EZ No Tools Brick Micro Batch Box Core (The Match Box) Plus Variations

Hi John -
Nice masonry job - for a fellow who is just doing his first project!   I'm watching with great interest - and plan on getting supplies this week.  It appears that you are building this INSIDE a pre-existing fireplace.  Is that correct?  (The shape and size looks right - but I don't see any evidence of a fire being burned in it - charring, black marks, etc).
You seem to have carpet pulled back, away from the block.  Will you replace it "around" the block outline, or just move it out of the way?  (I have a carpet 'problem', also)  ;D

Do you plan to post the size and amount of blocks and bricks you used?  It would be a great help!
Thanks!
Linda

Help please?  I lost the link to Peter van DeBerg's calculations.  Will someone send it to me?  Thanks!  
Linda

John I would lay out a dry stack of your bench and then build your rocket stove exhaust to that level for simplicity. if your bench lines up with the bottom of your bell then go with that, but if your bench lines up lower then build it that way.

i have used a chisel when i wanted a different block configuration on my rocket stove without breaking the blocks with ease. I would take the extra blocks as a sign to make your wood magazine LARGER! build it for top wood feed and Make sure you put in a bottom front air flow port and you will be happier with it. I have seen reports of batch box overloading and when batch boxes burn, they seem to have been documented to burn all at once without that desired bottom burning effect you get with the attractive rocket stove fires.

Linda Questor wrote:Hi John -
Nice masonry job - for a fellow who is just doing his first project!   I'm watching with great interest - and plan on getting supplies this week.  It appears that you are building this INSIDE a pre-existing fireplace.  Is that correct?  (The shape and size looks right - but I don't see any evidence of a fire being burned in it - charring, black marks, etc).
You seem to have carpet pulled back, away from the block.  Will you replace it "around" the block outline, or just move it out of the way?  (I have a carpet 'problem', also)  ;D

Do you plan to post the size and amount of blocks and bricks you used?  It would be a great help!
Thanks!
Linda

Thanks, this is not my first one, but my first install with an external mass.
I am not building into an existing fireplace, although there are existing stacks and exits here and there.   I'm installing it into a 20x20 concrete geo-thermal earth-berm, which is like basically a basement (mostly underground) which faces south.  The walls and floors and even the ceiling/roof are all 8" concrete.    It's like a bomb shelter/ root cellar/ earthship hybrid.   There was interlocking floor mats and then carpet on top, but it's all easily adjustable, it's only an area rug.   I used 18 insulative Fire bricks for the riser (so far) and 19 dense firebricks (for inside wood stoves) on the firebox itself.   All the firebricks are identical size.   Approx 4.5"X 9" and 1.25" thick are the size of the bricks.  So I've used about 37 firebricks on the core in total. Plus I've used 12 regular hollow 4" concrete blocks for the hearth/core base.   I have about 20 more 8" hollow concrete building blocks to use for my mass bench which I haven't got to yet.   I also bought 2 mortar types, sand, pipes, and 75 paver stones, I have not got to lay the pacer stones yet either.   I've got maybe 200$ into this project so far give-or-take.

F Styles wrote:John I would lay out a dry stack of your bench and then build your rocket stove exhaust to that level for simplicity. if your bench lines up with the bottom of your bell then go with that, but if your bench lines up lower then build it that way.

That's a good idea I suppose, I'm best at working and figuring as I go also.   Kinda my usual and preferred M.O typically lol

I would let the gases leave the bottom of the bell in both locations you show, with a plenum under the bell to allow them to easily move around and reach the mass channel(s).

I don't think putting a duct inside the blocks is a good idea; it will reduce the heat transfer considerably, and as calculated above, you want more internal surface area than one 4" or 5" x 5'+ round duct would provide. I am sure you will get better heating, and think that you will make better use of the heat, with one or two block cavity channels through the bench. If you make it to flow through both channels and find that it is extracting too much heat, you can always block one of the channels. If you use one channel and find you have leftover heat, it would be hard to retrofit the second channel.

Linda Questor wrote:Help please?  I lost the link to Peter van DeBerg's calculations.  Will someone send it to me?  Thanks!  
Linda

http://donkey32.proboards.com/thread/734/peterberg-batch-box-dimensions?page=1

I have not made huge progress, between work and chores I'm trying to take my time and do things nicely.   But here's a few more photos from last nights' progress.   I've also done some measuring and planning for my bell barrel.  What's the closest barrel you've ever seen to a wall or brick wall?    Mines going to be only 2.5" from the back wall, if I leave the hearth where it is.   It's not attached to the floor anywhere at all so I might be able to carefully slide the whole thing ahead if I must...  
What's the closest bell/ wall tolerance you've ever seen?   Is 2.5" okay for a 14"x26" bell to be only 2.5" away from an interior concrete block wall?  

