Questions about building my first J-tube RMH for a tiny house

Hello people on permies,

first of all: I purchased the book about building rocket mass heaters from Ianto Evans and Leslie Jackson (3rd edition) and read quite a lot in this book and in this great forum. But before I start building I have still some questions and I would be happy to hear some thoughts on this questions from people who have more experience.

My first question is: Is a rocket mass heater an adequate heating source for my tiny house in terms of space, building materials and the way I use this space?

Here is a short description of the space and my plans for getting an opinion on that question:

The floor of the house is concrete, a layer of polystyrene and a wooden floor above that. The walls are build with aerated concrete covered with lime plaster inside and outside (no additional insulation). The room inside measures 118 squarefoot (almost square shaped) with a ceiling height of 90". I use it mainly as sleeping and living room. Kitchen and bathroom is in a shared house nearby. In winter I heat it with a conventional wood stove a few hours a day, sometimes a hole day but it also happens quite often that I don't heat it for 2-3 days. Winter temperatures here (central germany) are usually between 25 °F and 50 °F infrequently it can be less than 14 or even 5 °F.

My plans on the design: Considering the space I have I thought building a relatively small 6" J-Tube design with a 30 gallon barell (riser formed with clay perlite mix inside) that (including a bench as piped mass) measures around 39" x 76". I would remove the wooden floor and the polystyrene underneath the rmh instead of rising it above the wooden floor. From my plans the combustion unit could have a 10,5" feed, a 20" burn tunnel and a 38" riser. The stovepipe inside the mass would have a length around 105", the exit pipe (the part inside the room) around 65". Since the place is just rented and I don't know how long I'll live there I want a design that is not too expensive. Materials that I get for free or cheap are bricks (some), sand, sandstone, clay and 20" x 20" concrete slabs. So I thought using this materials and spend around 300 € for 6" stainless steel stove pipe and maybe on a 30 gallon barrell on eBay if I don't get one at a junkyard or a car repair shop.

The benefit a RMH in my imagination could have would be: less firewood consumption, a cozy warm bench to sit on and more stored heat (so f.e. the space stays a little bit warm until next morning so I don't need to get up in a really cold room).

What do you think about that? If I get the impression that a RMH could be adequate I have some more detailled questions .

Thanks,

Jonas

Hi Jonas and welcome to Permies!
It sounds like you have a good idea in your mind as to how this would work in your space. A sketch showing the layout might throw up other issues. I'm not an expert, so hopefully others will chip in but I understand you have to be a bit careful using sand in the bench area, since it acts as an insulator rather than effective heat storage. According to rocket scientist Glenn here however loose fill pebbles can store heat and allow it to release so that might be an option for you.
Are you intending to use the existing wood stove flue? I'm assuming that is 6".
Hope this helps and good luck with your project.

Hi Jonas! Welcome to the RMH Forum,

I am going to be bias towards encouraging you to build one as I have loved building and operating several of my own.

Even though there have been innovations since Iantos book, I think that in keeping with your wanting to have it be an inexpensive build, I would follow most of your plans the way you described them. On thing I would change though is to eliminate the potentially expensive pipe for the bench and just make a bell instead - which is just a hollow chamber where the hot gasses can stratify more efficiently and with much less impact on draft. The shape is much more forgiving (especially for a small space) as it doesn't have to follow the straight lines set by the pipes.

Also, the 10.5" feed would mean that in order to keep the burning wood below the height of the feed for safety and potential smoke-back issues, your wood will need to be cut rather short. J-tubes tend to burn hot and fast so you'll be feeding it more often than with a taller feed tube and longer wood.
Not a deal breaker, just saying it upfront.

Pictures of your place where you are thinking of installing it and perhaps a drawing would also be helpful.

As you are already planning on a 6" system, I would advise making the J-tube dimensions larger so you can use longer firewood. Ideally you would make the feed tube the same depth as your standard firewood, but that might be a stretch. If your firewood is similar to the US standard of 16" long, proportions of 1:1.5:3 would give dimensions along the outside edges of 16", 24", and 48". This could fit inside a small masonry bell just as well as the tiny dimensions you mention. You would need a part of the bell to be at least 5' tall, but part could be bench height for one or two warm seats.

Wow, thanks for all the answers. I'm deeply grateful for your advices!

