Convert wood stove to rocket bell (& mass) heater, simply

After reading a lot, making a couple test J-tubes out of red brick then dense firebrick, making several iterations of plans on paper and in my computer, I've come up with what I think will be a simple way to convert my wood stove to a rocket mass heater, and would like any advice you can give me before I progress further.

I've got a fairly standard 1980's wood stove - a 31'x28'x28' Country Flame CMD-160 with no secondary burn apparatus inside, a small 6x6' square metal plate baffle inside about 4' under the flue exit hole, a fan on the back, an air channel along the back and top so the fan can blow hot air into the room, and an 8" flue, leading up 4' or so then out the wall to a 20' or so tall external 8" metal pipe chimney. It has heated my 1400 square foot house well for several years, and I'm fairly happy with it. One key downside is that it appears to be letting a lot of heat out of the chimney.

Adding dense firebrick inside appears to have made it more efficient, but I want to get more efficiency out of it. That's my main goal. I also have a lot of small diameter twigs to burn, and wouldn't mind making it easier to get enough firewood for each winter. I also want to stay within building and fire codes, and avoid making permanent or irreversible modifications to the wood stove.

So here's the idea: Make a rocket bell heater inside the wood stove, and leave open the option of adding mass outside in the future. In more detail:

1. Make A Bell: Cut out the internal square baffle, insert a 19" or less long, 8" diameter stovepipe inside into the flue hole, leaving at least 2" between it and the bottom. This should make the stove's body serve as a bell, and should retain more heat in the house. One question: will the temperature at the top of this bell be too great for the stove's steel ceiling and walls? I think it might not be because lots of rocket mass heaters use steel barrels directly above the heat riser. Nevertheless, I plan to attach at least 1" of ceramic fiber board to the stove's ceiling directly above the heat riser's exhaust.

2. Make A Rocket: Create a J-tube next to the right wall (when I'm facing the doors) which barely misses hitting the stove pipe, and is made out of an inner layer of dense firebrick, and an outer layer of 1-2" high temperature ceramic fiber board cut to fit. I've got enough room to make the internal dimensions of the burn tunnel (and whole J-tube) be 4.5" tall by 6" wide, so the cross-sectional area will be 27", which is comparable to that of a 6" diameter system. The feed tube will be completely outside the plane of the stove's door opening, resting on the external ash shelf, and the rest of the door opening will be sealed off with a steel plate. I wish I could have an insulated feed tube sticking out of one of the door's removable windows so I could leave the doors on, but can't see how to accomplish that well.

3. Add Mass: I don't intend to do this at first, but if the system heats the house quickly after steps 1 & 2 above, and if we stay here long enough, I'd be interested in reinforcing the floor joists and adding soapstone as mass flush against the stove's side walls, on top of the stove's air channel, and if I can get the masonry skills, make a soapstone second bell directly on top of the existing wood stove to capture more heat inside the house.

I welcome any comments, recommendations, or warnings you could give me. Thanks for your time!

Tim Blackk : A late welcome to Permies.com, our sister site, Richsoil.com, and a Big Welcome to the Rocket and wood stoves forum Threads.

It is hard to visualize the layout, and when you get all done you will have a 6'' system using a 8'' external chimney, It is kinda unlikely that you will get to
extract so much heat energy that your exhaust gases will stall in that chimney. It is likely that you will need a little help to get your chimney to draft well
when and if we can cross that bridge when we come to itEven a simple sketch would help Good luck and come back here often ! Big AL

Thank you, Allen!

allen lumley wrote:It is hard to visualize the layout

Here's what my current wood stove looks like:



And here's the chimney. I should measure it; I'm probably wrong that it's 20'; maybe it's 15'.



Here is the internal 6" x 6" baffle directly below the flue. I'd have to cut it out to put an 8" stove pipe inside the stove.



