New here, first post, I have been researching all different designs, and I came up with my own.

Hello,
I haven't been in the game for very long, but I built a rocket stove out of pavement bricks a few years back and was very impressed with the performance, so I have been looking into these casually for years. Recently I purchased my own place, and I have been slowly doing up the garden etc. I used to install various household electronics, so I know my way around a roof/floor cavity, and I noted when I moved in that my new house had ducted heating vents in the floor of every room, but no heater controls anywhere to be seen (heavy renovations before I moved in)

Long story short, they have blocked off the intake and the actual old gas heater is under a mass of ivy I recently poisoned, it is, as we Aussies say, cactus.

BUT, I do still have the ducting system and the vents... so I started forming an idea...

What if I built a mass heater type contraption outside, run some air ducting through the top section where most of thermal mass will be, above the bell section, and run it all back into the house with a ducted fan under the floor (I can get one with a remote control for about $100-200 aussie dollerydoos on ali express)

So, with all of this in mind, I really started researching these heaters, looking into the dimensions etc and started drawing.

I have a friend who is a legit mason, he could put this together from actual stones if he wanted to, but I was happy to just have it be made of brick and mortar for the most part, and I figured screw it, why not go for a pizza oven in there since there will be so much heat.

I went for J rocket design, because I have a few trees in my backyard, and I'm getting some fruit trees to plant around the place too, so I should have a ready supply of small sticks to dry out for the next year, and the existing trees are mountain ash, that stuff actually burns well fresh and wet, so it should be great mixed in with the other woods to spice things up when it's aged.

So, since this is the place to post the tinkerin's I figured I'd document the build and post it here for posterity as I go (it may be months to finish, I have to let the fire box set for probably at least a week, more like a month before I even start to test fire it, and then I have to build the brick mass around it with the air channels.

I drew up 3 angles of something I am fairly happy with, though I want to change a few things already so I may draw up a new set soon with things like a wood box or two at the front to store the next day or two of sticks in and keep them pre-warmed, put a masonry bell over the stacketc.

Fire box will be all fire brick and refractory mortar, since I won't be able to easily get to the chamber so I want it strong and long lasting, I also had the idea to mix the concepts of rocket stove and masonry batch heater, and restrict the flow out of the fire box and into the chimney, and then run a secondary air channel under the fire box, to a grate at the bottom of the riser, mixing with the expanding hot gas to hopefully burn again and get cleaner and more efficient.

I'll post the side picture, please note that this isn't the full 3D Insides, just a slice, the idea is that the smoke goes down out of the riser, along a wall at the back and front which has a few missing bricks at the bottom so it has a place to go, then back up and through a slightly winding path before going out the top of the whole thing, the front part is the same but circulates through the oven before the end, and that sits right on the hottest part of the fire box, with fire brick connecting those 2 sections to get the radiated heat from the hottest part for the pizza oven.

My biggest worry is that the thing won't work after I've bricked it all in for some reason, and also at the moment I'm wondering about the ratio I should go for with the secondary air intake. I'm, thinking about 1/4 of the diameter of the feed tube, and that will go to a grill to break it up a bit and also restrict it a little more just before the stack.

Any thoughts on making this thing run as well as possible before I actually build it would be greatly appreciated, diagrams are not fires and experience counts for everything in my book.

Thanks for reading the essay, I'll try to post regularly with updates on the build, so far I have literally only dug out the hold and framed out the place where the slab will go to start things off.

Also, the diagram doesn't include the air ducting yet, I figured I had better build the actual J tube and make sure it burns as clean as possible first!

Cheers from the land downunda.

Just the first couple of actual pics to whet the whistle.

The reo is only in the hold to size everything up, the piece I managed to find was almost exactly the size of the slab I wanted to lay anyway, only leaving about 100mm on the short ends over the actual reo, it almost goes to the edge on the long ends.

Slab will be 2m x 1.1m or so.

Oh, and I sourced a door for the oven today, someone was selling an old fireplace cheap. The look on his face when I said I'd unstitch the welds with a grinder and take it apart for the thick steel, then just use the door, was priceless.

Hi Miles;
Well, you get an A+ for thinking outside of the box!
This may work just as you are hoping but only time will tell us.
It may work well at first but then slowly get sluggish as ash collects.

We encourage experimentation.
I would only use a clay-based mortar with no concrete mortar at all.
When you experiment with Rocket Science, you should expect to need to tear down and rebuild if/when design flaws appear.