I see no issue with a bell close to a masonry wall, as long as there is enough room for air to circulate behind it. 2" minimum for a round barrel should be no problem.

Glenn Herbert wrote:I see no issue with a bell close to a masonry wall, as long as there is enough room for air to circulate behind it. 2" minimum for a round barrel should be no problem.

Whew* thank heavens.   I only have 2.5" but my system and bell are small, and I'd like to warm up that wall anyway to provide more mass and heat to the back room.   Since this is an interior wall there is another room behind this wall.   Thanks again Glenn for the helpful info.  

I'm going to lay some more pavers down tonight likely after work and I'm thinking about using the regular mortar but I do still have quite a bit of the pyromix, less than half remains of the 4L bucket.  

I think I will simply build an exhaust pipe collar onto the bottom/side of the bell barrel like I have done with my other two builds, but more downward and less horizontal, perhaps diagonal, this will make it easier on me to seal the exhaust system and I don't have to worry about my pavers and novice mortar job being air-tight.   I think I've got it all mapped out now.   I will show some progress when I get further along.  

Satamax - Thanks for the link!

John - Did I miss some of the discussion?  I thought Glenn had recommended that you use only the openings in the block as 'ductwork' - instead of putting a pipe through the holes - since heat would transfer better to the block without the pipe.  Soooo... Perhaps this pipe is meant to form only the EXHAUST pipe from the bell up and outside?  Is that right?  Thanks again - lovely work!    ;D

[quote=Linda Questor]Satamax - Thanks for the link!

John - Did I miss some of the discussion?  I thought Glenn had recommended that you use only the openings in the block as 'ductwork' - instead of putting a pipe through the holes - since heat would transfer better to the block without the pipe.  Soooo... Perhaps this pipe is meant to form only the EXHAUST pipe from the bell up and outside?  Is that right?  Thanks again - lovely work!    ;D[/quote]

Thanks for following Linda
Most of my work is experimental so it's only for entertainment, to watch.   I can't tell you what materials to buy or what to build, or what to do with your carpet.   Sometimes I seek advice or second opinions here but ultimately I sign the final blueprint and test my own theories.   I've tested some of these parameters and I believe as discussed with Peter, without the duct my mortar would have to be air tight, and most of my heat would be lost, stalking my final heat-risen exhaust stack.   I need some heat to reach my final stack in order to keep the rocket draft.   I've experimented running hot air throu cold bricks here and the heat loss via thermal conductivity is too much when the bricks are cold.   A simple insert duct will keep the flow smooth and warm.   I only have maybe 250F coming out of the barrel exit and I'd rather not take that risk.   Also my mortar may not be air tight in the mass because I'm only a novice brick-layer.   Too many risks.   I have a plan and a vision and I enjoy doing my own designs, even though I often ask for advice.   I've tested many of these parameters already.   Most builders use the duct in the mass and I will be no exception.   It will help with flow, and temperature regulation.   Thanks for following my build Linda, if that's your name.

I might take some of the advice from Peter, and some from Glenn, and some from my own feverish imagination.   I like Glenn's idea of running multiple mass chambers, and with 5" I will still get lots of heat transfer via thermal conductivity.

I felt that one more coarse of riser bricks would be too tall, and my gap between the riser and the bell would be too small, but I also felt that it was not tall enough for my preference, so sticking with the "no tools/no cutting" core idea, I put two more uncut bricks on the top, leaving the side gaps open.   It's different than any of my other risers, asemetrical but shouldn't cause any issues.  

I've also started making double-pavers, 4x4x8 sandwiched bricks.   These double-thick bricks will be mortared to the hearth base behind the core, and will support the back-side of the bell barrel.  I will show photos later when I finish some more progress.    Clearly the core is only a fraction of the work involved, and I will require tools for the install, but I'm happy with the "no tools, no cutting, no dust" core design.   You can see this core functioning wonderfully in episode #50 of my Rocket stoves YouTube series, with flames up to the top of the riser with the Ram's horns' double vortex.  