The old stovepipe is 4,5" and since I read, that RMH systems below 6" don't work properly I would install a new stovepipe and close the hole from the old one. It's located in an area that doesn't provide enough space for the RMH anyway. Thanks for the hint about using loose sand!

Attached you find 2 pictures. One that shows the position where the new stovepipe would be located on the outside of the house and one that shows the space where I want to install the RMH inside. The wardrobe on the left would leave and the bed would move around 11" away from the opposite wall. So the total space there could be 52" measured from the opposite wall to the bed (in the picture it is still 41") and around 77" from the wall on the left to the door. Because I want to keep enough distance to the bed I want to keep the combustion chamber size rather small. I think on monday I'll have some time to make some sketches and post them here. Maybe then it's easier to get an impression of how I imagine it.

About the depth of the feed and the size of firewood: In germany common measurements of firewood are 25 cm (9,84"), 33 cm (13") and 50 cm (19,69"). Since I cut my own wood I can choose any length, but the hint about the need of feeding it more often is a good point! But because I don't want the RMH to be closer to the bed than 13" (which would be the possible distance with the 20" burn Tunnel lenght) I can't really expand the burn Tunnel length. Which leads me to the following question: is it possible to increase just the depth of the feed and the riser and leave the tunnel 20" without loosing burning performance? I think in Iantos and Leslies book it is written that the burn tunnel should be as short as possible but the diameter of the barrel that sits on the riser and the length of the feed opening of course add up to a minimal possible length (around 20" in my case). So could dimensions of 15" by 20" by 45" work as well? Or I stick to your suggested 1:1,5:3 (instead of my original 1:2:4) and make it 13,33" by 20" by 40"?

About the bell: spending less on metal stovepipe and using more basic natural materials like cob and bricks instead sounds really good. I understand the idea behind bells and the principle of how they work but would need more information on how to construct them, important rules etc. Can you recommend a source where I get this kind of information?

Thank you and greetings from Germany!

Jonas

If a common firewood length in your area is 35cm, your suggested dimensions sound good to me. You don't have to orient the burn tunnel at right angles to the bench; that seems too close for comfort to the bed for my taste. You could point the burn tunnel about 45 degrees, directly away from the back corner, and have the feed almost within the outline of the bench.

Bell theory, design and construction is covered well at batchrocket.eu

I think 15":20":45" would work, but don't see a benefit to making the feed deeper than the firewood. 13.3":20":40" would most likely work well.

I have found with my 8" J-tube and bell that my temporary 6" stovepipe chimney (about 18' from firebox floor to top) draws fine. You may be able to use your existing 4.5" roof penetration, keeping everything before that to 6". Be prepared to seal that and go out the wall as you indicated if necessary.

Attached you find a sketch that I made today. I made it mostly to clarify dimensions and the rough shape of the hole RMH and for seeking the right position for the combustion chamber in it. So some things (especially the bell) are still poorly conceived.

The following questions came up when I drew it:

1. what do you think about the distance between outer barrell and wall (6") and exhaust pipe and wall (3")? Like I said the walls aerated concrete with lime plaster

2. to me it seems quite little space in the area where the hot gases enter the bell and where they leave. Especially if I want to install a bypass there, too. I thought maybe turning the combustion chamber around 60 ° instead of the 45 ° Glenn proposed leads to more space in that area

3. what do you think about the idea of the brick arch on the left of the combustion chamber? Do you think it is to close to the chamber? It would be nice to have some storage space near the feed.

4. As bell I imagine to build something similar to Marks construction described here: https://permies.com/t/154800/Completed-RMH-polished-lime-fresco what do you think about that?

5. what do you think how far should I remove the wooden floor around the combustion chamber so that the wood and polystyrene doesn't get to hot? I thought about quite some insulation underneath the chamber because the concrete under the floor is the foundation of the house and heating the foundation means losing heat in the earth underneath it, right?

Thanks a lot again!

Thanks for the detailed drawing. It allows for equally detailed advice. In general, I think you are on a good track.

The first detail I will mention is that it would be better if the feed and burn tunnel are both 15cm square rather than 15 x 12. It has long been advised by innovators that a burn tunnel should be taller than wide if it is not square. Ash will accumulate in use, and even a couple of cm will make a big difference in 5 cm of space. A modest bed of ash in the floor will help insulate, and it is not necessary to clean it until you get a noticeable buildup. The early idea that the cross section should remain exactly constant has been superseded by the observation that corners in a duct increase drag, and a square section is really about equivalent in flow capacity to the same diameter circle. Also, the feed tube will be largely filled with wood for most of a burn, so artificially restricting it is not helpful.