Good Luck, and keep posting pictures

thomas rubino wrote:Hi MM;
Well, you get an A+ for thinking outside of the box!
This may work just as you are hoping but only time will tell us.
It may work well at first but then slowly get sluggish as ash collects.

We encourage experimentation.
I would only use a clay-based mortar with no concrete mortar at all.
When you experiment with Rocket Science, you should expect to need to tear down and rebuild if/when design flaws appear.

Good Luck, and keep posting pictures

Thanks, and that is exactly why i am not building the whole thing in one go, that would be a fool's errand. I'll make sure the rocket part works as expected with a few dry runs before I put it together, and I have purchased the real stuff, 2000c plus rated refractory mortar, my masonry mate said it would be probably the best stuff to use for strength etc, he kept saying stay away from clay, not exactly sure why since it seems to be preferred in these circles, but I figure the stuff made for the job would be best, so I got 30kg of it to make up the firebox with about 40 fire bricks.

Very curious as to why he says stay away from clay though, may I ask why you recommend it? I think I'm speaking to one of the experts here, so I'd love a bit more detail in that regard, and I'm hoping the refractory mortar is not cement based, I would hope not being made for that kind of temp, but now I have to go check the bags! I might take a pic of the bag to post here even.

Hoping I can get a sweeper in the secondary air channel to get ash out of the chimney and air channel both, and there is an access in the back bottom for cleaning as well, with hopes that I can at least sweep out the worst of the debris if it does build up, and keep it going for as long as possible.

I haven't heard of anyone running air duct work through one of these things before though, which makes me wonder why.... I have either thought of something new, or something someone else has tried and it didn't work, that is the biggest worry at the moment.

Thanks for the reply.

Ok, it is cement based, and I may have exaggerated the rating, says 1000c on the bag.

It does say perfect for pizza ovens etc though, and my mate said all good, will this thing get over 1000c by chance?

Pics of the bag so yall can scrutinize, I don't want to mess this up!

Metal Monkey wrote:My biggest worry is that the thing won't work after I've bricked it all in for some reason, and also at the moment I'm wondering about the ratio I should go for with the secondary air intake. I'm, thinking about 1/4 of the diameter of the feed tube, and that will go to a grill to break it up a bit and also restrict it a little more just before the stack.

From what I've seen with our in-house "batch box" RMH builds, a secondary air source somewhere is a good idea. If you're already amenable to it, I'd suggest you build it in, bigger than you think it might need to be, then find a way to adjust the air flow at the intake. For example, you can slide bricks over the opening, or if you're really invested in the design then you can fashion a sliding door of sorts that adjusts the air flow as needed during different stages of the burn.

I think the suggestion for using clay in this build is so that, if things don't work well with this design for some reason, the tear-down is easier and you're not left with concrete waste. For instance, we build everything here with fire brick, which is modular, along with cob, sand, stones, and gravel. All those things aren't necessarily locked into a certain form factor and can be re-used multiple times as learning occurs and things are rebuilt. Using natural materials is one of our core tenets as well, so at least on our site we prefer to adhere to that standard.

Thanks for including that diagram. Best of success on your project...!

I am not a rocket enthusiast like many here but I wanted to welcome you to Permies none the less!

I'd love to see your project as it continues so consider keeping a running record here if you wouldn't mind letting folks live vicariously through your photos.

Timothy Norton wrote:I am not a rocket enthusiast like many here but I wanted to welcome you to Permies none the less!

I'd love to see your project as it continues so consider keeping a running record here if you wouldn't mind letting folks live vicariously through your photos.

That was the plan, I don't think I'll have the energy to take video of the whole thing, though I spose I could technically point the security camera over there and just download the footage at the end of a work day for timelapse or something... now you got me thinking....

To Stephen:
I just went though a video from a guy who was saying don't use cement because it won't expand and contract, and it will crack early and break up, plus clean up on the fire bricks if they are re-used. Bugger, Spent a bit of money on those bags, though I guess I can still use that further away from the fire box, or mix it with the bottom layer of what will probably be perlite/vermiculite concrete (haven't decided yet) for about 2 layers of brick. Clay/sand and a little ash seems to be the order of the day for the fire box I see. I'll look into getting a bag of clay, the sand is easy, local hardware has it for concrete mixes. This will make for an interesting conversation with my mason mate though, literally the first thing he said was "stay away from clay for the fire box" haha.