Here is the space that would need to contain both the short section of 8" flue pipe and the J-tube:

allen lumley wrote:when you get all done you will have a 6'' system using a 8'' external chimney, It is kinda unlikely that you will get to
extract so much heat energy that your exhaust gases will stall in that chimney. It is likely that you will need a little help to get your chimney to draft well
when and if we can cross that bridge when we come to it

The stove roars (and even starts resonating vibrations and pulses of flame and smoke out the intake holes(!), which used to scare my son) when I light a cardboard box full of twigs in it. So it should be possible to get a draft started somehow.

allen lumley wrote:Even a simple sketch would help

Ok, here's a quick sketch which is not to scale, but expresses my main ideas which I'm considering implementing. I intend for the heat riser to be 3 times taller than the feed tube, and the burn tunnel to be no longer than 1.5 times the height of the feed tube.

Edit: A larger view of this image is available at http://alwaysreformed.com/publicdocs/rocket_bell_heater/Sketch_for_permies-fullsize.jpg

Thanks for any critiques or recommendations; that's why I'm working things out on paper and posting here before spending money on supplies.

Tim blackk : You are definitely onto something ! What I am not sure, A true bell should be 4Xs the size of the smallest of the Cross-Sectional Areas CSAs,
of the Feed Tube, Burn Tunnel, or Heat Riser! you are close, You will I Think create a Plunger Tube went you place the 8'' pipe into your fire box, I
have just the man to ask! Lets see what ''Max'' has to say!

For the Good of the craft ! BIG AL

Max doesn't understand zilch. So no comment!

allen lumley wrote:You will I Think create a Plunger Tube went you place the 8'' pipe into your fire box

After reading up on plunger tubes, yes, I think my idea incorporates a plunger tube. I might try putting a slip collar on the bottom of the plunger tube to be able to adjust the distance from its bottom opening to the floor. I'll also plan to insulate the plunger tube with rock wool or something similar so as to keep heat from conducting through the metal wall of the stovepipe then escaping up the flue.

What is the best way for me to seal the firebricks in my J-tube without making them stick together so strongly that I can't get them out of the wood stove in the future if I want to remove them? Is mud (ours has a lot of clay in it and just about no sand) sufficiently good at sealing the cracks and handling the high temperature? Or do I still need refractory cement?

Satamax Antone wrote:Max doesn't understand zilch. So no comment!

Well, rereading this, and looking at the pics, there is no way this stove can be retrofited with a rocket without cutting the top.

Way to small for any heat riser. And not that intresting for a bell.

Tim Blackk : At the bottom of the Page find permies >> forums >> energy >> rocket stoves , this time click on forums, at the Forum(s)

page Scroll down to the Building Forum, find, and click on the Cob Forum/Thread section. You are looking for the Thread title :::-->

Making cob 101 (the basics of basics ) ( I have Bumped it up near the top for you! )

Click on and browse this thread and thread extensions, so you have a clue which ones to read for content The goal here is to learn how to test
and recognize good local clay!

The best answer to the original posters question is found in The book The Hand-sculpted house, also by Evans et all

Now a much more specific answer to your questionusing the cleanest batch of clay you have about the size of a loofa of store bought bread
dice this up into a 5 gallon pail and water to cover,plus 4 inches Next morning with a heavy duty drill and a paint mixing tool start turning the
clay and water into one homogeneous bucket of Clay Slip ! The bottom will have lumps ( this is your first batch and sand ) you want to have about
three gallons of clay slip about like thin pancake batter ! If Not sure practice with the real thing! finger push in the clay slip and then pulled out
should leave a dimple lasting several seconds!

Then you take a clean fire brick test it for proper fit, then dunk it in water for 5 seconds ,and then dip it in your clay slip and set your firebrick
into its new home ! Then just repeat as needed! Thats it !

When if you want to disassemble your Firebricks set a chunk of 2x4 smaller than a brick against a brick and tap the 2X4 with a hammer !

Making Cob is much harder, and small power tools are not powerful enough, keep this in mind. For the Good of the Craft ! Big AL

Late note :while it does not look like the draft damper in your Big Box stove could ever be accidentally closed, nether a rocket stove, or pocket
rocket or Rocket Mas Heater should ever be run with a damper in the system 99 % chance you WILL find out why ! A. L.

Let me try and explan without pics.

