RMH build Idea - one sided bell - that serves as a heated seat back

200 sq ft space/shop - small heater -  desire hot seat back and warm seat bench and good draft - simple design.  instead of using 4 sides to the bell -

Anyone think this would work ok?

Hi Carol;
After studying your drawings, it appears you are trying to combine a J-Tube design with batchbox features.
Although you can probably get this to work, I doubt it will reach a clean super hot efficient burn, however, I could be wrong.
At worst, it might stall while burning and smoke back into your home.

I suggest building a factory stock 6" J-Tube with a heated bench with a backrest.

Master Builder Peter Berg has a saying,  "Build a proven design and get proven results."

If you have the time, the place, and the bricks to play, then give your design a try and see if it roars!

I could be wrong, but the only add on that I took from batch box is the suggestion for the venturi. I dont see anything else taken from batch box design....As to having just one side of the bell active, I do realize that is unorthadox, but thought experiments seem to suggest it would work. To have the other 3 sides also open and working, as is done in standard brick bell arrangement shouldn't be much different ? but could be wrong.

You have a bend in the burn tunnel that will make the horizontal longer and more convoluted; I think that will make the burn and draft less rockety. I would suggest that it will work better if you simply make the feed directly at the end of the burn tunnel.

You show the sticks leaning back and trying to slide down the burn tunnel. From experience, the feed works better if the sticks are leaning forward so that air has to go between them to get into the burn tunnel, rather than being able to go over the top of the sticks and straight into the burn tunnel.

It may work okay to have the downdraft only on one side of the riser, but the space needs to be bigger or you will get friction slowing down the draft. As is, it is not any kind of bell but just a fast downward chimney. I would make the space there at least 8" wide in both directions, more would work better.

By the way, the red paver bricks will not be safe to use anywhere in the flame path or the top half of the bell. You need to use only firebricks there, anything else will not last and may crack quickly.

The thin bricks if not covered with several inches of cob will get unbearably hot at the top after a short burn. I have full thickness firebricks exposed at one side of the top of my bell, and after an hour or less of burning they are too hot to touch and hold safely.


Also, I see that you have the burn tunnel sized at 7" wide and 4 1/2" high. Recommended practice is to make the feed and burn tunnel square in cross section, or if that is not practical, taller than wide. Ash will build up on the floor, and a wide shallow profile will get choked quickly, while a taller profile can take a few inches of ash before it needs to be cleaned out. Note that the crooked burn tunnel layout will make it significantly harder to clean ash from the whole floor. You really don't want to have to open a cleanout for weekly ash clearing  and then reseal it.

thanks for the tips


... make the feed directly at the end of the burn tunnel.....the feed works better if the sticks are leaning forward ...
i can change that



....make the space there at least 8" wide in both directions...
can do that, but I initially chose the 3-4" to try and make that narrow to slow it down thinking that would make it more of a bell. but if it impedes flow, I guess it could be doubled in thickness on the back rest.



...The thin bricks..will get unbearably hot...
I assume you mean at the top horizontal cap. I can do two layers of firebrick there. And continue a bit with them on top end of the 'back' before transitioning to red clay


Also, I see that you have the burn tunnel sized at 7" wide and 4 1/2" high....
I can do two courses of fire bricks on their edge but that would be 9" high


...You really don't want to have to open a cleanout for weekly ash clearing  and then reseal it.

I was wondering how to put in those cleanouts without having sealed doors. I guess most people dont have any cleanouts in J tubes?  If not do people use a vaccuum hose at the front burn end? and never get into the bottom of the burn riser to clean I wonder?

carol othdac wrote:thanks for the tips


... make the feed directly at the end of the burn tunnel.....  

>> agreed make it straight. Easier to lite. Easier to clean.

...The thin bricks..will get unbearably hot...
I assume you mean at the top horizontal cap. I can do two layers of firebrick there. And continue a bit with them on top end of the 'back' before transitioning to red clay.    

>> I think all the thin bricks to the left of your pix will get to hot to touch.   Especially the top of the back rest.  That is the equivalent of the side of the barrel and the hot gasses are 1000°F at that point.  I expect the back rest and the top of the seat will need at least a second if not a third layer of bricks to make them cool enough to sit on.  But those will be simple to add after the first burn.   A very short fast burn could work as is.

