?

Cheers

[fuegos]

I can't comment on the riser/barrel distance question my friend or about where to buy CFB in Europe. Hopefully other members of the board will be along soon to help out.But your plans seem like there're almost complete , just a few details.

[lsch]

no problem of barrel holding (you will have at most 400 degrees on the surface).
the mass, if you want some, you will find some on the spot (pebbles piled up around the barrel will do the job).
you can have only 30 cm between the top of the inner chimney and the top of the barrel (so you can extend the chimney to have 10*the base)
the ceramic panel is not adapted to the box, too fragile, but may be suitable for the internal chimney.
the refractory bricks are in my opinion the best solution for the box.
a construction proposal could be to prefabricate a metal box in which the bricks come to assemble (dry,no need for gasket since the metal box makes the sealing).

Translated with www.DeepL.com/Translator (free version)

[bombadyl]

Many thanks for your replies.

It is good to know that the top of the barrel will not melt

The pre-built metal box is a good idea ! I will keep it as backup if I fail miserably with casting a core
I have made already a small vibration table using an old car tire and a motor with eccentric so I will give a try to the casting first.

I am also thinking about replacing the heavy riser made of refractory concrete by a simple steel tube of 150mm diameter with a 25mm ceramic blanket INSIDE to make a 100mm riser.
I have seen this principle in the video posted here: donkey32.proboards.com/post/33732/thread and i find it very practical in my situation where mass matters !
Any feedback on it ? Only good for a few fires or can it be a long lasting solution (only approx 30 fires / winter in my case) ?

Also if someone as ideas on how to assemble nicely 2 cut barrels of same diameter together ? Peter uses aluminium tape in his example for the 3 barrels tower, but are there solutions that "look" nicer without involving welding (it has to be made on site so only 12V solar power available) ?

Thanks !

[pigbuttons]

Advice:
1) make sure the bottom of the outlet from the barrel, the 150mm T fitting in your description, is at least one radius from the floor of the barrel. If not the air will not flow smoothly into the T and you will always have trouble getting a proper draft through the stove.

2) casting using refractory cement is better done, in my opinion anyway ( because that's the way the instructions for the refractory I use are written ), by ramming than by casting on a vibration table. It may require you to rethink the design of your forms but you can keep the excess water out of the mix and therefore reduce the chance of cracking. I just use a couple of steel rods, the large one is 16mm and the small one is about 10mm to get into the corners. My casting is going strong after two years of very hot fires. Also, this would allow you to carry all the materials to your cabin and cast it there instead of trying to carry your casting up without breaking it.

3) the ceramic fiber riser is a great option. 25mm will work if that is all you can get, but here in the US we also can get 1/2" which is about 13mm. It makes it easier to wrap close to the inside of the pipe and the seams of the first layer and the second layer can be offset so as to prevent leaks and again make the inside more of a true cylinder instead of sort of egg shaped tube.

4) to join the two barrels, you can of course cut the top off of one and he bottom of the other. Since I don't know the construction of the barrels it is hard to give precise instructions but I give you the general idea. If there is a strong seam on the top of the bottom barrel that protrudes from the side, cut the top out leaving the seam in place. Then cut the bottom of the top barrel removing the seam. Cut narrow, inverted, "V" shaped slots all the way around the bottom of the top barrel making the Vs about 30mm tall. Now you can bend the remaining tabs inward slightly so that they will fit inside of the top of the bottom barrel. Place a board on top of the top barrel and drive the top barrel into the bottom barrel as for as you can with whatever is the heaviest hammer you have. Drill the tabs and the bottom barrel and rivet them together. Then use fireplace caulk to caulk the seam between the barrels.

[bombadyl]

Thank you very much for your valuable advice !

1) Distance between floor and outlet : I will follow this rule and add a few extra cms to be on the safe side.

2) casting refractory cement by ramming : Excellent, i will try that. As I already have the vibration table, do you think there is an added value in combining both methods ? Add a first layer, vibrate a few seconds, then ram with the rods, and repeat for the next layer ?

3) ceramic fiber riser : definitely adopted then ! I did not manage to find 13mm thickness so far but i will keep looking.

4) Joining the two barrels : Thank you very much for this method, it is clever ! In the meantime I managed to find a metallic clamp that is used to close the top of barrels with a full opening. Unfortunately my barrels are slightly smaller in diameter (58cm instead of 59.5cm for full opening ones). But I guess I can cut some length from the clamp and solder it to match my barrels. This would have the advantage of easy opening for inspection/repairs. If I do not succeed shortening the clamp, i will definitely try your method !

Also I have found easily procurable vermiculite boards and I think it could save me one casting (the top part) , save mass, and simplify the interface with the ceramic fiber riser. With only 2 parts to cast, I would only need one 25Kg bag of refractory concrete instead of 2.

The boards i found are high density (1,2 instead of 0,6 for standard vermiculite boards) so they should be a little more resistant. They are given for 1100°C max.

Below are some pictures of the updated design with the vermiculite board on top:

And here is the picture of how the mould should look like:

[pigbuttons]

I'm so glad that you are making such good progress, it is looking very good.