Build your burn tube with firebrick, wrap it in steel on outside, with the riser .

then cut your square hole in the bottom of the stove, and add it to the outside , poking up threw the stove, getting within the needed ceiling clearance.

Aside from the fact that the person in that video seems to be talking about burning coal and not wood, there are serious mistakes in his plans.

Firebrick (particularly the splits he shows) does not have low thermal conductivity unless it is specifically "insulating firebrick", which is not the common sort you find at a hardware store. In any case, the "burn tunnel", which would not function as such with coal, needs to be well insulated with high-temperature insulation, not wrapped in metal which will suck heat out of it. And the heat riser he mentions cannot be metal, but needs to be the same inert and highly insulated construction as the burn tunnel. A big firebox like the whole interior of a woodstove will not function as a rocket.

It might function as a batch box if lined with firebrick and cut out in back for a connection to a substantial heat riser as per Peter Berg's designs, but at that point it would be easier and just as effective to scrap the stove and build the whole thing of firebrick.

Accually, coals, was said in video. meaning hot wood coals.
Not coal, singular.

Okay, coals from wood burning... still not a good model to follow.

Gentlemen : Tim Blackk has stated very clearly, the last sentence of the 3rd paragraph that he wants to leave the basic core of his Wood
stove conversion as intact as possible, the basic unit is capable of heating the core of his housing, And that is how he wants to leave it!

Perhaps some of the suggestions made so far could be restated for him to re-examine if and when his original line of work is unprofitable.
Richard The design as posted speaks of wrapping the firebrick in steel, this ignores the need for more insulation, also the combustion core is
nether clay Slipped to hold the bricks together, nor has a Clay/Sand type mortar been used to seal the bricks together! While both of these
issues are minor and easily caught, posting this material is likely to cause confusion in new members, many of whom will find this post over
and over again in the future ! Big AL !

Working with the base desire to fit a j-tube inside the existing stove chamber, the first critical question is exactly how tall the space is from base of doors to top of back section of stove.
Subtracting thickness of new materials and top clearance above heat riser will tell what proportional dimensions could be achieved and how those compare to the equivalent diameter of the proposed system (about 6"). A question for an expert with calculations is how much heat can be generated by the size of j-tube feasible here. I would guess it would be less than the current firebox can generate at full throttle.

In a short,

Wood stoves work, its' down to the percent of efficiency that is any debate
just like a bungle bee, it's not supposed to fly.

allen lumley wrote:What I am not sure, A true bell should be 4Xs the size of the smallest of the Cross-Sectional Areas CSAs, of the Feed Tube, Burn Tunnel, or Heat Riser! you are close

The CSA of the J-tube will be 4.5 x 6 inches, so 27 square inches. The stove's internal surface area, not counting its floor, is 2318.75 square inches. How does this compare to a properly-sized bell?

Satamax, thanks for your input; I appreciate it. I ask "Why?" below in order to make sure I understand your reasons, and so that I can improve the design to avoid the problems you see.

Satamax Antone wrote:Well, rereading this, and looking at the pics, there is no way this stove can be retrofited with a rocket without cutting the top.

Way to small for any heat riser.

Why? Do you think with the chimney won't generate enough draft, so the short heat riser won't be tall enough to pump air through the bell and up the chimney? I'm concerned that the heat riser might not create enough draft too.

So, I tried step 1 (insert a plunger tube to create a bell) of my project first, as an experiment tonight, and am pleased to see that the stove has enough draft with only a plunger tube placed inside. It's about 38 F outside right now, so a good night to test the plunger tube. The draft is comparable to burning the stove regularly without the plunger tube. Though the stove's doors are not sealed perfectly, smoke doesn't leak through them into the house, because there is enough draw up the chimney. Then CSA of the air intake holes is currently about 10 square inches, plus leaks around the door. So this gives me hope that the draw will be strong enough once the J-tube is installed with its larger CSA of 27 square inches, and a hotter burn temperature.