I was wondering how to put in those cleanouts without having sealed doors. I guess most people dont have any cleanouts in J tubes?  If not do people use a vaccuum hose at the front burn end? and never get into the bottom of the burn riser to clean I wonder?

 

>>  On a 6inch J tube you can reach in the feed tube with a small scraper and get under the rocket.  This unit is much smaller I don't think my arm would fit but I suspect your hands are much smaller then mine.  Which brings up the size of the seat / bell.  I count 27 brick faces on the top and sides of the seat. 8x3.5x27=756 square inches.  Peter van den Berg has shown a good (max) size for the bell is 300x the system cross section.
https://batchrocket.eu/en/building#dimension
If your smallest choke point is 3x4=12 sqin you will be putting a lot more heat up the Chimney then necessary.  12x200=2400sqin will draw good and stay hot longer after the fire is out.  I recommend at least doubling the seat area.
 Also make sure there is no choke point at the top of the rocket where it turns to go down the seat back.  This is 2 back to back 90° turns and those always cause big restrictions to flow.  Tom

have seen his size chart. But it's a bit hard to follow what he's referring to.

the category 'box dimensions'..what box is that? the horizontal burn chamber im guessing. So if the internal opening of the heat riser is 6" for example, the horizontal burn chamber should be 9" wide x 13" tall and have a length of 17" ( he is calling length,  'depth' Im surmising.


the port dimensions as well? is that the same term as 'chimney' ? and does chimney mean the heat riser or is that the exit pipe out of the building? or neither

for a 6" heat riser diameter he's got port dimensions as approx 2x9x2, which doesnt quite make sense if that is referring the the building's exit chimny. or is
port referring to the open space above the heat riser between it and the cap?


I appreciate the info in your post as well, but didnt quite follow, as you may be bouncing between a couple different areas and am not sure where you transition from mentioning the ratio of seat to heat riser, and when you mention ratio of the thickness of the seat back to the diameter of the heat riser. A guess is that you are suggesting the seat be larger in internal volume. 2nd, that the seat back be much more internal volume? 12" 'thick? by the 7" width Im guessing.  If so, the fireplace is starting to get larger than I'd like for the small space, but it could be accomodated if it's needed..will continue to try and decipher the details..thanks

For cleaning a J-tube, a sardine or tuna can with an 8" or so handle attached works excellently to get to the back of the burn tunnel floor (in a straight standard J-tube). Mine is 7" square cross section (as the recommendation used to be for an 8" system) with a 24" long total floor.

The narrow cross section you have at the seat back will speed up the flow and cause significant friction, while choking total flow volume. You want a bell big and spacious so the hot gases can flow lazily and give up all of their heat to the walls and ceiling.

I meant the seat back especially when talking about getting hot. 1 1/4" thickness of firebrick will get too hot to touch on the seat back probably within 20 minutes of good burning. Only the inner layer needs to be firebrick, the rest can be common brick as it will not be directly exposed to the flame/heat.

Heat absorption in a bell is figured on the interior surface area (ISA) of walls and ceiling (not floor, and not the side of the riser). A 6" batch box wants about 57 square feet ISA, though a J-tube of the same size has a lower heating capacity and wants somewhere around 30 square feet ISA. So 3' x 10' of internal surface would use your heat output best. Smaller would throw away a lot of the heat in your wood.

For burn tunnel dimensions, you can lay a course of split firebrick flat and then the vertical ones, for a total height of 5 3/4" which is pretty close to 6". Or you could use full fb flat, which would give 7" height, and make the burn tunnel 6" wide, and leave an inch of ash on the floor to give a good cross section.


How do you plan to insulate the riser? You need to keep the riser inside as hot as possible for best efficiency, so the sides not exposed to the bell must be insulated. It wouldn't hurt to also insulate the side facing the bell. Do you have a source of clayey dirt? With that, you can make cob and cover the outside of the bricks with a couple inches or so that will hold more heat and keep the surface from being dangerously hot. You can also shape the seat and back to be comfortable.