I think you are making the bell more complex than necessary. I would simply make the entire mass hollow with wall thickness as desired, and let the bottom of the barrel space be continuous with the main cavity.The stovepipe/chimney can drop in the cavity to something like 10 cm +- from the bottom. It would be good to put an obstacle between the hot entrance flow and the chimney exit, even just a sheetmetal deflector. A bypass would be most useful going from the side of the barrel directly to the chimney, and can be a size smaller than the main system as it is very short.

You will need a cleanout or two in the bell space. The end of a half-barrel bell is a convenient place, or it would be very good if you can put one next to the feed/burn tunnel so that you can reach the chimney connection.

Thanks again for your advices Glenn! I will alter the dimensions of the cross sections as suggested. I took the recomendation of using 5" x 6" as cross section from the book from Ianto and Leslie and like it was said before it seems a little outdated. So I think I will dive into some more recent theory and instructions about the build and post questions here if I keep having open questions.

I still got some time because I'm planning on having it finished when the next cold season starts (roughly october).

Hello again,

After reading quite something about bell theory and purchasing and reading in Ericas and Ernies Builder's Guide I altered the design a little bit and some new questions came up.

first the design changes: To get an ISA closer to the 5,3 m² that would be ideal for an 6" system according to batchrocket.eu I decided to change to 55 Gallon Barrels as bells (both: above the riser and in the bench) and because of the lack of horizontal space building a relatively high (170cm / 67") first bell with these over the riser, which should sum up to an ISA of 3,39 (side and top surfaces - inner diameter = 58,5 cm). The bell in the bench from my calculations could have an ISA around 1,34 so that would make a total ISA of 4,73. Attached you find some drawings showing the new design.

And my questions:
1. ISA: Is it correct that the surface area from the first bell around the riser counts as ISA as well as the surface of the bell in the bench does?
2. What do you think about this quite high first bell made of barrels? As shown in the drawing the riser doesn't reach close to the top because otherwise it wouldn't be possible to install the second barrell because of the ceiling height. So from my understanding I won't get a top surface that gets hot enough to cook on it but I don't want to do this anyhow. To store some more of the heat from this bell I would cover the outer surface pointing in directions of the walls with cob and a layer of perlite clay as insulation at the walls just leaving the blank metal in direction of the room and on the top of the barrell for radiation heat (see drawing aswell). To distribute the hot air above the top of the barrell I'd like to install an oven fan on the top (I already got one on my old heater)
3. What do you think about the idea of using a 1" slab of firebrick as material where the bridge of the burn tunnel rises to the top of the feed (see detail foto of the area on one of the drawings). Using normal brick her would make the burn tunnel around 2" longer and again ending closer to the bed which I try to avoid. I see 3 Options here: 1. using the 1" slab of firebrick that I got here from an pizza oven, 2. using normal brick and therefore ending up closer to the bed or 3. keeping the short tunnel length and using brick, which means that the lower barrel doesn't end with a clay sealing on the bridge of the burn tunnel but on the top of the raised "backwall" of the feed.I would be happy about opinions on that!
4. and of course any other hints and comments are very welcome !

Thank you and have a nice day!

The firebrick slab at the back of the feed tube sounds good to me, as does the partially cob-encased barrel. You would want that cob to be placed to allow the barrel(s) to be removed for maintenance. It needs to only go halfway around touching the barrel, though it can be as thick as you want near the barrel.

The lower partial barrel could be anchored and cobbed in place, as long as the upper barrel can be unclamped and removed for inspection.

Looks like you are getting lots of design help and feedback.  I’m glad for you.

I built my first rocket stove following the Evans and Jackson book, and a couple consultations with Ianto.  At the time, that was all there was!

I haven’t read every post as my time is limited this week!  Sorry if this has already been mentioned, but I want to make sure!

My first rocket had to be retrofitted with some rise in the exhaust.  I had put the exhaust horizontally, straight out the wall, and the wind was variable in that location, as opposed to some situations where the wind only ever comes from one direction.

