Walker tiny wood cookstove build

Ive been mapping out the passage ways in my head. The main question mark is..
-how to get the gases from the oven area to the small bench

The only solutions I see involve exiting the oven down low and going under the heat exchanger area, or under the passage from the heat exchanger to the oven. Either way, to do so would involve cutting out the bottom of the downdraft heat exchanger area, which is a conflict for my water heating needs. (I want to have a small thermosiphon tank that feeds a 250ft pex loop floor, and my domestic hot water needs) Alternatively, the stove could be taller, which is also an issue.

I think a compromise could involve a passage under the heater exchange area which is low, thin, and rectangular. I could maintain the stove height, maximize the water heater exchanger and marginally boost the oven residence time by having the gases exit "lower". I may require metal support to span the brick at the main body exit point. That's also where the hot water bypass would live..

Once I start playing more brick Lego I may stumble upon another solution, although that appears to be the main one. Maybe removing base layer red bricks for the under passageway and shortening the exchanger area could function, as a strategy, too. Then again, id be relying on the kiln brick to cement board to plywood connection in oven exit to be safe. I also would need to do some fancy angle grinder chamfer cuts, which sounds high risk. It could extend the years I could run the stove without having to cleanout the heat exchanger area(removing cooktop and a few bricks). As it would create a little indented ash trap in the bottom of the heat exchanger area.

I'm not sure how useful all this dialogue is. I'll be looking to take more photos to make it more visually clear when I'm messing around with arrangements. I have the morning to play around.

The other big question for me is the door mounting. My door is way oversized for the opening, and would involve glass that is looking at mostly kiln brick. I have thought about having a crack at fabricating a door, although this sounds like it could become a 10hr, $100+ operation. I may even be able to use my current glass door as the oven door. This would require building and mounting a second frame, which I also don't like.

Okay, well those are some of my thoughts on design. Today I intend to move another truck load of materials up, clean them up and place near where they will be used. I'll also get the cutting station setup and give a few methods a try on quarter bricks. I'll finalize the core, and figure out the required main red brick body cuts to accomodate for the core. This should also dictate the kiln brick and base layer cuts, although I can do them later.

Then I'll probably spend some hours sitting around looking at the footprint and plans, smoking tobacco, and trying to figure out which strategy to dry stack, which door to use, and how to do so.

Here's the thread for our build in case you haven't yet found it.

Matt Walker tiny stove build

There's a fair bit in there and quite a lot of Q and A.   It's a pretty standard build without any water heating or such, the only slight difference is it's mirrored compared to the plans.

If you don't have the size bricks that are in the plans, you need to start by getting the internal sizes of the core correct (my core is also built with firebricks), then make the stove body contain the core including insulation space around it, and then also make sure the internal passages in the stove are right.

The bench inlet/outlet are at the bottom of the stove.  Bench inlet (form the stove) is at the back of the descending flue (on the left in my build), bench outlet is at the base of the chimney.  On the plans, these are marked as optional if you don't plan to add a bench.
I'm reliably informed that it works as-is, but it's also possible to place a brick over the outlet from the bottom "oven" space into the flue so as to push more heat into the bench.  I'll be (finally) building the bench for ours soon, so no doubt there will be a few more posts added about that.

I saw a video of your build many months ago, looks great. I found the discussion about clay/mortar types to be really useful as I'm unsure how I'll source fireclay in my location.

I haven't given it a full read yet, but will tomorrow morning.

I was thinking to minimize the mortar use in the core. This is the opposite of what Matt did in his build,  I noticed he used large joints to shim up the internals to spec. I was thinking to use thin joints(1/8 or less) and a little 5/8" shim layer, and put a cast refractory paste I have on the firebox ceiling and shim area. I hope this would be able to improve combustion, reenforce the shims, and minimize reliance on fireclay(?)if I can't source it.

In the rough dry stack I have shim variance of 5/8" to 13/16". I was thinking to frame the shims and palm sand them down to a uniform 5/8". Or whatever gets me 10.25in internal(1/4in refractory paste)

Yes the passage way strategy you mentioned is what I also have landed on. It makes the heat exchanger cavity shorter, which I dislike, as I was looking to max out the water heating capacity. It also creates a bench "deadzone" at the back for my geometry/orientation, but I think I can just accept that and try to modify its slope to improve bench performance.

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Id like to see what you think about the firebox door/oven door, as it seems to be right up your alley.

I think for the firebox door I'll fabricate something and mount to the front of the firebox. I only have steel on hand , but will try to do a no weld frame and door similar to what Matt used on his tiny. I noticed most build mount it inside the front. Maybe this is easier?