A couple other notes on how it's working presently. The plunger tube doesn't have insulation around it yet, but I'll add some later. It's made of stove pipe which has a sort of snap-seam made of crimped sheet metal found along its length where you have to fit it together to make a pipe. In order to fit the plunger tube into the bottom of the flue hole in the top of the stove, I had to leave the pipe NOT snapped together, so the unsnapped seam is an air leak, and there is another air leak at the top of the plunger tube where it fits into the flue hole. These leaks will be sealed when I'm done. These leaks could be contributing to the strong draw, but I don't think they are very much, because pretty much all of the flames are getting sucked into the bottom of the plunger tube. The bottom of the plunger tube is 4.5 inches from the bottom of the wood stove, because it is propped up with a firebrick laid on its side.

For anyone who thinks they want to put ONLY a plunger tube into their wood stove and say they're done with the project, I suppose you can do that, but I've found one quite undesirable, and predictable, result: you get a lot of smoke in the house when you open the door to feed in more wood. The smoke wants out of the bell, and it prefers going out the open door over going down to the plunger tube's opening. I trust adding a J-tube and not opening the doors anymore will fix that problem.

Satamax Antone wrote:And not that intresting for a bell.

Why? I think what you mean is that the bell is too small, and/or not surrounded by enough mass, to work very well. I agree. I think it will work for my purpose, though, which is to function like a burn barrel in a "standard" rocket mass heater: radiate heat from the top, and let the cooler gases out the bottom. So it's a small bell. It won't work as a pump, though; I intend to rely on the existing chimney to serve as the pump. My hope is that a plunger tube plus a J-tube will equal less fuel consumption for the same heat output as burning the original wood stove.

I'm considering (in stage 3 of the project) adding mass around the stove, and adding a soapstone secondary bell on top of the stove, generally sized the same as the existing stove. I think this would give me enough internal area for a bell. Do you think so too?

Allen,

allen lumley wrote:clay slip

Thank you for telling me how to make clay slip! Just the answer I needed.

Glenn,

Glenn Herbert wrote:Working with the base desire to fit a j-tube inside the existing stove chamber,

Heh...it may be a "base" desire to put a rocket in a wood stove! But I don't think you meant the word "base" that way. So to answer your questions:

Glenn Herbert wrote:the first critical question is exactly how tall the space is from base of doors to top of back section of stove.

From the base of the doors to the top of the back section of the stove is 19.25 inches. That's the maximum vertical travel the air can make within the existing wood stove's box. However, because (unless the chimney's draw can lengthen it!) the burn tunnel should not be long enough to get the heat riser into the back of the wood stove, the heat riser needs to be in the front of the wood stove, which is shorter. So...

Glenn Herbert wrote:Subtracting thickness of new materials and top clearance above heat riser will tell what proportional dimensions could be achieved and how those compare to the equivalent diameter of the proposed system (about 6").

I plan for the inner dimensions of the J-tube to be as follows, and these are achievable inside this wood stove:

Rectangular CSA throughout: 4.5 x 6 = 27 square inches (the CSA of a 6 inch (circular) system is 28 inches)

Feed tube height: 4.5 inches (measured from lowest air entry to bottom of feed tube)

Burn tunnel max length: 9 inches (measured from near edge/opening of feed tube to near edge/opening of heat riser)

Heat riser min height: 13.5 inches (measured from bottom of heat riser to top of heat riser)

Heat riser max height: 13.7 inches (takes into account 1 inch ceramic fiber board insulation below and above heat riser)

Heat riser to ceiling, min: 1.3 inches

I'm able to adjust these measurements only slightly; the easiest modification would be to lengthen the burn tube. Please let me know if you think any of these measurements absolutely won't work.

Glenn Herbert wrote:A question for an expert with calculations is how much heat can be generated by the size of j-tube feasible here. I would guess it would be less than the current firebox can generate at full throttle.

I would like to know what an expert's calculations are on this. I agree the J-tube probably won't put out as much heat as the current firebox at full throttle, but I am pretty sure I don't need that much heat, too. I can get the house way too hot on a cold winter day at full throttle. I've taken to burning just a little wood at a time, placed directly in front of the air intakes, with the air intakes fully open to burn that wood hot and clean, and the stove's fan on the back distributes the heat through the house. I don't desire to boast by saying these things; rather, they just give me hope that a small J-tube might be able to fit the bill. But again, I'm here at permies to learn what problems my plan might have before I waste too much time on it, so please show me the folly of my ways!