More coming soon...

Hello John -
Sorry if I ask 'stupid questions'...but it's the only way I can understand how the build works and 'why'.  And Yes!  I am "Linda" - I'm an almost 78-year-old grandmother who prides herself in learning as much as I can - and using as much as I learn.  I'm really concerned about our future in the U.S.  I think there is a very good likelihood that we're going to be in really big trouble in the near future.  I'm trying to plan ahead for all possibilities...and that includes staying warm in the winter if we lose power in our rural area.  I've developed a lot of skills in my lifetime - as a lady home builder (designing homes, drawing blueprints & being on the construction site every day), as well as numerous arts and crafts, including woodworking - and knitting and quilting (my 'grandmother bit').  But HVAC, plumbing, and metalwork (including welding - wish I knew how to do THAT!) are my weak points, so I hope you don't mind if I follow along, and occasionally ask a 'dumb question',  ;D  Thanks again for keeping such a wonderful log of your ideas and work.  I'm sure you're going to create a great batch box core....and I hope to be able to duplicate what you do.
Best wishes -
Linda

Linda Questor wrote:Hello John -
Sorry if I ask 'stupid questions'...but it's the only way I can understand how the build works and 'why'.  And Yes!  I am "Linda" - I'm an almost 78-year-old grandmother who prides herself in learning as much as I can - and using as much as I learn.  I'm really concerned about our future in the U.S.  I think there is a very good likelihood that we're going to be in really big trouble in the near future.  I'm trying to plan ahead for all possibilities...and that includes staying warm in the winter if we lose power in our rural area.  I've developed a lot of skills in my lifetime - as a lady home builder (designing homes, drawing blueprints & being on the construction site every day), as well as numerous arts and crafts, including woodworking - and knitting and quilting (my 'grandmother bit').  But HVAC, plumbing, and metalwork (including welding - wish I knew how to do THAT!) are my weak points, so I hope you don't mind if I follow along, and occasionally ask a 'dumb question',  ;D  Thanks again for keeping such a wonderful log of your ideas and work.  I'm sure you're going to create a great batch box core....and I hope to be able to duplicate what you do.
Best wishes -
Linda

Nobody said "dumb questions"

Like I said before, most of my projects are experimental so I wouldn't "duplicate" any of my work.   I've tested most of this design and core and flow rates but as I said, my stuff is just for entertainment.   I'm glad you like this stuff and I agree about being prepared and self reliance.   If you want to construct a rocket mass heater, that's great also but you should perhaps use a "traditional proven design".   I would not encourage anyone to duplicate my build(s) until they are tested and finished the research and development stages.   That take multiple years or seasons to say "okay this build has endured x many hours and reached X degrees temperatures" etc.    There are several designs that are lovely out there and they have been tested for years and the builders have shared thier pros and cons and suggestions.   I would suggest something traditional and proven to someone interested in constucting a rocket mass heater.  

Alright back to the match box, here's a video, and a photo of some double stacked bricks behind the core.  These will support the bell.

Ep#51



The gap beneath my level is where the exhaust will transition from the bell to the mass bench.   I have not even started the mass bench yet because i have to remove my leather convertible futon-sofa.   Yes I'm going to remove the comfy leather sofa for some bricks.   Does that make sense?   I hope it will be easier on my heating bills and I can always make it more comfy as seen in the photos with cushions and such.

Okay I started working on the bell today.  It turns out to be different dimensions as my other lube drum build, which I used to reduce maple sap last spring.   This one is not 14x26.   This one is 13.5x28".    I don't think it will make a big difference from my previous 14" bell & brick riser tests I've done.    After cutting out the bottom to accommodate the riser and "kicker brick", I'm not left with much room on the bottom.   All my previous builds used side exits but I was hoping to do a downward exit on this one.   I may not have the room to do a downward exit from this particular bell but I will see what solutions I can come up with.

I have seen many people use these hvac duct boots on thier transitions so I figured one of these would help me on my transitions and I could put it on the outside-lower area of the bell pointed downward, where I can 90 into my mass area.

Why exit from the barrel/manifold so high, then go down to the duct? It would be less displacement of airflow to go all the way straight down under the drum and then horizontal.

Also, your sketch looks like you are thinking of putting the duct in the lower row of blocks in the bench. Is this true? I believe that the heat from there would barely ever make it to the top of the bench. Running the channel in the top row in the bench would let you go straight from the base of the barrel manifold without cutting into the base layers of 4" concrete blocks.