The oven door I may use that glass door, with the mounting stubs. I think this means cutting down the entryway slightly. It's also got an oval shape. Any ideas on this in general and/or how to do this and avoid welding?

Okay so the drystack has illuminated many things. I would highly recommend to anyone doing a build with even minor adjustments. I may even do a second dry stack due to the design issues I ran into.

Okay, so the core is good. Or so, I think. It fits in the red brick body with quite a bit of clearance on the back, and mortar joint clearance on the sides. I do have to plane out the shim layer, although I expect the dimensions to be 7.8-8in x 10.1-10.25in x 17.25in. I think this is quite close to walkers plans.

I am a bit concerned about the start up bypass functionality, so I'm giving up some cooktop gradient to optimize for startuo function. The sidewinder will be 2.5 * 10.15-10.25. I was thinking to chamfer the inside edges about 1/4in and use last o kite paste on the inside.

I plan to coat most of the inside of the firebox, about 1/8-1/4 thick. Should help to preserve the kiln bricks doing heavy duty, and should improve firebox insulation.

The under cooktop passage is 8.25*4.25. I am thinking to heavily chamfer the 180 turn, basically smooth it out over 2inches on either inside corner. This will bring the gases through the cooktop area faster(,less gradient), but help startup. I have just enough clearance to Jam in a second layer of IKB on the backside of the core, which won't help on startup, but is great in full masonry mode as the heat protruding out the back of the stove and back of stovetop is lowest value given my space. I have some extra space on the top right side of the undercooktop area to sandwich in all my tiny IKB offcuts, and some mortar(maybe the IKB cutting paste from the cutting station too, so 0 waste), should also help with full masonry mode.

There is a 90turn from the under cooktop area to the top of downdraft passage, which I would also like to have a slightly oversized CSA with chamfer edges and kiln brick.

The start up bypass will live almost directly across from the under cooktop exit, at the top of the downdraft heat exchanger area. I also am thinking to use kiln bricks in that area, and chamfer/carve them to suit a transition to stove pipe.

I'm hoping my modifications will make startup bypass mode functional. I'm asking for an extra sweeping 180, sweeping 90, and 18in of straight in addition to the plans. No experience here with how much my mods will impact startup flow. AI helped guide me, but I don't really trust it on this, so still feels like a gambol.

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Ran into issues when I realized my downdraft entry channel to  oven port and oven exit to bench entry point were so close that I could not create seperation. I thought about using 1" CFB to box off the oven exit. I then played around with stacking bricks on edge in the oven cavity. This works, but creates a strange CSA geometry. Around 3*12 preserves the oven cavity and allows for routing. I would be concerned about cookware pushing out the passageway, so maybe need to think of a way to re-enforced the oven passageway walls. Also unsure if asking for a 3*12 passage is going to create too much friction? I could do more square, but would shrink the oven depth below the opening width. This was a head scratcher, I'm still unsure if it will provide function.

Also, the bench entry, I ended up finding out the entry point creates dead space(stairs are behind the stove) so this led to slight mods, including moving the entire footprint a few inches in. The "bench" area also had more depth than I was anticipating, so I am able to incorporate a 24in*24in seat. I was thinking to have a hinged leaf, so as to make it optional lounging bench that is 40-46inches long.

I also ran into complications on the chimney pillar. I started to mock up the pillar at 8*4 internal CSA and then realized I eventually have to switch to 6in round stove pipe. This posed an issue with strange sideways transition and would make for a weak chimney pillar support beam. So, I have decided to change the entire pillar to be 6*6 internal CSA so the transition up high is smoother, and my cross beam on the pillar can be better supported.

The pillar and bench mock-ups are very rough, mostly used as a footprint pilot, as I was running out of bricks and didn't want to make cuts.

The hot water mode bypass at the downdraft heat exchanger area to bench/chimney plenum area is proving to have multiple functions. If going from start-up bypass to full masonry mode doesn't draw well, I could use the hot water mode to create more draft and prime full masonry mode.

Also, another fail-safe is my bottom of chimney cleanout can be used as a place to put kindling to establish draft if startup mode is too laggy..

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I'm feeling pretty excited about the layout/mods. It's still feeling quite ambitious and now the stove requires significant operation skill to function well.

There is now a break in the build. I have to pickup the last load of bricks, the cooktop, and figure out the door situations. I also need to source fireclay or some sort of mortar that will function. The next load is on the 19th, so this will give time to reflect on the mockup and perhaps make more adjustments and decide if I need another dry stack.

Alright so had a dip in motivation, today. I managed to hang a plumb bob from the desired chimney penetration location. It wants to be quite far from my dry mockup. Around 10in in one plane and 3 inches in another plane away from my dry stack location.