"....(in a straight standard J-tube) ... 7" square cross section (as the recommendation used to be for an 8" system) with a 24" long total floor."

am not following - your horizontal burn chamber (you are call it 'the J tube I think. is 7x7x24   The recommendations for a 6 or 7" heat riser in previous posts is that I make it 7x12x24   I must be missing something there, or opinions differ on that point perhaps


"the seat back especially when talking about getting hot. 1 1/4" thickness of firebrick will get too hot to touch on the seat back probably within 20 minutes"

that is sort of the goal, to have a hot seat back, but one can always layer red clay over the seat back later no?

"Heat absorption in a bell is figured on the interior surface area (ISA) of walls and ceiling (not floor, and not the side of the riser). A 6" batch box wants about 57 square feet ISA, though a J-tube of the same size has a lower heating capacity and wants somewhere around 30 square feet ISA. So 3' x 10' of internal surface would use your heat output best"

3' x10'  is unclear what that is referring to - I assume we are talking the volume of the seat back - what you are referring to as the bell?  Am planning on upping that volume to 7" x 9"  (which is the width of the stove x thickness of two bricks on edge (9") which is also the 'top' measurement of the bell no?  . the length of the back is say 24" . not sure how to take those figures and calculate ISA and what 3 ft x 10ft is referring to




"How do you plan to insulate the riser? "
was thinking about vermiculite around the riser core held in by a thin sheet of aluminum wrapped around it?


"You can also shape the seat and back to be comfortable."
yes I was thinking of later building out the lower part for lower back support.






also, the peterberg chart seems to have been developed for batch box burners, and then given to RMH builders as a guide?

In any case, the  'port' dimensions is referring to  the opening at the end of the burn tunnel right where it turns up 90 degrees to the tower?  and if this is for batch box , it seems many have not adopted this for RMH'ers . the port of 2"x9" (assume 6" riser), is essentially the venturi idea no? and many RMHeaters are not using this and instead leaving the port the full size of the burn tunnel . The only discussion on venturis for RMH is from peterberg and his believ and trial suggests that the venturi (narrowed port) is not beneficial and likely problematic for RMH that use the non batch box/open ended feed tube....?

The J-tube is the whole combustion core, feed tube plus burn tunnel plus heat riser. The standard recommendation for proportions is 1:2:4 or 1:1.5:3 feed:burn tunnel:riser. The feed and burn tunnel should be the same, approximately square cross section, 6" x 6" for a 6" diameter system. The burn tunnel length is the total floor from near edge of feed to far edge of riser. Feed and riser height are measured from the burn tunnel floor. As 16" is a standard length for firewood and it is safest to have the wood completely inside the feed, length dimensions of 16":24":48" work well. The wood can stand up in the feed and a brick slid over to control the draft. I find that I want the top of the feed around 3/4 closed to still get plenty of draft. In case of emergency, you can close the top of the feed completely and stop airflow.

The riser can be 6" square or 6" diameter, both work about the same. Square is easier to build from brick, round is easier to build with ceramic fiber blanket. The lighter weight the internal riser is, the faster it will come up to temperature. A brick riser will stay hot and keep drafting while the fire burns down to coals.

Since you will have to wrap the riser with insulation, to keep the same outside dimension, you can make the seat and seat back space 12" wide inside giving 16" or more actual seat width. More would be better. Bell volume is irrelevant; what matters is the internal surface area. At 12" wide x 12" deep, you could have 4' of internal seat back, 2' of seat bottom, 5' of seat back height x 2, 2' of seat bottom sides x 2, 1' of seat bottom front, 2' of bell top above riser and seat back,  and 3' of upper bell sides around/above the riser, giving a total of 26 square feet of heat transfer surface inside after the riser. This contemplates making the bell enclosure around the riser and seat back 5' tall inside. It would be fairly close to the theoretical desired ISA of around 30 square feet, and you could make the bell top a foot taller if you have the space and get more heating from it.