If your existing stove has significantly more heating capacity than you generally need, you may get enough from the J-tube. That is good news

I did a dry-fit test run outside today. It seems to work OK, but I wonder if I need more dense firebrick mass inside (probably not, I'd guess) or more inches of insulation outside (probably so, I'd guess). In the pictures I forgot to insulate one wall of the feed tube. The burn tunnel is half as long (4.5 inches) as it could be (9 inches), by the numbers. The insulation is 1 inch ceramic fiber board. I've got some clay slip made up to prevent air leaks, but haven't applied it to the bricks yet.

Today is a windy day, and the wind direction tries to reverse the flow of the air through the J-tube. But most of the time the J-tube draws the air the correct direction, despite the wind, which I think is a good sign.






A short video of the test run is at https://www.youtube.com/watch?v=knKepTnXsCI&feature=youtu.be

http://www.handprintpress.com/featured/greenhouse-heater/ What about something like this?.......

Rj Schneider wrote:http://www.handprintpress.com/featured/greenhouse-heater/ What about something like this?.......

I like the idea of building a bell on top of the wood stove. I wish I was already good at doing that; if I was, I'd do it now.

I'd have a couple questions about that design:

Will the stove's metal burn out, or is the fire box not intended to get hot enough for that to happen?

Will the cement in the bench disintegrate from the heat?

The purpose of my design is different: I want a small insulated J-tube inside the existing wood stove so I don't burn out the metal of the wood stove; I want to be able to revert to using the wood stove as it was originally designed in the future.

I believe the stove will act normally...It would be an advantage to insulate the inside of the stove with fire brick and or a castable refractory....also the outside of it as well, notice ,they used some type of ceramic fiber on its outside, so as to get as much heat to your exhaust to run the bench...regular concrete in the bench is ok...read the article for particulars.....its no rocket stove, but, it will heat a bench for extended residual heating. and you get to use the stove that you have.....I don't think that trying to make a rocket out of the stove you have is viable. You will have a iron box with a insulated riser on top with a barrel....

Rj Schneider wrote:You will have a iron box with a insulated riser on top with a barrel....

That's not my plan. I'm not sure if you meant that comment for me, though. My insulated riser will be INSIDE the iron box, and there won't be any barrel. The iron box will act like a barrel, though.

After my test burn, the fire went out. The bricks were still hot. So I put some more relatively thick pieces of wood in and didn't try to light them. When I came back 3 hours later they had completely burned up, except for one unburnt end which fell to the side so didn't burn. That's 'cool' that the bricks retained enough heat to light the new fuel like that; gives me some hope the system will work.

wouldnt it be easier to add fire bricks inside the fire box, to act as thermal battery? maybe stack bricks on top of the stove for additional thermal mass. or even bricks stacked behind the stove against the wall.

where are you collecting the heat from with this insulated insert? it seems that just setting that in your stove would only super heat the flue. forgive me for my ignorance if im missing something. its early. lol very interesting subject for sure.

OOPS i re-read your first post and see you already added bricks inside and out. sorry bout that. i really need to wake up before babbling.



nice looking link RJ.

I'm afraid people might be confusing themselves with this thread. To help clear up that confusion a little, let it be known that this thread contains three heater designs, which are not the same, and I'm only attempting to construct the first of these designs:

1. My design in the original post:



2. Richard Woods' design: https://permies.com/forums/posts/list/40546#316538

3. The design Rj Schneider mentioned: https://permies.com/forums/posts/list/40546#321581; the details are at http://www.handprintpress.com/featured/greenhouse-heater/

ronald bush wrote:wouldnt it be easier to add fire bricks inside the fire box, to act as thermal battery?

That would probably be easier, but I'm trying to capture some heat in a bell in stage 1, then add mass in stage 3, as described in my original post above at https://permies.com/forums/posts/list/40546#315689.

ronald bush wrote:maybe stack bricks on top of the stove for additional thermal mass. or even bricks stacked behind the stove against the wall.