Thank you for the replies, I will at least post pics here as they come.

EDIT, oh, and the air intake, yes, I was thinking the exact same thing as you, like maybe make it 1/3 of the burn chamber diameter, and then add a steel sliding baffle on the front or something, more control over the 2nd burn that way, should be able to tune it a bit better. Thanks for confirming my thoughts are probably the way to go!

Hey Miles;
1000 C  = 1832 F
The riser of a J-Tube runs in the 1800F range; do not use your refractory there.
The core bricks will not get that hot; your mortar will hold on to them, but...
As Steven mentioned, it is semi-permanently attached if you use anything other than a clay mortar.
With respirators and chisels, refractories can be removed, but it sucks to do so.
Clay-based mortar pops right off into a bucket. Add water to rehydrate it, and use it again! Try that with a cement-based mortar.

All mortar will crack, even clay.
When clay mortar cracks, a wet finger dipped in a small amount of clay and sand will fix it immediately.

thomas rubino wrote:Hey Miles;
1000 C  = 1832 F
The riser of a J-Tube runs in the 1800F range; do not use your refractory there.
The core bricks will not get that hot; your mortar will hold on to them, but...
As Steven mentioned, it is semi-permanently attached if you use anything other than a clay mortar.
With respirators and chisels, refractories can be removed, but it sucks to do so.
Clay-based mortar pops right off into a bucket. Add water to rehydrate it, and use it again! Try that with a cement-based mortar.

All mortar will crack, even clay.
When clay mortar cracks, a wet finger dipped in a small amount of clay and sand will fix it immediately.

Would the mortar be better suited to the perlite mix for the bottom insulation layer to prevent my slab exploding maybe? or save it for the pizza oven possibly where it will be nowhere near as hot as the firebox or riser?

EDIT. I want to make this thing as much as possible to last without needing to be torn apart, so any durability advice with the hot parts is appreciated.

This is good, I'm learning, thank you.

Ok;
Here is what I recently learned about mixing refractory.
You must mix the entire bag at once; any less, and you will not get the proper mix of ingredients.
The amount of water used is critical, close enough will not work.  I weighed the water I added to my refractory to be sure I got it right.
As I was warned, it would seem way too dry but absolutely do not add more water.
There was no "pouring" of the mixed refractory. Instead, handfuls were stuffed in our form boxes and then placed on a homemade vibrating table, which was vibrated using a variable-speed saw. (saws-all)
Care must be taken not to over-vibrate as well.

Could you return that mortar?
If not, the pizza oven would work fine, or as the slab.

Clay mortar is made with one part dry clay and three parts dry sand.
Fireclay is suggested, but other bagged clays may work.
Screened medium-graded sand is highly recommended.  Concrete sand is not usable without significant work.
Free local sand is usually too fine and always has rocks and debris.

Here is my thread about mixing.
https://permies.com/t/254174/Casting-Large-refractory-Slabs

thomas rubino wrote:Ok;
Here is what I recently learned about mixing refractory.
You must mix the entire bag at once; any less, and you will not get the proper mix of ingredients.
The amount of water used is critical, close enough will not work.  I weighed the water I added to my refractory to be sure I got it right.
As I was warned, it would seem way too dry but absolutely do not add more water.
There was no "pouring" of the mixed refractory. Instead, handfuls were stuffed in our form boxes and then placed on a homemade vibrating table, which was vibrated using a variable-speed saw. (saws-all)
Care must be taken not to over-vibrate as well.

Could you return that mortar?
If not, the pizza oven would work fine, or as the slab.

Clay mortar is made with one part dry clay and three parts dry sand.
Fireclay is suggested, but other bagged clays may work.
Screened medium-graded sand is highly recommended.  Concrete sand is not usable without significant work.
Free local sand is usually too fine and always has rocks and debris.

Here is my thread about mixing.
https://permies.com/t/254174/Casting-Large-refractory-Slabs

To my mate's credit, as soon as I told him why not cement, he agreed. He's a mason after all.
Looking into some fire clay and sand, I thought concrete sand was best because it was sharp grains, was I lead astray? (that info was from the same video I watched that told me why not cement by the way, so he mixed some on the spot with fire clay and concrete making sand and recommended it) What would I be looking for as a purchase if that is the case? Would landscaping sand etc fall under that category or should I just get a bunch of random sand and a medium gran sifter and make my own?

ok the word i was missing: Silica.
I seem to have found some pretty good localish sources, and one of them had the clay as well.