For the seat, remember that if you made the internal space 16" high from the floor, the seat surface would end up maybe 20" above the floor. Also, you do not want the interior space directly on the concrete slab, but insulated from it, so the internal floor will end up at least 2-4" higher than slab level. So I would plan on a space under the seat of about 12" high giving a finished seat about 18-20" above the slab.

thanks so much. Trying to sort it out with the two bricks I have - a fire brick 4.5x9 and clay 4x8 . You lost me a bit wen totaling TSA but trying to do a more accurate to scale layout of approx bricks, and estimating at your figures I ended up with this diagram . The two variables that I can play with are the seat depth and the 'thickness' or the open space of the seat back. Currently have the seat depth at 24" and the seat back space at 8"

A picture being worth a thousand words...

Nobody has ever recommended that a J-tube burn tunnel be 6" x 12" cross section - maybe you were thinking of a batch box firebox dimension. That is a different animal and not relevant here. The feed tube for a 6" system should be 6" x 6" x whatever height you decide (I recommend 16").


For supporting the seat over the 12" wide clear space, you can use concrete pavers which can be gotten in 16" x 16" or 18" x 18" or some other sizes, by 2" thick. These will work fine in this relatively low heat location.

i see. 6x6 or some suggest adding another 1.5  (7.5) by laying a layer of brick on their face to accomodate for ash buildup but Im not certain on that or if I have enough fire bricks.  Also, I now see the horizontal burn tunnel is much shorter than I was thinking, as its measure of 18" includes the 6" inside the burn tower.

Today got the fireclay, sand, and some ceramic fiber. So am comfortable to start with that diagram. Will put a 3 to 1 mix of sand to fierclay as the minimal insulation layer on the concrete slab, then a layer of firebricks as the floor, and then build up and wrap the riser with ceramic, and the short horizontal tunnel top and sides. Thanks for all the help. The material cost for this little heater is around $650 if anyone is wondering. More than I thought it would be but then again, everything is 2 to 3x the cost these days.

firebrick $375
clay brick $100
fireclay 50 lb $50
Sand $25
Ceramic $50
Stove pipe $50

temperatures these days are around 40-50 F during the day and 25-30 at night. From searches, concrete is barely able to set at 40 but I dont know about sand/fireclay. Would like to build this now as opposed to spring however so am hoping that mix sets up. The natural temp in this little shop probably adds another 5-15 degrees from sun absorption as well during the day but that disappears at night.

If you have a circular saw and a masonry blade for it, you can cut some of the firebricks in half lengthwise to make the added height.

Clay-sand mortar doesn't set, it just dries. So as long as you keep it from freezing until it is dry, you will be fine. The  colder it is, the slower it will dry.

now that the great freeze of 2024 is over....Sifted out large particles. And did more research.

The problem we had is a perfect storm of a few variables, which on their own are not dealbreakers but when added together, caused failure

The thin firebricks and red pavers laid soldier style are weak
the firebricks are used
no wet saw to cut the firebricks nice. have to hammer them , and they are not meant to be
The sand had too large particles
the weather was too cold

I can rectify the last two

So am still on the fence of trying the weak  but proper sand/clay method, or shifting to a heat cement method. Has anyone successfully laid 1.25" brick in a soldier fashion, using sand/clay? The fact that I have not seen this done, let alone, successfully, is the main cause for concern

Carol,

I think you demand too much from the bricks. I would never lay even the full size bricks as sailors (vertical with the largest side exposed). If you are trying to do it with splits and on top of that they are used and it's not a perfect weather for drying clay mortar, you will only be frustrated.
The splits are used as replaceable cladding and not the structural medium, unless they would be laid as stretchers or headers.

I mistyped. I was laying them as shiners...but still, your comment probably applies..
The main section in question is the heat riser stack. ( 4 ft tall)
you are suggesting I lay them broad flat (strecher/header) ? If so, that will increase the brick count many times what it currently is....but yes, that would be stable....I may have to do that...