I spent some time in consideration with a friend of the pros/cons of shifting the entire footprint. To frame in a opening in the attic is not terrible, but I'd prefer to avoid it. I also am wanting to avoid using elbows to route around the chimney pipe. I'm asking a lot from the little firebox and rather give it a very straight, tall, insulated run. I ended up settling on moving it back 2inches, and inwards 6inches. This will open up a 9in cavity between the wall and stove, which I think may be enough to fit some sort of tall wood storage device.

I'm thinking I may do a second brick skin between the water heat exchanger downdraft area and the chimney pillar. That would give me straight chimney run. It would be a nice boost of exposed masonry during hot water mode. My climate is pretty mild(southern BC, winter sees -5C-5C for the majority of the season), I like to sleep cold in the loft, and have the stove fairly appropriately sized for the space/building envelope, or so I think. Given these factors, I expect many days of a strong morning firing, and expect to use hot water mode often on milder winter days and shoulder season.

Doing a second skin on the outside of the downdraft would involve more bricks, a bigger footprint, and startup bypass traversing an extra 4.5inches. a small price to pay.

I also have decided to move the footprint back 2 inches as a way to provide more space to the high traffic zone (kitchen/hallway/living room transition) and give less space to the loft stairs. I don't see too much of an issue cutting down the stairs to 26-28inches.

I didn't anticipate spending so much time and energy on the footprint location..however with a stove of this size in a 24*16 space, I think it's well worth the time to consider.

So, I dismantled/sorted the bricks from the drystack. I'll need to do another one. Id like to get some of the hardware at least partially sorted on my second dry stack.

After much consideration, I mortarted down the footprint in place,(not a total dry stack !) with a bit of extra in certain areas incase I make any fine tuning or mods on the wet stack.

I laid out the kiln brick base. In the first dry stack I found my base red brick layer protruded over the kiln brick and this time decided to give larger mortar joints in that orientation to get it closer to flush. I may change it back on wet.

I then laid out the base red brick, this time giving myself a more realistic 1/8in-1/4in gap. I started my first course and have got to the point where the bypasses will live. Felt pretty low energy and unmotivated, so called it a day. I could have continued, but found myself a bit scattered. Head wasn't really in the game. Better to save this sort of work for more focused times.

Tried posting a few times, times out and lost what I wrote. I'll keep this one short and only a few photos.

I did a second mockup. Feeling good about it. Clearances for the footprint are all within an inch or two, more than enough to play with. The stove is the one that is harder to alter. Stairs on the back, stairs going up and over both have clearance, bench has 24" and a couple inches before stairs, 12-13" cavity by the wall, chimney pillar is over a stud by re-enforced joist by only 6-8inches. It all works out.

The bypasses, in theory, seem to be working. I used angle iron scrap or flat metal scrap, depending on the opening I was aiming for. I have yet to try and build the bypass plates. I was going to use CFB, but it appears too fragile to me, so I'm going to apt for steel. The chimney start bypass I will have slide horizontal, and the hot water mode bypass will be up and down, and the rod will be sticking down through masonry. I may sleeve it to make things easier.

I didn't fit the cleanout, but I think that will be relatively simple. I found one in my scrap pile that I can cleanup to use.

Found some flashing for the stove pipe transition, too. It will have to be cut/modified. It has a gradual taper, which I like, but it doesn't sit flat. I either have to indent a seat, or cut it down. Unsure which to do. Using the inner taper seems like a good idea for my anticipated startup difficulties.

Somehow in the second mockup shuffle I ended up stacking the core differently. Although, I realized when I was doing it that it solved one problem and perhaps made a second. My cooktop is 36 * 21, and my first mockup core had a 21.5in span. This core is a 20in span, so I can gasket and have continuous rim support for the cooktop. The door opening is 7.5*6.5, and the firebox projected size is 8.25 x 10.25 * 16.25.

Okay, more photos on this reply.

So I sourced the ceramic glass, the glass sho

My plan to fit the 1" ceramic board can be seen in the photos. The CFB is really fragile, I may have to skin all sides for the oven door., I don't have a good plan yet to seal it, I was planning to use Matt's strategy with pins.

The firebox door in even less confident about. The 2" CFB I have is even more fragile. I am looking to use that with 2in alum angle, and a steel front face with corner supports. I am thinking to use thin stainless through bolts with large fenders and catch the corners. I don't trust the CFB to hold much more than oversized fenders. I got a 3*5in glass offcut from my glass shop, and I should have enough CFB to make wedge controls for airflow, or I may try to repurpose a sliding air control I found on an old camping stove.