Then Ianto recommended Ernie and Erica, saying he had “handed the torch” on to them.

Following their generous advice I put a riser on my exhaust, to get some draw, get it up higher than the roof.  It was an improvement for sure, but it was always iffy.  

I see you have access to some of E and E’s plans and publications, but still I want to say, you need some height to the exhaust, and I think it’s WAY better if the vertical is part of the original design, and gains as much height as possible inside the building.  Going horizontal to the exterior on the the same level as the burn channel then going vertical once it is outside creates plenty of difficulties.

Good luck!  

Jonas Fritzsche wrote:Hello people on permies,


Here is a short description of the space and my plans for getting an opinion on that question:

The floor of the house is concrete, a layer of polystyrene and a wooden floor above that. The walls are build with aerated concrete covered with lime plaster inside and outside (no additional insulation). The room inside measures 118 squarefoot (almost square shaped) with a ceiling height of 90". I use it mainly as sleeping and living room. Kitchen and bathroom is in a shared house nearby. In winter I heat it with a conventional wood stove a few hours a day, sometimes a hole day but it also happens quite often that I don't heat it for 2-3 days. Winter temperatures here (central germany) are usually between 25 °F and 50 °F infrequently it can be less than 14 or even 5 °F.

 

Thanks,

Jonas

Not to be Negative Nilly, but.....what is the R factor of your aerated concrete walls?
And you must have a fantastic relationship with the owner of the house, to undergo this project? (which ever way it goes)
I know of few landlords that would  allow such a internal structural change.  Just curious

I'll start by saying that using an open bell design for the bench and the extra high radiant chamber (or first bell as Jonas labels it) adds a couple of variables outside my grasp of RMH conventions, but I'm wondering about the idea of cobbing half of that radiant chamber up like the last design proposal. The rapid cooling of the gasses going down the outside of the riser is enhanced by radiating off the surface. Won't the cobbed surface be much slower to absorb heat than open air allowing it to radiate would be? Won't this slow the draw in the feed tube?

Again, my general understanding is based upon the single barrel radiant chamber into ~35'-45' of ducted mass designs featured in the E&E book. I've seen that stratification exhaust systems struggle with draw in those designs (even with ducts well short of 30'), but no idea how all of the extra open spaces being proposed here will affect the system...

First, Jonas is proposing about 1 1/2 barrels in height, so cobbing some of that still leaves a lot of radiator surface. Bells typically are mostly if not all masonry, aside from the shop versions designed for fast heating. The drastically lower drag from switching from ducts to bell means less boost is needed.

Stratification/bell systems do not have any significant horizontal ducting, and vice versa, so I am not sure what combination examples you are referring to.

I've had a bit of experience with the "juice box" systems at basecamp, in construction and operation. I've witnessed problems there, but no experience with the lowered exhaust pipe in a more open "bell" type chamber.

Thekla McDaniels wrote:Looks like you are getting lots of design help and feedback.  I’m glad for you.

I built my first rocket stove following the Evans and Jackson book, and a couple consultations with Ianto.  At the time, that was all there was!

I haven’t read every post as my time is limited this week!  Sorry if this has already been mentioned, but I want to make sure!

My first rocket had to be retrofitted with some rise in the exhaust.  I had put the exhaust horizontally, straight out the wall, and the wind was variable in that location, as opposed to some situations where the wind only ever comes from one direction.

Then Ianto recommended Ernie and Erica, saying he had “handed the torch” on to them.

Following their generous advice I put a riser on my exhaust, to get some draw, get it up higher than the roof.  It was an improvement for sure, but it was always iffy.  

I see you have access to some of E and E’s plans and publications, but still I want to say, you need some height to the exhaust, and I think it’s WAY better if the vertical is part of the original design, and gains as much height as possible inside the building.  Going horizontal to the exterior on the the same level as the burn channel then going vertical once it is outside creates plenty of difficulties.

Good luck!  

Thanks for sharing your concerns and experience! I know that a chimney works best when going just vertical and ending above the roof. Unfortunately my roof contains asbestos, so I'm not going to touch it or even cut a hole in it... So my plan is to rise the chimney close to the ceiling inside the room, let it leave through the wall (ideally with a slope of up to 45° upwards, not horizontally), and outside turning again to a vertical orientation and ending above the roof. So I hope that will work.