You could also rip the split brick and you would create something that could be called "split-soaps" and then lay them flat, but not having masonry saw it would be too messy.
I really recommend buying some saw or at least a good angle grinder with a diamond blade. It will open new horizons for you and you will be less limited by the material that you have. It will allow you to build what you want and not what the brick size dictates.

yes thought about it, Would have to rent a wetsaw ....doing it with concrete disc on angle grinder yields too much of the silica dust- are unhealthy and overly dusty - REALLY dusty. I wish I could buy split bricks as that is what I need...is a diamond blade for the angle grinder also super dusty? I wonder  if not it may be worth trying. It is alot of cutting though. Maybe I could get a big fan from behind and drive all the dust quickly away. I will think on it

I wonder which blade to get?  There are some for 'concrete' and some for 'ceramic stone'...

Am unable to find accurate info on my bricks but I believe they are ceramic. So ordered one of them....will try it !!





just mocked the riser up with flat laid firebricks. I dont think flat will work. It uses too many bricks, and makes the heater too large in size and mass for a 200 sq ft space.

Only option I see (if I dont custom cut them) is to lay them on edge and use heat cement..When the joints crack, hopefully the ceramic wrap will hold it together and the outer layer of clay bricks keep the escaping smoke contained....may have to try and limit the burn temps somehow to try and minimize that...Shouldn't need a ton of heat for this space...I dont know. Did not anticipate any of these issues

For my 14" masonry saws I use:
-regular segmented blade for cutting regular bricks and concrete dry
-turbo blade (segments are at the angle) to cut granite with water
-smooth edge to cut roof tiles, ceramic tiles, firebricks with water

For the 4.5" angle grinder I love to use this blade:

4" diamond blade with diamond face

I do quite a lot of brick cutting, sometimes carving and I consider this blade indispensable. It can cut and also grind with its flat face. Its light weight does not strain my muscles and it's free of vibration.
When cutting wet, dust is not an issue. Dry cutting should be done OUTSIDE. Wind or mask can help.

Am not certain, but seems to be going well. The diamond blade cuts like butter, but produces so much dust, even after soaking.

The little squaring cutoff jig helps keep things true and this really helped the strength when mortaring.

filtering out the large pebbles in the sand is key

and the temps are in upper 40's 50's which seems to help

Changed the mortar for the firebricks to 1 part sand, 1 part fireclay, 1/8 cement

for red clay, it is 1 part cement, 1 part sand, 1 part fireclay

Could use some opinions on this ambitious idea.. Regarding the seat back. Am thinking of making a plywood jig and pour in some concrete. The idea is to integrate the back and the top cap over the heat riser by making it a one piece 'L' shaped slab

Two reasons, one is because stretching firebricks over the top to form a cap seems difficult if not impossible without some creative support structures? and so by integrating the top with the back, this can be negated.

2nd benefit is that a lumbar can be more easily integrated.

3rd is that concrete is not as good an insulater compared with brick, so this will allow the back to get nice and hot, which is a top priority.

The only downside is that the top cap is concrete in this idea, and that is a very hot surface and may crack....So If I drop in some firebricks when I pour so that a percentage of the concrete slab has firebrick in it. Would this help minimize the cracking?  

Carol,

Even refractory concretes like to crack in such a setting. If you use a regular one it will crack for sure and the embedded bricks may make it even worse, because of different expansion ratios.
You could build "stairs" on the back support part and use t-bars (at least 3/16" steel) to support the bricks on top (and also behind the stairs). Put a little bit of refractory wool between bricks and the steel, to separate differently expanding materials
Then you could use cob with good amount of chopped straw to form the back of the seat, but this is not a good solution.
The proper one is to have double skinned bell at the top and then cut the red bricks precisely to lay them at the angle to create smooth seat back, but it's rather too late for this, because the bricks are laid as shiners from the bottom up and there is nothing to support double skin.

You said you are using 1:1:1 cement:fireclay:sand. It's much, much too rich. It's a waste of cement. Regular mason's sand has voids volume of around 2-2.5 times the volume of sand, so loading more cement will make it weaker. 1:3 ratio already creates a mortar almost as strong as concrete. Sometimes I use 1:1:3, cement:lime:sand for smooth and also very strong mortar to build cornices. I like to use some cement in clay mortar to accelerate drying, when it's not too warm outside and to stabilize the mortar, so it will be more water proof. I ever use more than 10%. Less cement = more flexible mortar.
Also I see few of vertical joints not being crossed by next course of bricks. In heater building it's very important to never do it, but it will work, at least for a while.