I'm concerned about the bypass plates warping and causing issues.

Also anxious about my copper pipe plan to bend/fit in place during the wet stack. It's soft copper, one continuous piece, so I rather try and use that even if it's mortared in, and avoid soldering joints or around the heating zone. I have a pipe bender and I'm unsure if I'll go for 4 or 6 straight sections. I have 24in * 22in to play with in the downdraft exchanger area.

I bent over and recieved the $80 shipping fee for two bags of fireclay. It will arrive in 4-5business days and will probably be the time constraint for the wet stack. If I had been prepared on this, I could have placed an order with my local school and paid half the shipping.

Photos show 2nd dry stack progress. I have ran out of useable bricks. I'm picking up around 150 tomorrow and expect that to be enough to finish her off

Okay, more photos on this reply.

So I sourced the ceramic glass, the glass sho

My plan to fit the 1" ceramic board can be seen in the photos. The CFB is really fragile, I may have to skin all sides for the oven door., I don't have a good plan yet to seal it, I was planning to use Matt's strategy with pins.

The firebox door in even less confident about. The 2" CFB I have is even more fragile. I am looking to use that with 2in alum angle, and a steel front face with corner supports. I am thinking to use thin stainless through bolts with large fenders and catch the corners. I don't trust the CFB to hold much more than oversized fenders. I got a 3*5in glass offcut from my glass shop, and I should have enough CFB to make wedge controls for airflow, or I may try to repurpose a sliding air control I found on an old camping stove.

I'm concerned about the bypass plates warping and causing issues.

Also anxious about my copper pipe plan to bend/fit in place during the wet stack. It's soft copper, one continuous piece, so I rather try and use that even if it's mortared in, and avoid soldering joints or around the heating zone. I have a pipe bender and I'm unsure if I'll go for 4 or 6 straight sections. I have 24in * 22in to play with in the downdraft exchanger area.

I bent over and recieved the $80 shipping fee for two bags of fireclay. It will arrive in 4-5business days and will probably be the time constraint for the wet stack. If I had been prepared on this, I could have placed an order with my local school and paid half the shipping.

Photos show 2nd dry stack progress. I have ran out of useable bricks. I'm picking up around 150 tomorrow and expect that to be enough to finish her off

Can't seem to upload many photos without timing out. Have many.

Matt Walker has been very helpful and identified a major potential issue in my second drystack. My understanding is that with my proposed downdraft exchanger cavity that the flue gases would lose too much heat to the coil by the time they get to the oven, meaning the oven and masonry would not be able to absorb enough heat to effectively heat the space. This is a major issue, and there are a few solutions.

My understanding is a second downdraft channel without an exchanger coil, would be the cleanest solution. This would also require a fair amount more width and bricks.

We are exploring alternative solutions that are less clean, but might work and only involve expanding the downdraft width a few inches and changing the oven port entry location. This would help a lot with my floor plan constraints, floor enforcement footprint, and chimney alignment. Maybe even keeping the existing port locations, but offsetting the coil to one side and widening the channel.

I think in any event there is no use in completing my current drystack at this stage, and I should carefully dismantle / sort and wait for more information

It looks like I'd would be wise to do a teardown and 3rd drystack with the new load of bricks in my truck !

You're doing way more experimental dry-stack than I did!  I basically just built it, making sure the internal spaces in the core and within the body came out right using the bricks I could source.  However, you're planning things like water heating which I didn't do.

As for storing heat, you store heat in the hot water and I assume your hot water tank is inside the house. If you find you want more heat in the stove body/bench then you can just burn another batch of wood.  Without the bench on mine I found that more than 3 consecutive batches (i.e. one fire-box stacked with wood) started to overheat the chimney, so was wasting fuel.  I expect/hope that with the strat-chamber bench which I'm adding this year, I will be able to to do an extra batch or two without losing too much heat.

At the base of my chimney is a single-skinned 45° bend before it switches to double-skin going through the wall.  During the first and second batches on each burn that bend is merely warm to the touch, showing that the flue temperature is nice and low and the heat is remaining mostly in the stove.  At the end of the third batch it starts to get hotter although you can still touch it without burning yourself.  At that point I usually let the fire go out, since burning any more would send too much heat up the chimney.

Just to add I'm trying to create a distinction between "batch" and "burn".  The way I see it, when you fill up the firebox with fuel and light it that is a batch, and that's what distinguishes a batch-rocket (stove) from something like a J-tube where you feed it little and often.