I used Kast-o-lite 30 and the instructions on the company's web site gave a weight of water to weight of the bag of mix. After mixing in that amount of water the mix seemed way too dry, but once I started ramming it in place it held together just fine. Some people get "castable" refractory that comes in a bucket and is pre-mixed and is not quite 'pourable', almost like wet clay for pottery. If that is what you have then vibration may help. But if you take your time and layer in the properly moistened mix about 20mm at a time, ram it thoroughly, add 20mm more, ram it, until full, you will have the strongest, most durable product possible without vibrating it. Excess water is not your friend.

I like the idea of the vermiculite boards for the roof of the box. Are they still somewhat flexible or are the completely rigid?

Rather than trying to shorten the clamp for the barrels, I would recommend putting a sheet metal shim between the clamp and the lip of the barrel. I don't think cutting and soldering/welding is going to work well with the expansion rate of the barrels and the stresses that the clamp is under when closed. You can hammer the sheet metal to the shape of the barrel lip and it should fit snugly enough for the low pressure inside so as not to leak. 

[bombadyl]

Thanks for your encouragement and advice !

I have ordered a bag of refractory concrete but I have not received it yet (could not find the correct material in shops around).

But it is not pre-mixed, they recommend 15% water so 3.75L for a 25Kg bag. They say that before casting you should make a "ball test" to check if the quantity of water is correct : make a small ball in your hands and launch it about 30cm high. Let it fall on your flat hand. If the ball breaks or cracks, it is too dry. If the ball flattens it is too wet. Ideally the ball should remain almost spherical. Have you heard about this test before ?

About vermiculite boards, from what I understood they are supposed to be completely rigid. Still waiting to receive it but I will confirm as soon as I have it in my hands.

I like the idea of the sheet metal for joining the barrels. The only one which is flexible enough and easily available would be the cut parts from the bottom of the barrel itself. I will try to cut small bands and hammer them to the shape of the barrel lips.

The clamp itself however was really too loose (more than 1cm gap on diameter => too many sheet metal too compensate !).

So I still needed to shorten it but I have used rivets instead of welding. It seems at least as solid as the old rivets already used on the lock mechanism. I have adjusted the length so that the tension is just enough for a firm grip. But I will still need the sheet metal to hold the 2 barrels together because the clamp is wider than the lips, so the 2 barrels can still move by a few mm when pushed.

In the meantime i have started the build of one mould. The sides are cut from 2mm thick synthetic glass (using utility knife for clean and precise cuts). The filler is made of a 60mm thick extruded polystyrene foam board. And I have also used a spare piece of BA13 plasterboard underneath because I needed a total thickness of 73mm for the big flat part (inner side wall of the firebox).

[bombadyl]

I forgot to ask : for the refractory casting, it says you only have 15min to use it once it is mixed with water. It seems to me very short to properly pour it and ram carefully in 20mm layers, especially with the complexity of the shapes. Is is fine to mix only small quantities several times for the same casting ? Or is it preferable to have the full quantity mixed at the same time so that it starts drying also at the same time ?

[fuegos]

it's looking good .Hopefully someone will be along soon to comment on the refractory cement question.

[martyn]

In an ideal world you would have a vibrating table, it takes but seconds to vibrate the cement into a one piece void and air free casting.
You really need some form of mechanical vibration to get a good casting, you can scan youtube for inspiration ....
A cheap obital sander might do it?
I cast with refacrory quite regularly and yes it will set very quickly and get very hard in 24 hours.
For a more guaranteed long term result you can add plastic or nylon burn out fibres to the mix (your product may already contain some) they will aid moister escape when you heat the cement.
Moister is the main reason that fresh cement will crack when heated as it will boil and expand forming cracks.
Another additive is stainless steel needles that can add reinforcing and further prevent longterm cracking (your product may already contain some)
There will be a specified amount of water that needs to be added to the mix, you will find the mix is dry and not very pliable but it is important to not add to much water!
Of course you can just fill you mould and pock it with a stick but it will not be easy if at all possible to form a air free result.

[Vortex]

Molds have to be quite strong as the castable is very dense, yours don't look like they could take the weight. Also they have to be watertight, as otherwise they leak the water and dissolved cement part leaving the grog behind and you get crumbly corners and edges. You can use the top of a washing machine on spin as a vibrating table, someone posted a video of that here once.

[bombadyl]

I do have a sort of vibrating table : there is a motor with excentric bolted under the wood board in the middle of the car tyre you can see on my pictures.
But pigbuttons was suggesting I would get better results without vibrating, only ramming.
I guess I will try a combination of both !
Any feedback on the 15min usable time ? Is it fine to make 2 or 3 mix within an hour for the same casted part ?

The concrete I bought already contains small fibers made of SECAR 71 ( ? ). They claim it helps getting the water out when drying so it matches your description.

You are right, my mould is not the strongest... The translucid boards are quite flexible in the middle so my plan was to add an outer frame.
For the inside, i hope the extruded polystyrène will resist the weight.
I plan to add cheap acrylic joint to fill the small gaps.
Do you think I shall paint the extruded polystyrène to make it waterproof ? I was hoping the mineral oil for unmolding would be sufficient.