It's great that you are building, because you are gaining experience. If it cracks you will build better.
Good luck and keep posting images!

not sure what to make of that info....searched double skinned bell and only found info on double bells...i assume you are talking about the top cap. Lay one layer one direction and another on top the opposing?  to support individual bricks, I'd have to cement on some small pieces on the outside of the heat riser, at the top, to support the center of the bricks? and if so, that would mean high temp cement which I don't have, and also seems like that may even break lose there?  I don't know how to support the bricks on that top cap.

If I went with the concrete idea, no bricks inside....Let's assume it cracks...Couldn't I then add another layer of brick on top of that cap, if and when that happens?

Yes, I wish I had lime and high temp cement but only learned of them recently and am trying  to not put any more $ into this,  which is why I've tried to use what I have....for the rest of the build, it sounds like I should not use sand however and just use cement /clay, in a 1 to 10 ratio.... if I understood.


this is the only way I could think to support the top bricks. by resting one end on some cleats - shown in red - but if they would break off, even with heat cement, that may not last, and the top cap would cave in


To use metal - that may be ok, I may have some on hand..so the metal would rest on the red brick bell outside rim, (would it be cemented on?), then a layer of insulation on top, then a layer or red bricks?

Carol,

For the mortar I meant it is too rich in cement. The best would be, let's say 2-3 parts sand, 1 clay and 0.5 part cement max. You always need sand (grog) or refractory aggregate.

Peter Van Den Berg, batch box build
At 2:45 you will see the T-bars used as the support of the ceiling bricks. You could also use angle bars to support bricks on the seat back.

ok, I see that idea. seems quite involved and alot more bricks. Bricks to support the angle iron, bricks for layer one, then bricks for layer 2....Will think on that one..

would almost like to try a simpler idea. Could i just cut some 1" strips of insulation and set in around permimeter of bell, then lay a piece of metal on top of that. then a full layer of insulaton on top of the metal.

the lower insulation would keep the smoke from leaking, the metal is a one piece structural solution., and the top layer of insualtion would keep some heat from radiating through the metal?.. if needed, a layer of bricks just dry set on top of the insulation. Plus I could remove it all easily if necessary since its just held together by weight of the materials

The top is the hottest, so it's most critical to material selection and the workmanship.
I shared the video to show you the idea of t-bars. The simplest solution would be to get some angle bars or t-bars and use them wherever you have problems with spanning a large opening in your bell in the current setup. This way you will keep using red bricks all the way. You already use the grinder for cutting, so you can cut the red bricks in such a way that they will form smooth seat back without any stairs. If you find them later to be too hot, you will think about insulations.

seems like you are mentioning two situations. the top , then the lower back section?

regarding top and the use of angle supports....the question is how to attach..seems like heat cement will give way?  and if not, where would they be attached. to the outside rim of the heat riser I would think

the other area you mention (if I follow) is how to create a lumbar support contour...you are suggesting I angle cut red bricks? I can do that, but the concrete idea seemed more simple. Since the back itself is less hot than the top. maybe I could come up with one solution for the top, and still use a poured concrete back with lumbar for just the vertical back. In hopes the temps there would not cause a problem.....

how about concrete for lower half of back where temps are less, and firebricks on top of it?
or would the two different expansions there cause that seam to crack I'd guess....the reason for trying to use concrete idea is to minimize any more brick cuts than necessary. Already have many more to do just for the shell and seat etc. so to add more for custom angles for the back, may be more dust than i can tolerate...will think on these issues i guess

I would use a thick angle bar where the bottom of the seat back starts and go all the way with red bricks.
Try to interlock the sides of the bell with the seat back.
Then cap the top with more t-bars, firebricks on the steel and then red bricks on the top.
Please do not rest any part of the bell against the heat riser. It will break the bell, guaranteed.
I understand that cutting bricks with the angle grinder is not the most pleasant endeavour. Masonry wet saw is pure pleasure.