A "burn" is composed of 1 or more batches and is the time during which the stove is alight until you decide not to feed it any more.  This, I think, conforms to the idea of a "burn" in a J-tube.  And if it doesn't, I'd like it to.  So for example you could do a 1½-hr burn in a batch stove which used 3 or 4 batches, and after that the stove and bench would all be nice and hot for the rest of the day/night.

Austin Shackles wrote:You're doing way more experimental dry-stack than I did!  I basically just built it, making sure the internal spaces in the core and within the body came out right using the bricks I could source.  However, you're planning things like water heating which I didn't do.

As for storing heat, you store heat in the hot water and I assume your hot water tank is inside the house. If you find you want more heat in the stove body/bench then you can just burn another batch of wood.  Without the bench on mine I found that more than 3 consecutive batches (i.e. one fire-box stacked with wood) started to overheat the chimney, so was wasting fuel.  I expect/hope that with the strat-chamber bench which I'm adding this year, I will be able to to do an extra batch or two without losing too much heat.

At the base of my chimney is a single-skinned 45° bend before it switches to double-skin going through the wall.  During the first and second batches on each burn that bend is merely warm to the touch, showing that the flue temperature is nice and low and the heat is remaining mostly in the stove.  At the end of the third batch it starts to get hotter although you can still touch it without burning yourself.  At that point I usually let the fire go out, since burning any more would send too much heat up the chimney.

Yeah, I think 2 partial dry stacks and a third drystack is probably far more than most people would mock up! Even as such, there are a couple things I haven't cut out yet that I will during the wetstack, such as the cleanout and firebox port.

Its certainly a slow process for me getting things figured out with builds, in general. This is often the case as I collect salvaged materials and make a pile near the build.. I often find myself asking "Okay, what are my needs/goals? How can I use these materials to achieve this?"
This is the case in this build, too. 5 sets of bricks(all different sizes/conditions), thick insulated kiln brick(2 sizes), random metal bits(I looked to source metal and lol'ed at the prices), unusual ceramic glass top dimensions(What I could find used/free), my own clay deposits, sourcing sand from local deposits, etc... I think I've got a good handle on how to arrange most of the materials now, and feel relatively confident about putting everything together.

I think the core internals is the one I'm trying to get as close as possible. Even still, my drystack is slightly different. At 16.5" internal firebox depth, the ceramic glass that I have is flush with the inside of the kiln bricks at the top support, it would have no bearing or seal around the front and back perimeter of the ceramic glass. The ceramic glass is kind of a strange size. Super wide, and narrow. I think my preferred option is to shrink the firebox to 16" to be able to fit the ceramic glass on the supports, and very mildly bridge in two layers to get it closer to 15.5"-15.75" for a ~1/2" support/seal. I was thinking to open up the other dimensions by 1/4"-1/2" to compensate, which I know doesn't translate into reclaiming combustion performance, but I imagine the + volume would help more than hurt. There are other solutions that I don't prefer, and some I haven't considered, ...but yeah, this is the sort of stuff I've run into.

The water tank would be in the attic. I'm trying to +thermosiphon flow(only 3/4" copper), and be able to use my domestic hot water system on gravity feed, so I lose out on some of the radiant heat properties. It's still within the insulated envelope, and I have a little circulator pump to push the water through the masonry floor. So yeah, I'd still be reclaiming some of that heat for radiant gains. I'm really unsure how it will perform in practice, lots of variables involved. In regards to what you were saying, after 3 loads the chimney is getting hot and you could use more masonry... Yeah, I really don't know what to expect with this in practical operation for my stove. I only have a hot water downdraft exit, or oven/bench route. The water downdraft channel arrangement is custom, and starting water temps/flow would have a pretty big influence on chimney temps. I was thinking I could get a couple probes with a display to be able to figure out optimal operation, similar to what you are describing with observing the 45degree stove pipe. Now that you mention the stove pipe gauge, I like the manual gauge concept that you have going with the pipe. Similarly, I think when I'm assessing "Do I switch from hot water mode to masonry mode?" I could perhaps touch the cold return on the copper pipe and a couple "gauge bricks" and get a feel for the tank temps relative to masonry.

I was looking at the way you and Matt both did bridges over the support, with mortar joints over the span. I wish I saw that earlier, I didn't even consider such a thing and have been reclaiming old bed frames for angle iron supports.

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A friend and I spent time working with the aggregates.

I have two sands on hand. One is a black sand, angularish, but lots of flats. We harvest 1.5cu yd from a nearby deposit. It's got lots of magnetite and its also shaley. I'm concerned its too weak, structurally, for most of the stove. The other sand is more of a standard sharp silica/quartz sand, from a local quarry. I used it for concrete. I did some test batches, testing with both sands and fireclay. I'm currently drying them(oven-assisted!). I accidently washed the IKB before mortaring, and its drying very slowly. I want to fire them when dry, see if/how they crack, then stand/jump on them and see what the bond is like.