They have the sand in 100, 200 or 350 mesh, I assume 200 is medium but would a little 100 course mixed in work better do you think? I will get a bag of each if that is the case.

Clay:
https://www.blackwattle.net.au/RMFireC/FIRE+CLAY/pd.php?ccode=FIRE+CLAY+20kg

Sand:
https://www.blackwattle.net.au/RMSilica200/SILICA-200/pd.php

Price is right, though I haven't checked delivery yet because it's one of those that wants a full login before they will tell you, so if that suits I'll check shipping on the both of them.

Um.... I found something worrying for it being sold in Australia, but wow, this might be a good liner for the fire box and chimney...
NUCLEAR REACTOR FIRE RETARDANT GOOD TO 2700c!!!

WTF is this stuff?? I don't even know if I want it in my yard! haha

https://www.artisansupplies.com.au/product/rtz-washcoat-1kg-nuclear-reactor-cladding/

Miles Teg wrote:Um.... I found something worrying for it being sold in Australia, but wow, this might be a good liner for the fire box and chimney...
NUCLEAR REACTOR FIRE RETARDANT GOOD TO 2700c!!!

WTF is this stuff?? I don't even know if I want it in my yard! haha

https://www.artisansupplies.com.au/product/rtz-washcoat-1kg-nuclear-reactor-cladding/

ok i found the msds for this... I wouldn't want it in my backyard for sure...  I saw someone on reddit posted using it to line his pizza oven 3 years ago...

i reckon he dead now... no one posted any replies and neither did he! I wouldn't want to chow down on a pizza made in that crap!

nope nope nope

https://www.stonhard.com/ProductFiles/stonhard/RTZ%20Skim%20Coat%20ISO%20SDS%20-%20en_US.pdf

Nope nope nope nope... sample: Fatal if inhaled... he dead now.

https://www.reddit.com/r/Blacksmith/comments/so84wi/i_bought_some_rtz_wash_coat_from_mathews/

not confirmed, but he dead now.

Well , it is guaranteed that advice coming from this community on Permies will keep you on the side of the angels .....not up there playing along on your own harp!

Seriously though, I know many of us are influenced by videos or advice from at times. But honestly mate, you can't go wrong by consulting folks here.  Emphasis on safe experimentation,  using mostly or completely natural resources,  and suggestions with a smile. You can trust folks here, specifically Thomas, Fox, Gerry, Glenn, and The ultimate friendly genius Peter

In other words:  do not look at those shoes behind the curtain, I am the real Oz!

Hey Miles;
I agree. No, No, No on the nuclear stuff!
Yes, the fireclay and silica sand that have been screened are perfect.
You need 3x of the sand than you do of the clay.

I doubt it is available there, but this is the sand that I use.
https://www.homedepot.com/p/SAKRETE-100-lb-Silica-Sand-65200396/207136556

Rico Loma wrote:Well , it is guaranteed that advice coming from this community on Permies will keep you on the side of the angels .....not up there playing along on your own harp!

Seriously though, I know many of us are influenced by videos or advice from at times. But honestly mate, you can't go wrong by consulting folks here.  Emphasis on safe experimentation,  using mostly or completely natural resources,  and suggestions with a smile. You can trust folks here, specifically Thomas, Fox, Gerry, Glenn, and The ultimate friendly genius Peter

In other words:  do not look at those shoes behind the curtain, I am the real Oz!

I'd already be a buildin'
and explodin' and a killin'
if I only had a brain.

Thankfully I recognize that brains can sometimes get in the way, really, that's why I'm here, so any advice anyone has to throw in, happy to listen.

Still looking for a supplier of silica sand and fire clay that doesn't charge 3x the price of the goods for shipping... Australia... we're big and empty...

did manage to source a place selling the ceramic blanket in 25mm 7.5x.5 sheets which might suit well for the chimney insulation, with some to spare for an idea, since this thing will evolve until I actually build it, based on suggestions here of course, I was thinking on the cleaning aspect, but it may reduce the efficiency of the heater, though I have an idea there too.