You are absolutely correct Glenn.   The transition exiting the bell should be as low as possible.   I agree.   It was just a crude 10 second sketch to show the duct boot transition idea.   I would also agree with the mass duct, it should run through the top row(s).   Excuse my sloppy sketch   I mainly wanted to roughly demonstrate this bell exit transition visually.   But I agree 100% with pretty much everything you said.  


I'm using 8" blocks on my mass bench.  Do you think the 5" hvac duct boot would suffice?  There is a 6" but it's larger and would require a 5" adapter post-transition.   I think the 5" duct boot should work but I had mentioned a 6" transition earlier in the build.

I suppose I could exit out of the bottom of the barrel but there isn't much room there left on the barrel bottom, and I'd have to make sure all my external paver stones are mortared air-tight/no gaps.   I'm almost out of hi-temp mortar but I still have lots of regular mason mortar.   I agree a single 90 degree downward-outward transition would be best but I'm not sure how I will transition quite yet.   I might just do the mortar seal on my base and pavers and do it like you said but I will have to check my flow clearances and such before I decide.

Here's an episode of burning the paint off the barrel.
Ep#52

There you go with all that safety gear again. no one lives dangerous anymore. geesh.

I  agree, go lower on the bell and higher on the bench. using regular cement on the out side should be fine, even close to the bell. keep in mind you may off set your bell to one side and it will work fine, especially if you want to come out of the bottom of the bell to one side. in all seriousness i think that would work best to come out the bottom side and off set your bell and cement in a duct out the bottom. the lower the exhaust the more push pull you have and make your manifold larger than your system core.

I agree with Linda, really rough times are coming and one of the reasons i built a rocket mag stove that heats my water. i can heat my water with my rocket stove since my water is gravity spring fed non pressurized. when i get the water heating part set up, i will take pictures.

I agree one day the power might go out, or propane might not be available, like the zombie apocalypse for example, or maybe an asteroid bombardment from nibiru gravity pull, either way it would be nice if you could somehow provide your own sustainable heating and cooking fuel, and hot water will be a luxury in the xombie apocalypse FStyles, so you will be king lol.   Anyway I thought you already the water system running?

I didn't get much done last night aside from burning the paint off the match box bell, I also sanded and painted it afterward partially with hi temp stove paint.  Having a painted bell + an external mass bench, are going to be two new experiences for me this build.  My original compact RMH sits outside and it's pure steampunk rust patina lol since I never painted it with hi temp paint.  I might fire it up again someday.  

Now that the match box bell is mostly prepped and cut I can actually set it ontop my core and riser and see where I'm at with a moch-up.  I have yet to do more mortar and ensure a good seal at my core/base.  We just went through daylight savings fall-back and it seems like my days are shorter now.   Dark by 530-6 now here in Ontario Canada.  I will have more free time this weekend and I will likely start on the mass bench and exhaust, maybe more by Monday.

Speaking of the "offset bell" FStyles.   I had miscalculated the bell diameter by 1/2", and I have a kicker brick inside my bell, outside my riser, so I did end up offsetting the bell forward about 3/8" or 9mm.   Im used to the common 14"x26" lube drum, but this one is 13.5"x28".  The tolerance between the bell and riser will be closer than I expected , but I do not anticipate any major differences from the previous 14" bell test fire which I have already tested to ensure functionality.   That test was episode 48 I believe, in my rocket stoves YouTube series.   I tested the brick rectangle riser with the 14" diameter bell and it worked amazingly well.   So using the same riser configuration and dimensions, and a 13.5" bell, shouldn't be much performance differences.

Anyway I will definitely got some stuff done this weekend with much more free time, so keep an eye out for progress and such.

Happy Remembrance Day

Anyway I thought you already the water system running?

I have a water tank at the end of my line to suck out any heat that is left over. Needing 140 F in the up stack is a bunch of bull. its 100 F in my up stack and my system runs amazing.

the last part of my horizontal run will heat the water tank in my last bench to about 70 F only and 80 F if i really push the system. its definitely better than the 45 F water that comes from the spring although not living like a king. My 2nd water heating solution will have a copper line coiled around my bell to heat another water tank for really hot water. my spring gravity water is a perfect set up for rocket mag heating. i will have multiple safety valves on it for sure just in case of a catastrophic failure of magnanimous proportions. when i get it built i will post pics.