I have 3 clays on hand. I have a pureish shiny blue I scored from a nearby excavation, a native blue-brown vein on my land, and purchased fireclay (lincoln60).
I'm mostly testing the fireclay for the stove, as I plan to use it in pretty much all areas, except the chimney pillar, and maybe the bench.
I have lots of plaster projects in the near future. My friend also happens to be great with color assessment, and generally strong observational skills... so we used the opportunity to test the aggregate combinations / color mixes for plaster work / other work.

I think the next stove build stages for me will involve prepping the space and wet-stacking the base layer / first layer. I've got the kiln brick base tight knit and dry on the cement board, and expect that to be fine as is. I think I'll seal up any small seals as I put down the red brick base.

I am seeing lows of 3C-4C, in the coming week. I'm wanting to get started soon. I foresee the wetstack to take a couple weeks. Any unforeseen holdups during the wet stack will bring me into very frost-sensitive times. In the last few days I was doing some plumbing/waterbox work and secured a rental agreement on one of my cabins. Now that those are taken care of, I've moved this stove build to top priority !

I'll be interested in your results with clay and sharp sand.  On my build, because I didn't have any sharp sand to hand the parts that are done with clay mortar (mostly the internals of the core firebox and the top layers of the stove casing, the latter so it can be got apart if needed)... Those parts are clay mortar made with what is locally called yellow sand which is what you would normally use for rendering, not bricklaying.  It proved not all that good and it tends to come apart too easily.

The most part of the stove casing is done with some bought-in refractory mortar which I think they sell for building brick ovens.  That means I can't dismantle it easily but then I have no intention of doing that, I'm more concerned with longevity. I can access the core by lifting the top off the stove, if I need to.  In fact I did end up needing to as a couple of the firebricks cracked and had to be repaired.  I don't know if they are inferior quality or if that was just bad luck.

Austin Shackles wrote:Just to add I'm trying to create a distinction between "batch" and "burn".  The way I see it, when you fill up the firebox with fuel and light it that is a batch, and that's what distinguishes a batch-rocket (stove) from something like a J-tube where you feed it little and often.

A "burn" is composed of 1 or more batches and is the time during which the stove is alight until you decide not to feed it any more.  This, I think, conforms to the idea of a "burn" in a J-tube.  And if it doesn't, I'd like it to.  So for example you could do a 1½-hr burn in a batch stove which used 3 or 4 batches, and after that the stove and bench would all be nice and hot for the rest of the day/night.

Yeah, that's how I interpreted what you wrote earlier. Yup, I could see how with too many batches in a burn, the chimney temps keep rising, and it's lowering system efficiency. I suppose that also means that having the stove sized well to the space is helpful for efficiency, too.

I think bypasses makes this interesting, too. As chimney temps rise in one "mode" it can be bypassed to a new mode for more heat exchange potential. I suppose with a water exchanger there is another aspect, too. There can be a "heat dump" hot water system, such as a hot bath. Then, the new cold water will strip heat faster from the gases, keeping the chimney temps lower. Well, at least in theory.

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I'm so focused on this stove project. I forgot yesterday was my day off. A friend reminded me and I took the afternoon off. I still managed to put in a decent morning session.

The wet stacking is way fun. That said, I've run into a couple challenges. The cleanout I have it partially warped and has a tiny lip. I didn't quite fit it perfect, and I accommodated for the bulging in a way that kept the plug happy. It seems to work pretty well as is without the mortar cured, so I think it will be pretty solid in a couple days.

I'm realizing with all my different brick sizes, and my prioritizing the inside channels being consistent, I'm going to have a pretty jagged outside face. It's also going to be like 6 shades of red. I think I'll just continue and try to keep it close, within 1/4in. Maybe ill consider a finish on the outside tile or plaster that smooths out the irregularities.

I lay the first couple courses and I'm about halfway through the third course. At this point, I'm looking to embed the copper coil in the downdraft chamber and figure out the bypass plate. I have pipe bender for 3/4in pipe. I started to use it to bend the copper... Not so easy. Much more difficult of a job than I anticipated. Its stiff and has a tendency to mini-kink at sharper bends(6 in radius). Also, its hard to fix in both planes while shaping, and the other 20ft of pipe is sort of flopping around, in the way. I thought with a proper pipe bender, it would be easy to shape. I was wrong. I called it early and I'll have to re assess how to shape it before proceeding. I think I'll have to get a proper table setup and involve in a clamp on the actual pipe bender and use open cupped palms and moderate force to shape, a few inches at a time. I'm not trying to get it perfect, but it does need to be a general shape and size without restrictions. Maybe filling the pipe with sand and using gentle heat will smooth it out and make it easier? I may try this, tomorrow. I have 4 ft of 1.25in straight copper that I can use if I mess up part of this coil moreso than I have.