Scott Weinberg wrote:

Jonas Fritzsche wrote:Hello people on permies,


Here is a short description of the space and my plans for getting an opinion on that question:

The floor of the house is concrete, a layer of polystyrene and a wooden floor above that. The walls are build with aerated concrete covered with lime plaster inside and outside (no additional insulation). The room inside measures 118 squarefoot (almost square shaped) with a ceiling height of 90". I use it mainly as sleeping and living room. Kitchen and bathroom is in a shared house nearby. In winter I heat it with a conventional wood stove a few hours a day, sometimes a hole day but it also happens quite often that I don't heat it for 2-3 days. Winter temperatures here (central germany) are usually between 25 °F and 50 °F infrequently it can be less than 14 or even 5 °F.

 

Thanks,

Jonas

Not to be Negative Nilly, but.....what is the R factor of your aerated concrete walls?
And you must have a fantastic relationship with the owner of the house, to undergo this project? (which ever way it goes)
I know of few landlords that would  allow such a internal structural change.  Just curious

I can't find an r-value for aerated concrete blocks and since I din't build this cabin I also don't know the manufacturer to ask about the insulation values of the material. But I know, that modern aerated concrete have quite okay insulation values. But from my experience of living there since 3 years and heating it with a conventional woodstove I can say that it is not that well insulated but possible to get it cozy even at -10°C outside with at least some hours of storing heat inside. And about the landlord: This small building is located on a big property with a main building with 3 floors. So he don't care that much for this small cabin. He actually don't care too much at all as long as he gets his money ...

Glenn Herbert wrote:First, Jonas is proposing about 1 1/2 barrels in height, so cobbing some of that still leaves a lot of radiator surface. Bells typically are mostly if not all masonry, aside from the shop versions designed for fast heating. The drastically lower drag from switching from ducts to bell means less boost is needed.

Stratification/bell systems do not have any significant horizontal ducting, and vice versa, so I am not sure what combination examples you are referring to.

Thanks about the hints how to use the cob around the barrels but keeping the possibility to remove the upper barrel for maintenance. cobbing the lower barrel and "surrounding" the upper barrel just as much that it is still possible to remove it (and not letting the cob stick/"glue" to it while wet) sounds good to me. And since it's almost 2 barrels (170 cm instead of around 180 cm, which would be the full length of two) I also hope to get a good ratio of radiation surface and surface that absorbs and stores heat.

By the way: I decided to really build it this summer and at the moment I'm gathering material, test cutting bricks, drawing more detailed plans of some sections and things like this

I got two more questions:

1. Should there be a layer of compacted rubble between the concrete foundation slab and the perlite-clay layer under the RMH? Ernie and Erica write in the builders guide that they almost always start with such a layer to protect the cob from damp. The cabin is located in a forest in humid climate. Or does the perlite-clay layer (that I'll put under the whole structure anyhow for not losing too much heat into the foundation) act as a drainage aswell? And if a layer of compacted rubble is recommended: would a layer of around 3/4" sufficient?

2. About the term and translation of "fireclay": Sold as powder in bags I did't find a german equivalent, only wet masses of pottery clay like this one: https://www.gerstaecker.de/gerstaecker-tonmasse-weiss-fein-schamottiert.html . So I need some clarification about the term to get the right material. Does fireclay mean pottery clay with a high alumina content (= tempered with fired clay but almost pure clay without sand or silt)? So far my understanding of the topic is the following: If you have a pure clay and you temper it with fired clay, then you get a more heat resistant high alumina content material (firebrick is based on that principle?). Is that right? Assumed that this is correct I would use this material as follows: a mixture of clay slip from this kind of pottery clay and perlite (without any further addition of sand) for the perlite-clay riser and insulation (recipe from the builders guide). For the clay-mortar between the bricks of the firebox I would use the same pottery clay mixed with sand (in a tested ratio that doesn't crack). For the cob structure of the bench and all around the insulated firebox I would use a cob mixed from a local clay rich soil, because this material doesn't need to be that high heat resistant. Does that sound good? In Germany we have seperate words for a clay rich soil (= "Lehm") and pure clay respectively pottery clay (= "Ton") but in english both is often referred to as "clay" so I'm a little confused about what is actually meant...

Thanks go out to this nice community again !