Anyway, thinking on the house side (on the drawing I posted you would be looking from the house, it's the long edge) in the middle, I was thinking of an access door of some kind, big enough to get at least half a body into so I can get around the chimney somewhat for cleaning. Like I mentioned previously, I want this thing to last for quite a while, and getting in to replace the fire box and chimney if something should go wrong sounds like a good idea to me, I wasn't sitting well with the whole thing being completely encased in brick with just brush and shovel access hatches where I thought ash would accumulate the most, but I purchased a whole wood heater cheap because the top has rusted out, but i mainly wanted it for the glass fire rated door on the front for about 1/6th of the price of a new one with no fireplace around it. EDIT: I didn't make this clear, the glass door is for the oven!

I was thinking I can use 2 of the steel panels off that, unstitch the welds with a grinder and sandwich a sheet of the blanket between them, that way I have at least a manhole sized access port to the stack etc just in case, and for better cleaning.

Was thinking put a fireproof handle of some kind on there and a decent hinge planted in the brickwork, then I am hopefully minimizing the heat losses (I know, less thermal mass, trade off town here) by having the reflecting blanket sandwitched between some pretty hefty steel from an actual wood heater, Being lower down on the outside of the bell, how hot do you think that would get? I imagine cooler than the stack but cool enough not to degrade the steel? Or would a double layer of firebrick in a steel frame with the blanket on the inside be better maybe? Heavier, but a good hinge would make that fairly easy to open and close anyway I would think, as long as it doesn't warp too much.

As with everything, this is only in the thought stages right now, input always appreciated, there are probably hundreds of combined years of experience here, that is why I joined!
Worth it in the long run to add something like that do you think? And thanks for replying.

thomas rubino wrote:Hey Miles;
I agree. No, No, No on the nuclear stuff!
Yes, the fireclay and silica sand that have been screened are perfect.
You need 3x of the sand than you do of the clay.

I doubt it is available there, but this is the sand that I use.
https://www.homedepot.com/p/SAKRETE-100-lb-Silica-Sand-65200396/207136556

Wow, apparently we are china now, that link is forbidden for me to access according to my internet. I don't get that very often!
Go Aussie Go! :(

I agree with your agreement, I posted on that 3 year old reddit post and asked him if he was dead! And then I checked his profile, and he is still posting as of today, so somehow he survived eating pizza from an oven coated in that stuff, that says it will kill you if you breathe any part of it, not just dangerous to inhale, straight up says lethal if inhaled, no ifs no buts.

And the pic shows him stirring it up bare handed... how is this person alive today??

Anyway, rant over, I was just shocked that this person used that stuff for THAT application!

Nope nopedy nope, thank you very much, don't need one of the neighbour's kids dropping dead of an asthma attack and the EPA tracing lethal dust to my yard thanks. Not to mention the PPE required just to open the bloody container! Wow, great thermal protection, but wow now I know why they only use that in nuclear reactors etc!

I found 200 grade silica sand, but damn shipping.... looking for somewhere more local to see if I can get it here cheaper maybe or pick up, but this place is devoid of this industry really. People start asking questions around here when you want specific stuff that most people who aren't already in the trade don't ask for. I may just have to pay the  $90 shipping for $30 worth of sand and clay! (10kg total, half and half though I know I will have clay left over)

Oh also, I was looking into what makes fire clay better, and saw mention of adding alumina oxide at about 30-40% as well, and also adding a little grog for some larger particles to stop the clay shrinking so much as it dries, would like thoughts on adding these, less inclined to add a lot of oxide, sounds a bit dangerous to burn all things considered, but grog might be ok in small amounts maybe? Or would the straight up sand/clay/water mix be best, and throw out the rest? If so, you would be saving me lots of research into the subject that I am currently doing. so thanks in advance.

Last thing, I saw somewhere that having some different grades of sand helps with the strength and shrinkage too, as in maybe 80% medium and 10/10 slightly finer and slightly coarser, if I find a good supplier, willing to try that out if people here think it would be better. Long lasting is the goal for me.

Cheers from tha land downunda.

Miles, the core is the only place where it gets hot enough to spall steel.
Your mass, especially down low, will not be that hot at all.

Rather than hinges mounted in brick (they always work loose), most of us use a slide-in door with some stove gasket to seal the flange.

The plan here, is if you build your RMH correctly and use dry wood to burn, most of the ash is incinerated.
Only an improperly built RMH will create enough ash to quickly plug up the system.
Typically, most of your ash will remain in the feed tube and partially up the burn tunnel, where you will clean it out every few weeks.

thomas rubino wrote:Miles, the core is the only place where it gets hot enough to spall steel.
Your mass, especially down low, will not be that hot at all.