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I've heard back from Matt Walker about my proposed downdraft mods. Its not his recommendation, and he's unsure if it will function as I'm hoping, but would not rule it out as possible. I would say he's not discouraging my mods, so that is enough of a green light for me to gamboool on my proposed design. Worst case, it will be a fail, information for the community + require a difficult retrofit. I'm hoping for a best case, fluid dynamics work in my favor(!?) and gases do the stuff that I'm wanting them to do, I get my goals met in terms of performance, stove footprint, and y'alls get a successful innovation.

His main concern is that the downdraft chamber will behave bell-like and gases will spread and stratify In the top of the chamber when the oven/masonry route is opened. They would prefer to do that, rather than get pulled through the oven/bench passageways. To be fair, I didn't even consider this initially, although it makes sense. I think very likely he is correct. The chamber is essentially an open bell with a much larger CSA than my system csa, and the draft is looking to pull the gases a fair distance through the system.

I have been considering options. I've landed on a couple adjustments which I hope(basically just guessing at fluid dynamics, here) will allow for a good function in all 3 modes(start up, water heating, masonry). My plan is to keep the downdraft entry to oven entry passage short, insulated, and partially defined path. I will do this by lining the inner front left wall with kiln brick, and recessing that kiln brick inwards, slightly.

In addition, I plan to put a divider brick at the top of the downdraft chamber entry, on top of the top coil pass. The intention is to discourage gases from entering and expanding in the top rear of the chamber.

I'll also be looking to improve overall system draft, which I suspect will help overall, and also help to reduce stratification relative to flow path "suggestions". I will use less 6in stove pipe, only a short double wall transition, and more chimney pipe, and may have to go a bit higher with the chimney. Continuing chamfering inside corners at corners of passageways, and making the chimney pillar to stove pipe transition gradual and smooth(not sure how, yet).

How will this play out in practice? Dunno. Complicated and too many unknown variables, plus my lack of skill in laying brick and fabricating aren't helping. I'm hoping my conceptual knowledge and luck will produce good results . In a not so favorable outcome, I'll need to design a sliding plate baffle to create more seperation within the chamber. I have a hard time imagining the gases preferring stratification if most of the passage is closed off relative to the shorter oven path. I hope this is not what will be needed in practice, as it involves more fabbing and additional operational controls/complexity.

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I tested the sharp silica sand and fireclay joint on kiln brick. I dried it, then fired it in my woodstove. It was about 1/8in-3/16in thick, a bit more than my planned joints. It fired well, no cracks. It's got enough compressive strength, me standing on it with my heel was fine. It held bond. I could hold one brick sideways or from the top and let the other hang. Sideways, gentle finger taps were okay. As soon as I tried to pull it apart with gravity assisting, the bond broke.

This tells me high compressive strength, low shear strength. I need to re-enforce the firebox area. Fuel entering the firebox that could bump a brick could easily break a bond.

I'll test my black sand kiln brick joint tomorrow. It's closer to 1/16 - 1/8in. I expect it seperate on the hanging gentle finger tap test. I have a thick blob on top to assess cracking.

I'm using my native clay to make a weak clayslip for use as my "water" for the mortar mix. Im using less fireclay, closer to 3.5-4:1 sand:fireclay. I like this strategy, as I can bypass screening my native clay, and still get benefits. Im also using a bit of black sand with the silica sand, in the mix, currently. As far as performance goes, I've had a few bonds break from pretty heavy swings with a mallet when fitting other bricks, adjacently... but they generally seem to be bonding pretty strong light taps don't break bonds. I plan to use less clayslip and less black sand, as I approach the core. I think for the core, I'll screed ash for the mix.

Id like to try and complete layers instead of mortar batches when possible.. the bond breaking situations are happening when I restart work on a partially finished layer and start tapping adjacent bricks to get level/flush.

Also, I would like to note that on the bricks I popped loose, I've simply painted / sealed the joints with mortar. I imagine this is not ideal for bond strength, but probably okay if it's only in a few places.

Also, this morning I managed to figure out the copper coil situation. I have an electrical conduit bender, not a soft copper bender. I think it may be physically possible with my bender, but difficult level is very high and I'd need to set up a station to mimic the proper tool. I ordered the proper tool for bending, which is due to arrive in a week.. unfortunately. I think in going to wait. I've already made a couple kinks that will need to be cut out and coupled.