Yes, that's stage 3.

ronald bush wrote:where are you collecting the heat from with this insulated insert?

In the bell, which is made of steel and has an air channel outside which a fan blows air through to convect heat into the rest of the house. That's just stages 1-2, though. Stage 3 would collect heat in soapstone placed around the outside of the first and second bells.

ronald bush wrote:it seems that just setting that in your stove would only super heat the flue.

Maybe, unless the bell works to keep the hottest gases from going out the flue.

I think the biggest obstacle to this build being effective is the short height of the riser and small stove/bell, which limits the amount of time for the fire to burn completely and heat to be absorbed before it goes out the flue. It is a worthwhile experiment, even if only to test the limits of how small this can be and still work well.
The best experiment would be to build a 6" J-tube with masonry bell, all in the footprint of the stove, and compare the output of that with the output of the stove setup.

Here's an update on this project.

Pros:

It works. It drafts, it heats the house, even on cold days.

It burns clean.

It uses less wood than the original wood stove did.

Cons:

It puts too much smoke in the house! Not much, just little wisps of smoke, sometimes a constant feed of a little bit, but too much for our comfort level. More on that below.

I have to split wood for an hour every day or two.

It takes burning hot all day to get the house up from 50 to 72.

Here are some pictures of what it looks like:

Help! It puts too much smoke in the house. What can I do to fix this?

Here are my attempts so far:

1. To try to fix this problem, I first put a short section of stovepipe around the vertical metal rods which keep the pieces of wood upright in the feed tube. You can see this short section in the pictures with the kids. This worked fairly well; it prevented most of the smoke from coming into the house. But we get smoke when the wood heats up enough to either:

burn with flames above the top of the short section of stove pipe, or

put out unburnt(?) sweet-smelling (not acrid like smoke, but acrid smoke still has a little of this same sweet smell too) gas from the wood grain (the cross-cut or cross-section of the wood) at the top of a piece of wood

2. So I thought maybe the problem was that the cross-sectional area of the short piece of stovepipe is greater than the cross-sectional area of the burn tube itself. So I removed the short piece of stovepipe, and tried wrapping the vertical metal rods with aluminum foil to make the cross-sectional area about the same to a height greater than that of the pieces of wood, in the hope that would channel all the smoke downward and prevent any smoke from coming upward. The result was that smoke still came out of the feed tube, partly because the wood stove's fan blows air through the wide horizontal vent directly onto the top of the aluminum foil. So I covered part of that vent with a piece of black-painted sheet metal, which stopped the fan from blowing smoke out of the feed tube. But I still get occasional wisps of smoke and sometimes a continuous feed of a little bit of smoke coming out of the feed tube.

It makes us and our house smell like we smoke cigarettes, and hurts my throat just a bit. It's too much smoke. I think what I need to do is increase the draft.

Here's what I can try next:

3. Wrap and tape aluminum foil or something around the bottom of the external chimney, where it has rusted through around the creosote cleanout door. This might improve the draft, because the hole probably lets cold outside air get pulled up into the hot air which vents out through the top half of the chimney, and the resulting mixture of cold and hot air has less pulling power as a result of its lower temperature, and the fact that it can pull air not only out of the stove, but also out of the hole at the bottom of the chimney. If you look really closely at the picture above, you might be able to see that there's a hole in it. That hole has since gotten bigger.

4. I can try raising the height of the bottom of the plunger tube. It's currently about 4.5 inches off the bottom of stove's floor. Raising it from its original 2 inches or so to its current 4 inches increased the draft, so maybe raising it more would increase the draft further.

What do you think?

P.S. One more "pro" to add to the list in the previous post (which it seems I can't edit anymore) is that the bell does appear to be helping keep some more heat in the house than the stove's original (non-rocket) configuration did, because the bell gets hotter at the top than at the bottom, and the stovepipe above the stove no longer gets as hot as it used to, nor is it as hot as the top of the bell.