Yeah I bet coils would produce much higher temperatures.    I had considered making a coil when I had considered putting the RMH beside my hot water tank, but I decided not to place the RMH in that location.

Anyway I removed the leather futon sofa today and that's where the brick bench is going.   I brought some bricks inside to climatize and I'm pumping hot air through them to test the mass heat loss and thermal conductivity with a small electric heater on low.  I can feel hot air going in one end and only warm air coming out of the other.   The hottest end seems to be warming up the blocks slowly but Ive just started this experiment so I'm sure they will warm up eventually.   Some of the blocks have different sized and shaped holes/voids so I will have to keep that in mind when I permenantly set them in place.  For now they are going to be my couch tonight, since I removed my leather one, and I mostly just want them to climatize in prep for the bench build.   I will likely get some of this done this weekend if I get enough free time.

Check out some of these photos.
I mocked-up the the barrel bell for a simple test fit and visual.   And somehow I'm happy with it.  I was concerned when I realized it was 13.5" diameter but it turned out well, giving me slightly more room behind the bell and enough room around the riser

Barrel bell heat exchanger radiator, whatchamathingy photos.  Clearances and visual.  It's going to go a little something like this.   Yes I only painted part of the bell last night, and I did it in the dark, so the bell paint needs to be addressed still but this is only a mock-up test.

I can only upload 3 photos per post?   Is that correct?
Anyway here is a few more photos, it was a long day helping family so I have not accomplished a lot.  

I did another variation in the bench where the bricks are stacked ontop one another...  Measured out an approximate bench placement and marked the floor for positioning.   The floor is intentionally uneven, not level because of the floor drain, so I will likely have to use mortar on the floor if I want my bench level and flush.   I also bought four 5ft lengths of 5" hvac duct, three 5" elbows, one 5" tee, and adapter from 5" to 4", and some 4" sil-gasket.  I was going to use the sil gasket under the bench to help make it level, but now I'm considering using the mortar under the bench to help make it level.  I still have not wrapped my head around the transition area but it will come to me eventually.   Tomorrow (Sunday) I finally get a day off to myself so I plan on addressing some more of this stuff.

A few more development photos

my suggestion would be to come out of the bottom of the barrel which would make the exhaust hole a half circle shape for you so you can then go directly into your bench at the height you want without having to do more crazy bends creating more drag for your tiny system, drag for your system is bad. it would be easy to bend and configure your pipe to make a half circle and come down out of the barrel. from the pics i saw you may have to cut into the bottom of your barrel.

F Styles wrote:my suggestion would be to come out of the bottom of the barrel which would make the exhaust hole a half circle shape for you so you can then go directly into your bench at the height you want without having to do more crazy bends creating more drag for your tiny system, drag for your system is bad. it would be easy to bend and configure your pipe to make a half circle and come down out of the barrel. from the pics i saw you may have to cut into the bottom of your barrel.

That is correct, if I exit out the bottom side of barrel, I will have to cut the barrel again.  Here's some photos of where I'm at now.

I ended up using my sil gaskets different than illustrated.   I used two stacked gaskets at the front of the bottom row, and one single gasket at the back of the bottom row.   That made the front slightly higher to compensate for a drain-level floor, and no mortar.

Placing the 5 ft pipe had to be done before the blocks.   With a wall on one side and a stove on the other, it would have been impossible to slide the pipe in later.   This part was tedious and I only used a single sil gasket on the front (cold side) of the top row in the bench.   This helped with the level/consistency of the the entire bench. I ended up sliding the pipe into the first two top row bricks closest to the stove, this part required planning to figure out because of the lack of room to slide the pipe in afterward.   I lubed up the pipe with vegetable oil and a paint brush.   I didn't want to scratch off the galvanized coating on the rough bricks, so I prepped one of the voids in each top row brick with a file, and made sure the pipe path was smooth and such.   It was a pain doing it this way, but after planning and lubricating the pipe with a non toxic vegetable oil, I managed to slide all the bricks over the pipe without disturbing the sil gasket.   It was not fun but I did it.

So far I have used zero mortar on the mass bench.  

More pics

I purchased a stove-top tea kettle for my caffeine addiction.