appreciate the ideas and help. It's not fully clear yet. Thinking out loud here for my own reference mainly......a main issue is that I dont have confidence in sand/fireclay as a structural solution. Am fine with it now for the inner sections but, for outer sections, and for sure, the back, I'd need a mortar to be fully structural on its own. So to use bricks there means mortar. Also, not sure how I'd support the angle iron if I used that to support top bricks, as the video seems to show that they needed tu run an entire layer of bricks all around the bell, to support that angle iron, and its too late and too many bricks in any case for my situation. Will look into other ways to support the iron though as it seems like a good idea. Also, regarding the top cap. I am wondering why the idea I presented of insulation/metal/insulation is not viable. It would also allow me to pop the metal top to clean or to prime the start fire, I'd think .......

for the back. If used bricks, I'd only be confident with high temp mortar and that is more $ so is not first option. . ...Will have to reread all the advice so far and do more searches because If I do metal supports, or lime mixes, or other ideas I havent researched yet, means sort of starting over on new ideas. I just found a nice piece of steel 20x16 I could use for the top cap, which also is a point in that ideas favor, as it is free.

I also am wondering why the idea to make the 'L' shaped, all concrete back/top wouldnt work, If I first span the top with a layer of fire bricks, (if I found a way to support them), then rest the top concrete section over that, at which point it should be fine for heat...the only area I'd wonder about is the upper back section. I dont know how much heat will be there or what concrete can take before issue..When I search 'temp , cured concrete, cracks etc'. all I find is info related to, immediately after the pour, and how fragile it is, seems that is cracks at 100 degrees or less !  Obviously that is not good, but after it cures for a few weeks or longer, I wonder what is the temp at which problems arise? If its over 150-200, it might be fine.

best i've found so far is this study - If am reading correct, 400F is a limit. Which is well above what the back should be seeing. but another quote below it may suggest otherwise. It wasn't clear in that article however what exactly they where referring to.

Carol,

I did not mean lime to be used in heater building. I was talking general about mortars. Lime mortar would be even less temperature stable than cement.

Your wall instability issues are caused by choosing to lay bricks as shiners. Shiner/soldiers/sailors are used mainly in decorative brickwork and will never be any substantial self supporting structure, no matter what. People were using just clay and sand for hundreds of years to build long lasting masonry heaters.

If you want you can use insulation/steel plate/insulation. I generally follow the idea that "metal is doomed" in heaters, but exceptions are doors and aforementioned angle or t-bars for support that would otherwise need a lot of bricks (corbeling) or unreliable large firebricks or even worse - cast in place refractory cement. Bricks are simply the best - the smaller they are, the better distribution of stresses. Basically the lower number of various materials used, the better. It applies to heaters and buildings.

I just realized that resting your back against unstable and heated structure may be not the best idea. Think about building it straight up with proper bonding.

In general, setting split firebrick in shiner configuration would be unstable, but a heat riser will have no outside disturbance and, if wrapped in any sort of insulation that is held in place from the outside, should be stable for the foreseeable future. It is a common design for brick J-tubes.

(Note that this applies to a riser that is inside a chamber - barrel, brick box, etc. If the riser is exposed on one or more sides, it will need mechanical protection as well as insulation.)

The heated seat back is another matter. Whether vertical or curved ergonomically, if it is wider inside than the brick length it will depend on the mortar strength to avoid collapse. If double-layered with staggered joints in both directions, it could be strong enough to be safe.

Temperature is also a concern, in that a single layer of any kind of brick beside insulating brick would get dangerously hot near the top within 15-30 minutes or so with a good fire going. I am talking about burns, not just discomfort. You really need to have a double layer at least.

went with brick and refract cement on the back and top bell.
still need to finish bench.
Pop the top and prime the riser with blowtorch for 10 seconds and she draws fast and clean so far, out the 4x8" cutout leading to the bench


since it draws so fast, thinking of creating some kind of  tunnel rout in the bench to slow it down.  There will be some bricks in the center that serve to support the bench seat bricks, so would want to incorporate them already being there, in the tunnel layout..not sure how to rout it though....the opening coming into the bench is already committed to, and is at center bottom of bench. Not committed yet, but would like to have wall exhaust outlet to outside, located at bottom of bench , to the far side, so that it doesnt interfere with human feet, which will be in the vicinity.....see pic at bottom. From these 3 givens, would need to devise some routing scheme to add on,

this is hard to draw. but here is the best idea so far as to how to slow things down in the bench without choking the draw too much.