Yikes, these be the type of hold ups I was concerned about.

On mine I was building pretty much exact to Matt's plans with the focus on making the internal spaces the right size; since the bricks I can get here are not quite the same size as the US ones the outside dimensions differ a bit.  As I have no reason to suppose I need to take it apart completely, I used some bought-in refractory mortar which is, I believe, used to make brick ovens and barbeques to build the body of the stove.  The last couple of layers are done with sand-clay mix so I can get the top off for maintenance should it be needed.  I've also got a tube of some fire cement stuff which I think is intended for sealing metal flue pipes and the like.  That's proved handy for replacing some of the top-edge bricks which didn't want to stay put with the clay-sand mix.  To be fair, I made snazzy 45° angles on the front corners which means small triangular brick cuts which don't have much area to bond to

Austin Shackles wrote:On mine I was building pretty much exact to Matt's plans with the focus on making the internal spaces the right size; since the bricks I can get here are not quite the same size as the US ones the outside dimensions differ a bit.  As I have no reason to suppose I need to take it apart completely, I used some bought-in refractory mortar which is, I believe, used to make brick ovens and barbeques to build the body of the stove.  The last couple of layers are done with sand-clay mix so I can get the top off for maintenance should it be needed.  I've also got a tube of some fire cement stuff which I think is intended for sealing metal flue pipes and the like.  That's proved handy for replacing some of the top-edge bricks which didn't want to stay put with the clay-sand mix.  To be fair, I made snazzy 45° angles on the front corners which means small triangular brick cuts which don't have much area to bond to

This sounds like a sensible plan, to me. Mine can be taken apart fairly easily, and I have a second bag of fireclay, so I'm okay to mod, if needed. Although yeah, this experimental stuff greatly increases the chances I'll have to make mods, particularly reaching down from the top layer. I have some leftover fire cement from previous pipe fitting projects.. I am curious to see how the thick mortar joints hold.

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Okay, so I did an update in the last week, or so I thought. I think the photo uploading process on permies doesn't seem to be working with my weak Internet connection. Thus, I will write text, then post photos, to avoid losing my writings.


The pipe bending process was unfortunate. It took several hours, resulted in a couple kinks from accidentally using the wrong tool, and ultimately ended up being barely passable when I used the "right" tool. I got a ratchet pipe bender for 7/8" copper. It turns out it is not nearly heavy duty enough, despite having some thick alloy pieces. It broke when I attempted the first 180. I then was stuck with a broken tool to do the rest of the job, or come up with a solution.

I decided to try to use the tool and I set up a system which used a angle iron lever and clamp to secure the snapped piece, and my body weight to use the ratchet. Getting setup took quite a bit of mental effort and physical skill. Even as such, the result I consider barely passable. I would not use as a thermosiphon in most cases.. in this case I have a big vertical and decent differential, so I think it will overcome the "hump" and small kinky aspects.

For pipe bending, I think if I were to do it again I would only attempt with a proper hydraulic bender setup, or pay a fab shop to do it. Looking back, and given knowing the difficulty, I think I would have paid $150-200 for a job well done, or have purchased a proper bender(rated for higher, much higher, esp if Chinese) and resold on marketplace. On the onset I thought I could do a decent job for $0, and a couple hours, but it ended up costing $60 and a broken tool, plus 4hours+ of time, for a meh product. Pretty substantial fail.

The lower bypass ended up working out pretty well. I used sandwiched cement board in a steel frame with re-enforced edges. I threw it in a burn pile I measured to be around 450-500F and it held up fine. The cement board probably will eventually get quite brittle, but I think the metal should be okay in that lower heat zone, down low.

All the angle iron supports on the left side of the stove made the out of level situation to be worse than the onset. In the 5th course(the one before the core), I had to have some massive mortar joints in the 'low' corner to level out the core platform. Probably around 1/2".

The oven was fitted with supports for racks that I salvaged. They fit in decently, and are easy to remove. The oven opening size ended up being 12.15-12.25in high, and within a quarter inch of square/plumb. This works out quite well given my CFB piece. It fits in the opening with a tiny bit of extra space for me to add a skin.

I found the 4th and 5th layers to be quite difficult, lots going on including cutting passageways for pipe, chimney pillar squaring, many cross supports.. there was plenty of ways to mess it up. I ended up re-dry stacking it before using mortar and taking a good look. I think that I got it right. Many of the CSAs are landing coser to 35-37in, which seems to be within tolerance.

Okay, now I will attempt to upload photos. There is a chance I will have to wait until I visit civilization to get wifi.

Pictures