Thanks for updating so comprehensively! Glad it's working reasonably well. I think both of your suggested alterations would be good - try them one at a time so you know which one had more effect. I'd plug the chimney leak first, as that has no downside of less heat stored.

Having gone through this trial, do you anticipate trying to further tweak this stove, or building a standard type RMH with bell for next winter? (How sturdy is the floor under the stove? Can you reinforce the supports if needed?)

Glenn Herbert wrote:Having gone through this trial, do you anticipate trying to further tweak this stove, or building a standard type RMH with bell for next winter?

Presently I plan to continue to tweak the stove, but the smoke is making me consider just taking the J-tube out and using the stove as a regular wood stove again. I don't want to put in a standard RMH because I don't have space for a big bench next to the existing chimney, a permit could be hard to get, and it would take a lot of work to reinforce the floor. So for now I'll see if I can do something to increase the draft.

Glenn Herbert wrote:(How sturdy is the floor under the stove? Can you reinforce the supports if needed?)

I could, but it would mean building a 4 foot tall platform off the dirt floor under the joists (and subfloor, hardwood floor, and floor tiles). For now that's not worth the effort for me. In other words, I'm not up to doing stage 3 of this project yet.

This might be what I need to try: https://permies.com/forums/posts/list/32099#253850 A heat pipe. Maybe copper tubing wrapped around the metal rods to cool the wood above the top of the feed tube, then run into a metal bucket to siphon the water through the copper tubing. (Note that would not be a sealed system since I don't want an explosion!) Not sure if that would siphon correctly, though.

Here's why this might solve the problem. The smoke only rises out of the feed tube area when the flames burn up on the wood higher than the top of the feed tube. Part of the reason the flames get that high is because the upper parts of the wood get hot, because the feed tube emits so much radiant heat. I doubt the copper pipe could combat the radiant heat, but it might be able to keep the wood cool some via convection and conduction. Another part of the reason the tops of the wood burn is that flames (still pointing downward because of the draft) slowly creep up the sharp edges of the wood. Maybe the copper pipe could stop some of that from happening, if it could cool the flames by contact with the flames.

I think the aluminum foil reflects too much heat back onto the wood, making it easier for it to burn higher up and so put smoke in the house. I'm not sure what to do about that yet. Maybe instead of aluminum foil I should use a steel box, and wrap that with the copper tubing.

I had the feeling space limitations were pretty strict, which is why I suggested a masonry bell instead of the long cob bench. You could build a small "masonry heater" with rocket core without too much reinforcing required. Masonry heaters are already in the International Building Code, which some states at least use as the basis for theirs (New York does). So that could be permittable with just a bit of leeway from an inspector.
But if the existing stove works, or you can tweak it to be smoke-free, and your wood budget is not too big, you might be justified in sticking with it. Your current modifications are no more "legal" than a new RMH would be, just less visible.

Glenn Herbert wrote:I had the feeling space limitations were pretty strict, which is why I suggested a masonry bell instead of the long cob bench.

Ok, I see. I thought you meant a "bell" made out of a metal barrel. So to answer your question, I don't plan on building a masonry bell just yet. I've got to solve the smoke problem first, then would need to decide how much floor reinforcing would be needed and how much I'd want to do, plan and price masonry work for a bell (which might be beyond my abilities).

Regarding building permits, I'm hoping my current modifications would not require a building permit, since they are easily reversible. But my biggest concern of that sort is to get the design to a point where it will not be a fire hazard, and can satisfy my homeowner's insurance agent. I believe to do so I'd need more tile in front of and around the feed tube to catch any sparks.

Having read (and re-read) this I believe the primary cause of the smoke back issue is the height ( or lack thereof) of the "plunger tube" above the (stove) floor base. This is the exit into the chimney flue - right?

While raising it will lower the efficiency of the stove a bit there may still be substantial gain overall. Try raising your "plunger tube" to a height even with or slightly above the burn chamber roof.

If that doesn't work well enough - raise it a bit more to a height even with or slightly above the feed tube inlet.

If I'm reading this right, it appears that the low plunger tube exit is creating the conditions which are contributing to the excessive smoke back.