Rather than hinges mounted in brick (they always work loose), most of us use a slide-in door with some stove gasket to seal the flange.

The plan here, is if you build your RMH correctly and use dry wood to burn, most of the ash is incinerated.
Only an improperly built RMH will create enough ash to quickly plug up the system.
Typically, most of your ash will remain in the feed tube and partially up the burn tunnel, where you will clean it out every few weeks.

Thanks, I did just watch a video of a guy who said he'd rather clean the J Tube than the batch box because he just shoves a shovel (funny that) with an angled handle down in there and scoops most of it out, that sounds fine to me, and I assume the left overs are fairly pure charcoal, and probably OK to spread over the garden for a better fruit crop?

So sliding door is better, noted, would it be worth double thickening it with the ceramic blanket wedged in? I guess that may screw up the sliding mech after a while huh? Maybe just 2 of the plates welded together to get something thicker but still able to slide will be better there.

And I am hoping to build this thing well enough so the 2nd burn (Tunable will be the idea there with sliding baffle and a larger channel than needed as well I think) will take out most of the bad stuff. I heard somewhere that making every opening past the J box larger in area than the J box for the outlets was the expected wisdom, so the plan is to make those channels at least 3 or 4 bricks wide to make sure it's more than the burn tunnel.

I doubt I'll get to pouring the slab for at least another week or two anyway, and that will need at least a week to cure, preferably a month before I start laying brick on it, and then another layer of the perlite concrete mix to insulate, which I assume needs about as long to cure before moving on. It may we well into winter before I have finished this thing (we are in the last month of summer here by the way, my chili plants are starting to bear some good fruits finally, Carolina Reaper is getting bigger by the day and covered in flowers and little skin melting fruits)... just an aside for kicks.

I'll maybe post a pic or two of them now and then while I post the build, I know it's not really the place, but these things are rare (at least down here) and I managed to get them to fruit, and this is partly a gardening forum as well! (I'll keep that to a minimum though, here for the fire, not the fire, if you get my meaning.

(if the chili pics are not wanted, please just say the word and I might make a thread in a more appropriate area here, while I'm here :)

Sorry to double post, but this one is a quick question about adding the ash to the clay mix, I hear it's for the lye content, but here in OZ we have Caustic Soda which is pure lye (I looked it up on the MSDS once because I'm a nerd like that)
Would that be better, or the charcoal content helps as well?

And one more thing before i possibly pass out from a long day (I worked 8 hours and then planted 4 fruit trees in the backyard this afternoon)
For the oven: I was thinking for a "white" oven I could just add a baffle to stop the gas coming into the chamber, and let it flow straight out, considering the fire bricks, I assume, will get plenty hot for an oven over that burn chamber, and it might even need the metal baffle with the airflow to cool it down some!

Just some things rattling around in my head all day today. Thanks.

As you might be able to tell, I am very eager to learn a lot before a brick is laid.

So Miles;
A couple of thoughts for you to ponder.
To start, there is no need to wait weeks or even months to build after pouring.
For the slab, pour it one day, and two days later, start mortaring bricks.
It's the same for building the rest; no need to wait before firing.
I build these things, slop on the last cob to seal it all up and build a fire.
Clay is terrific stuff; no need to baby it.
When stacking bricks with clay mortar, there comes a point where you should go work on something else.
Until it dries, it is easy to bump bricks out of place.

Now, the next thing I want to talk about is your J-Tube.
Dimensions are specific; you must stay within the established parameters.
Looking at your drawing, the burn tunnel appears to be too long.
The roof of the burn tunnel should be 10"-12" long; it should not be longer, or you will create excess ash.
The size (area) should remain the same throughout the feed tube and burn tunnel,
Feed tube depth should be no more than 16".

Now, let's talk about your secondary air tube... you do not need it.
J-Tubes do not use a secondary air tube...  first-generation Batchboxes use them.
If you deviate from the build specifications, then you are delving into experimental rocket science, and all promises are off.
The J-Tube design has been tweaked by Peter to be as hot and clean burning as possible.
Look on my website for Peter channels to see what they look like: https://dragontechrmh.com
There is also a burn tunnel roof modification called a trip wire you can incorporate into a J-Tube build as well. (search Permies)
This is why Peter moved on from J-Tubes and started refining the batchbox design to be as hot and clean burning as possible.
Any changes to a J-Tube design will not be as efficient as Peter's improvements and could be much worse...