the grey color in the sketch, is some aluminum diamond plate. The center bricks are two sailors tall. All but the end will be full height (2 sailors). The last will be just the lower brick. So the travel is:
Smoke comes in at right and immediately hits a angled set of bricks, forcing flow to the left side of the bench (full floor to seat flow). It then comes to the left end, and is forced on top of the back (right if looking from above) side of the bench. This side of the bench is divided in two by the diamond plate , which is sandwiched in between the two bricks down the line. Once at the other end if flows down into the last section and then out the bottom right - green circle.....Anyone think this is a reasonable plan? I should be able to dry mock it up and actually test it I'd think

I don't think you need to get that fancy. As you have found that it drafts strongly, all you need to do is limit the draft; the brick enclosure plus the central support spine will absorb all of the heat without adding a contorted path. You should have a cleanout access for the under-seat area, so you could just put a brick partially obstructing the exit, and adjust it until you get good flow without smokeback.

did a rough mockup. It did affect the draft....maybe it would work once it's all tight, but maybe the benefit of slowing it down that small amount is not worth the lessening of draft, so will leave it open for now...
here was the mockup


overall, it seems like this basic heater may work. Still need to finish.....the top metal plate gives some instant heat which is important..Also the horizontal burn tunnel is giving alot of quick heat. I may insulate that little section and put some bricks over it .

Am getting correct rocket draft to the chair seat, but getting back draft, unless I pull the metal seat a bit to create some air (2" ). So the current chimney must be the cause I guess. It's currently 4" dryer vent. Am waiting for 6" to arrive, but in the event that size difference is not the cause, am starting to envision other solutions...Currently am going out the bottom as per the original design, then it goes up 4 ft and takes another angle out the side wall. I know going through the roof is best, but would like to exhaust all options before I do that..so this is the next idea. Any chance the cause is because the current chimny is exiting at the end/bottom of the seat> and that moving it to the top of the seat would rectify the draw ?

the other idea and is something that can be changed/may have been mistake is the opening from the bell to the seat? I am not seeing that as the issue since it seems I'm getting nice rocket draft past that point. but since that detail was not figured in the original plan, am throwing that out there, as being something to change..see 2nd image

carol othdacReborn wrote:Am getting correct rocket draft to the chair seat, but getting back draft, unless I pull the metal seat a bit to create some air (2" ). So the current chimney must be the cause I guess. It's currently 4" dryer vent. Am waiting for 6" to arrive, but in the event that size difference is not the cause, am starting to envision other solutions...

So you are exhausting a 6" J-tube into a 4" chimney? The cross section area difference between these two is huge, 28.3 sq in. and 12.6 sq in. respectively. In other words: the carrying capacity of what you have now is less than half of what is required... A couple of bends and a bare steel pipe outside the house envelope could make matters a lot more worse again.

carol othdacReborn wrote:Currently am going out the bottom as per the original design, then it goes up 4 ft and takes another angle out the side wall. I know going through the roof is best, but would like to exhaust all options before I do that..so this is the next idea. Any chance the cause is because the current chimny is exiting at the end/bottom of the seat> and that moving it to the top of the seat would rectify the draw ?

It might, just might help a bit, but won't provide the necessary draw, I'm afraid.

carol othdacReborn wrote:the other idea and is something that can be changed/may have been mistake is the opening from the bell to the seat? I am not seeing that as the issue since it seems I'm getting nice rocket draft past that point. but since that detail was not figured in the original plan, am throwing that out there, as being something to change..see 2nd image.

A better approach to a bell system is to view the whole of it as one space, so the bench is actually an integral part of the stratification chamber. Which implicates that the  opening between bench and bell should be as wide and high as the inside of the bench. And I mean exactly, even a concrete lintel, to support the bell's wall, that is lower than the bench' seat is impairing the flow of gases.

I've learned all the above the hard way, won't make that kind of mistake again.