I shoul have a tee between the bench and the mass I suppose

After the mass, the duct will go vertical interior.  I need to build up to ground level before wall-exit, since this is building is well below ground level.  One more brick needs to be prepped and placed before the vertical 90

i would design your exhaust to have a straight shot to be able to push a chimney sweep brush straight all the way to the base of the bell or you will wish you had later. after your bench at the 90 elbow where you change direction add a TEE instead of just an elbow so you can insert a straight chimney brush to clean it out good over the years of use. Not having any way to scrub it after years of use may get you into trouble. keep that in mind. I designed mine to have TEEs instead of elbows at every 90 degree bend so i could access all the straight aways.

I only have one tee that's why I asked which end to place it, but I suppose I could buy anothe and do both.  Although my end wall is less than 10" from where the exhaust will be at the bench end so a sweep wouldn't fit regardless either way.   I've never intended on using any sweeps, only a vacuum cleaner to remove the ashes.


I had intended a barrel side exit, I had not intended to exit the bottom of the barrel, so before I cut the bottom of my barrel and build any transition: IS THIS enough room for all the rockety airflow between the riser and paver stones?   I'd prefer a barrel side exit as intended but if this is enough room I will build a bottom exit and transition.   The transition is becoming the biggest planning challenge for me, aside from the duct boot plan on the side of the barrel, as I had planned and previously drawn and shared.  This bottom gap doesn't seem to be as big as my Venturi, (1.75x9") but it does seem larger than the front door intake hole (1"x4") so I'm confused whether this gap would be enough under the barrel.  Consider ashes and it's sketchy at best if you ask me.  I know the barrel side exits work well because that's what method I have always successfully used on my builds.   Input?  

they have small pipe brushes to scrub your pipes out in case you have any build up and or want to look in and clean it with a hose or such. I would suggest getting both, you will thank your self later. i would suggest you make the exhaust out of the bottom of the barrel, it looks as though you can make it large enough with available space, exhausting there will have less bends and work more efficiently while matching your bench much better.

John McDoodle wrote: my end wall is less than 10" from where the exhaust will be at the bench end so a sweep wouldn't fit regardless either way.   I've never intended on using any sweeps, only a vacuum cleaner to remove the ashes  

Well I put a 90 at the end of the bench and there is more than 10" between the pipe and the end wall.  

I also noticed with the mass testing with the electric heater and the bench, that the bench is not getting as hot as it was withdout the duct.   I mentioned previously it would help keep my draft and final stack temps warmer for optimal flow, and I was correct.  the duct holds the heat slightly better in the exhaust, but the bench still heats up.  

I will be exiting out of the bottom of the barrel and I will try that first.   I don't like the narrow gas chambers under the barrel, and I usually exit the barrel side, but I will try the bottom exit first.  Trying a few new things on this one.

You may notice I'm taking my time on this build.   I do t want to rush anything and I don't get much free time due to work and chores and alike.  But it's coming along, slowly but surely

You have some space, maybe 1/2"-3/4", on the other two sides between the riser and the barrel support bricks, right? That space will add to the flow capacity, and I think it will make enough combined with the 1 3/4 x 8 main exit to work fine.

Glenn Herbert wrote:You have some space, maybe 1/2"-3/4", on the other two sides between the riser and the barrel support bricks, right? That space will add to the flow capacity, and I think it will make enough combined with the 1 3/4 x 8 main exit to work fine.

Yes the core Venturi is 9x1.75 and my transition is now 2x8. The air intake in my door will be about 1x4".

Yes I have the main outlet side which I increased to almost 2", and there is a long narrow gap behind the bell, but it's only about 3/4" maybe.   The opposite side has another 1" gap between the riser and pavers.  The front would only be able to flow around the riser.  

Thanks for easing my mind a little Glenn.  I was worried about the tight spots around the transition and fly ash therein but the design already has 3 clean-outs so it will all be accessible with a vaccumm cleaner from 1 the batch box core, 2 the removable bell lid, and 3rd clean out will be the tee-post bell, just before the bench.   I might even add a 4th at the end of the mass as suggested by FStyles, but my main concern was the transition after the bell.  It's good to hear that you think it should be fine

Just a question,how did you fit the duct so perfectly to the inside of the cinder blocks?

I think the majority of gas flow will be toward the largest gap, so most of the ash buildup will be beneath that. There will probably be some buildup at the bottom of the narrow gaps where flow is most sluggish, but that will not likely affect the flow unless it builds up deep. You might need to work up some sort of thin vacuum end to get down into those gaps if you need to do thorough cleaning... maybe a lightweight metal tube hammered to a flat oval, and duct taped onto the end of the regular hose.