Miles;
When making clay mortar, nothing but fireclay and clean, graded (sharp) sand is used.

When making cob, you can add all sorts of things, with ash being one of them. (straw, horse poop...)
Also, when making just a cob mix, you can use any clay and sand with debris included; it is just a filler.

thomas rubino wrote:So Miles;
A couple of thoughts for you to ponder.
To start, there is no need to wait weeks or even months to build after pouring.
For the slab, pour it one day, and two days later, start mortaring bricks.
It's the same for building the rest; no need to wait before firing.
I build these things, slop on the last cob to seal it all up and build a fire.
Clay is terrific stuff; no need to baby it.
When stacking bricks with clay mortar, there comes a point where you should go work on something else.
Until it dries, it is easy to bump bricks out of place.

Now, the next thing I want to talk about is your J-Tube.
Dimensions are specific; you must stay within the established parameters.
Looking at your drawing, the burn tunnel appears to be too long.
The roof of the burn tunnel should be 10"-12" long; it should not be longer, or you will create excess ash.
The size (area) should remain the same throughout the feed tube and burn tunnel,
Feed tube depth should be no more than 16".

Now, let's talk about your secondary air tube... you do not need it.
J-Tubes do not use a secondary air tube...  first-generation Batchboxes use them.
If you deviate from the build specifications, then you are delving into experimental rocket science, and all promises are off.
The J-Tube design has been tweaked by Peter to be as hot and clean burning as possible.
Look on my website for Peter channels to see what they look like: https://dragontechrmh.com
There is also a burn tunnel roof modification called a trip wire you can incorporate into a J-Tube build as well. (search Permies)
This is why Peter moved on from J-Tubes and started refining the batchbox design to be as hot and clean burning as possible.
Any changes to a J-Tube design will not be as efficient as Peter's improvements and could be much worse...

That is all very interesting, thank you, I was going by some site that said concrete slabs need a week to walk on, a month to full cure, that was where I got my numbers. if I can just start firing small fires straight away, all the better to get this thing done on time for the cold weather to come in!
And I plan to build the whole J tube and fire it slowly and well before I start building the top part up around it anyway, so that sounds good.

As for dimensions, I was pretty sure those were a little off yes, none of this was final dimensions I plan to draw at least one or two revisions before this is all over.

Thank you for the actual dimensions rather than just ratios etc, I will make another drawing with some of the improvements soon I think, when I have the energy! I think bringing the chimney forward and maybe even adding another up down baffle behind it might help some? I'll figure out the oven somehow, it might just be a little smaller than anticipated.

Thanks for the links, I may keep the "air" tube down there, but mainly for getting ash out of the system if it builds up at the bottom of the chimney, if you don't think that will cause issues that is, if so, it gets perlite concrete down there under the bricks, and maybe a layer of ceramic blanket under the brick and on the perlite. Baby steps.

Looks like I have some more reading to do! I might find some answers to those rattling questions, so I thank you.

New drawing will probably be incoming in the next few days... maybe... possibly.... :)

thomas rubino wrote:Miles;
When making clay mortar, nothing but fireclay and clean, graded (sharp) sand is used.

When making cob, you can add all sorts of things, with ash being one of them. (straw, horse poop...)
Also, when making just a cob mix, you can use any clay and sand with debris included; it is just a filler.

Just to make sure, is the 200 grade silica sand good? I was looking for the specific correct stuff to get for this and it seems that was the consensus on everything I could google, or some other type? (Just that sand could be technically made of anything ground up fine enough, so the correct material I assume is a little bit essential?) I fully admit I could be completely wrong about that last statement by the way.

I probably won't use cob just because I have a mason, and no easy cheap source of clay apparently, paying $1000 to get a tonne of it delivered will be cost prohibitive in my case, it will be brick like the Russian etc ones I think. He tells me he can get free unused new bricks from rich a-holes who have them left over from new builds and want them gone a lot of the time. Sometimes half a pallet at a time!

I also may shorten the whole thing (I assume if the burn box is too long, the stack can also likely be shrunk a little) because I was thinking a 2 meter tall fire filled brick structure suddenly springing up in view of the neighbour's kitchen window might not be the best idea, maybe I'll keep it under the fence line for the most part. :)

Yawn, good morning, one more quick question I was just curious about, has anyone thought to run an air duct through the mass in a mass heater to get warm air to somewhere else?