"Fireclay" does not mean that it has been fired or contains fired components. It is a high-heat-resistant type of clay which typically will not expand significantly when heated (thus not putting stress on neighboring materials). It will be high in alumina (around 30%). Firebricks are made from generally pure fireclay.

If you cannot get "fireclay" from a supplier, pottery clay like porcelain or stoneware should be fine for your purposes. Pottery clays often have grog (particles of ground-up fired clay) in them for wet strength or thermal shock resistance or appearance; you would want as little grog as possible.

Your usage ideas sound correct to me.

Now I actually found a bagged product from a german manufacturer. Its a fine clay mortar for oven building (grain size 0-1mm but mostly <0,3mm) that can be used for a layer thickness between 2-8mm (around 1/13" to 1/3") and is rated for 2200 °F. It is this one: https://www.primus-ofenshop.com/produkt/universalmoertel-plus-keramisch-sack-25-kg?_gl=1*jntsc5*_up*MQ..&gclid=EAIaIQobChMIntXY8aSJ_gIVhM1RCh3FHAaPEAQYCCABEgKSGvD_BwE

Infos about the manufactuerer from Wikipedia: Wolfshöher Tonwerke GmbH & Co. KG is a manufacturer of refractory fireclay for craft stove construction, ceramic chimney inner tubes and shaped bricks for heating inserts

So I think I will use it in its thicker consistency as mortar for the bricks but also mix a clay slip from it for the construction of the riser.

And are there any opinions on my first question?

Jonas Fritzsche wrote:

1. Should there be a layer of compacted rubble between the concrete foundation slab and the perlite-clay layer under the RMH? Ernie and Erica write in the builders guide that they almost always start with such a layer to protect the cob from damp. The cabin is located in a forest in humid climate. Or does the perlite-clay layer (that I'll put under the whole structure anyhow for not losing too much heat into the foundation) act as a drainage aswell? And if a layer of compacted rubble is recommended: would a layer of around 3/4" sufficient?

Jonas Fritzsche wrote:1. Should there be a layer of compacted rubble between the concrete foundation slab and the perlite-clay layer under the RMH? Ernie and Erica write in the builders guide that they almost always start with such a layer to protect the cob from damp. The cabin is located in a forest in humid climate. Or does the perlite-clay layer (that I'll put under the whole structure anyhow for not losing too much heat into the foundation) act as a drainage aswell? And if a layer of compacted rubble is recommended: would a layer of around 3/4" sufficient?

I'm thinking this is mostly about your analysis of your own build site. Wouldn't expect measurable amounts of moisture to come from the RMH in any form of condensation. How is the roof and foundation? Any reason to think precipitation could cause a problem in that area?

Thank you for you answer Coydon.

Water from the roof can't enter but I'm not sure wether the concrete foundation slab stays dry in our humid climate respectively I don't know how it was built and if it has a good drainage. At the moment there is a sheet of plastic between wooden floor and the polystyrene floor insulation on top of the foundation.

So I think I would like to have such a drainage layer just to be sure. I was just hesitating because I thought that it might be an unstable base for the heater because its loose material. On the other hand: if it is well compacted so that it can't move it should work? I once built a rubble trench as foundation of a strawbale-building, so I have some experience but the difference is that in case of the rmh it would just be a thin layer. I'm lacking of more detailled information or experience on such a layer and don't want to mess up the whole project with this first step... So any experience if rubble can be a stable base (under both: the bench and the firebox) and how thick this layer should be will be welcome

I have no building experience on this yet, but having looked into such things some I've worked out the need to focus on material specifics. You will have better luck dry stacking rocks than stones. When paving with bricks you want to use mason sand, not beach (playground?) sand. Sharp things end up more stable than round ones. If I'm reading the situation right you need to use gravel, not pebbles, but I'm not sure how nuanced the distinctions get...

I would say crushed gravel, which is angular and will lock together, is more stable than round pebbles. Of course, if the whole thing is contained so that it can't ooze out to the sides or sink into the ground, any stone material would work fine for this relatively minor load. I think all you really need for damp isolation would be one layer of fairly uniformly sized material, and a sheet of cement board or something equally rigid and noncombustible to keep perlite from sifting down between pieces of gravel.

Thanks again!

I purchased 6" stove pipe and now got the question where to plan the connection of the pipe for the bypass to the radiant chamber. Glenn wrote "at the side of the Bell". But which height? I suppose that closer to the top oft the radiant chamber respectively the same height where the riser ends means better draft in cold starting conditions, right? But since I want the upper barrell to be removable a connection there would make it necessary to remove this connection for maintenance aswell. So I would prefer to place the bypass connection right at the end of the first barrell which will be in a fixed (cobbed in) position anyhow. So the hot Gases would need to sink down around 16" from the top of the riser to the middle of the pipe. See drawing attached. Do you think that will work?

I tried to find out where other builders placed this connection. If I understood it right Thomas rubino f.e. placed the connection on one of bis builds quite at the bottom oft the radiant chamber and said that it works good. Its described in this post: https://permies.com/t/159700/Retrofitting-Piped-Mass-Bypass-Install . I think the last pictures show the radiant chamber at the left of the pictures.

The chimney will rise outside above roof level after leaving through the wall and end at a height of around 11,5 ft. See the attached second drawing of the chimney planning.

Obviously a bypass would work best coming from the top of the barrel, but I think your location on the middle side would work fine.

Is there a reason why the perlite riser shouldn't have a thin layer of fireclay mortar on the inside to prevent it better from crumbling in this area and for a smoother and more long lasting shape? The mortar I would use is rated for 2370°F. On the outside I will use a metal cylinder so crumbling on the outside won't be a problem. Someone got experience with that? From my understanding that layer would'nt absorb much heat if it has a thickness oft just 2mm or something like that?

Today I formed a little riser to test the building technique and to find the right mixture. It seemed to work but It's still enclosed in the cylinders I used to form it so I'll see the final result when removing the cylinders. I could test the idea of coating the riser with fireclay mortar with this small scale model.

And I found out that the ready mixed fireclay mortar doesn't work as clay slip for the perlite mixture since it contains too much sand and therefore isn't sticky enough. So I used a clay slip from fine stoneware pottery clay instead

When people talk about fireclay and perlite, they generally mean powdered fireclay which is as fine as dust. Stoneware clay slip (especially finer material with little grog) should work well with perlite. I would make sure it is cone 10 stoneware (typical gas firing grade) rather than cone 6 (typical electric firing grade).

From my experience, removing the internal form from a perlite-clay riser would disturb and crumble the inner surface. Burning out the liner would work much better, and would start to set the clay component. A thin clay coating on the inside of the riser would probably be good for further stabilizing it. You would want it sloppy so it soaks into the perlite-clay rather than remaining as a discrete layer on the surface.

Thanks! Yes, it's Cone 10 and I also thought that a quite liquid consistency would be good for the inner coating. Maybe even as liquid that it would be possible to lay the riser on the side (after the liner is burned out or removed in another way), splash a line of the mixture and cast it on the surface by rolling it. The dried perlite clay will soak a lot of water I think so it shouldn't remain that liquid for long and will possibly build a more stable connection to the perlite clay as you suggested also. I will see and test it with my small scale version.

Since I have an electric kiln at home, I bisque fired the dried perlite-clay riser with its stovepipe outer form and cardboard inner form, and got the whole thing decently sturdy for installation. Just burning out the liner would work, though it would not do anything to strengthen most of the material.

I finally build my perlite clay riser today. I decided to form an inner tube from cardboard stabilized by rings of plywood, so i can burn out this whole inner structure when dried. For the outside i used a sheet metal container (a ventilation duct) srewed in plywood as bottom for the perlite-clay. To keep the inner tube centered I added a form of plywood on the top that was removed when I got close to the top with the perlite-clay. Attached you find some pictures.

I found your ideas for working through the practicalities and their visual representations/ drawings/ quite informative.

Since it's already a few months past, at the onset of winter, did you get around the complete the project?
Have you found a supplier for the bypass valve/unit in EU ?

Hello Linden,

thanks ! Yes, the heater is working. It's not finished yet because a lot of the bench shaping and adding more mass still needs to be done, but the performance is already quite impressive to me. The bypass damper I finally build by myself because I have access to a good metal workshop and I developed a kind of construction that I considered easy enough to build on my own.

Attached you find some pictures of how the whole RMH looks at the moment. I will add some pictures from the bypass damper as soon as I have them on my laptop.

Great to hear that it is working well for you! The drawings and pictures could be very useful for anyone else who wants to build a similar system.