Just saying, I can't be the first one, I'm never the first one, so if anyone has tried this, please chime in!

EDIT: I have the day off today so I may actually level up the form, back fill the dry sandy dirt I have back in there and pack it down. I might even get to a slab today, it's warm and sunny so I'm a bit motivated... and the wheelbarrow is just staring at me asking "where is my concrete mix master?"

Could be more pics coming later on if I actually make progress today

I teeeeel you whut...

Did I ever tell you the story of how I have luck that could win lotteries, except it's always bad?

So, I tamped down the dirt in the frame after back filling it nice... went to the hardware store and got some 20cm reo spikes to hammer down into the ground about 15cm or so, leave 5cm above ground, balance the reo on that and tie it on with a few wires just to hold it while the cement sets...
So I put the reo back in the form and put the spikes down right up against it so I could just pick it up and move it over 5mm to have it sit nice on top of the spikes... 4th spike in and my bad but lottery level luck kicks in, I hammer the spike directly down through the one piece of water pipe in the whole backyard, going to the tap about 2m away from the slab location.

Plumber came and shook his head because the guy who did the renovations before I bought the place used bloody 20mm plastic sprinkler pipe for the underground stuff... The kind that is soft enough to punch through with the plastic spike on the sprinkler ends so you can run it through the garden and water a whole bunch at once... that stuff...

All fixed now, but I lost motivation to do more on the slab today, now I have to back fill... again... and then tamp down again and put the reo spike literally 1 inch to the left of where it was and I'm fine... There is nothing for 1 meter either side of this pipe minimum, it's like I bullseye'd it from 6 inches away blind and hammered direct through the middle of it...

Sigh, I think I'll go pull that heater apart for the steel and door, at least taking out welds with a grinder should be fun... watch me hit a bloody water pipe in there now XD

Well, Miles, quite the luck you have!
Nobody else could have dead-centered the water line as well as you did!
Brownie points for you! Oh and an apple...

Oh well, drop a gear and keep moving forward, you will get there.

As far as the sand goes, I will guess the 200 is good.   We do not grade sand that way.
If it is a sharp silica sand, screened & graded to be uniform it will work.

As far as piping extra hot air out of your mass, maybe if your bell is undersized, you might get this to work.
There is a limit on how much hot a J-tube can push without stalling.
But you will get less heat into the mass, and you will need to insulate (or bury in cob). the pipe.
Metal pipe sheds head rapidly.

thomas rubino wrote:Well, Miles, quite the luck you have!
Nobody else could have dead-centered the water line as well as you did!
Brownie points for you! Oh and an apple...

Oh well, drop a gear and keep moving forward, you will get there.

As far as the sand goes, I will guess the 200 is good.   We do not grade sand that way.
If it is a sharp silica sand, screened & graded to be uniform it will work.

As far as piping extra hot air out of your mass, maybe if your bell is undersized, you might get this to work.
There is a limit on how much hot a J-tube can push without stalling.
But you will get less heat into the mass, and you will need to insulate (or bury in cob). the pipe.
Metal pipe sheds head rapidly.

Thanks, they had 100/200/350 as the grades, higher is finer.

I guess 200 is "medium" as suggested, thanks.

Took off the door from the fireplace to at least get something done, and cleaned up/painted it with some high temp paint.

At least THAT came up OK

Oh, and looking into those mods suggested by someone, it sounds like the first one (P-channel?) is easy enough, can drop it into the bricks and still be able to close the door, I can use some of the fireplace metal for that actually, maybe even the baffle I got with it, it's hefty.

As for the 2nd mod, (P trap?)  I couldn't find an actual diagram of it, but it sounds like it's just a 5-10mm bump in the top of the J tube?

Hiya Miles;
I was looking at your drawing again... maybe I shouldn't do that, eh?
Take a look at this thread
https://permies.com/t/270559/Stratification-chambers-Bells-explained

You really do not need the European design with serpentine channels.
You and your buddy can whip up a strat chamber in no time!

Is the door you salvaged today for the pizza oven?

Miles Teg wrote:
I haven't heard of anyone running air duct work through one of these things before though, which makes me wonder why.... I have either thought of something new, or something someone else has tried and it didn't work, that is the biggest worry at the moment.

While I haven't looked to see what other people have tried, as someone currently working in the HVAC field my initial thought is that, while it might be less efficient then a rocket mass heater with the mass inside the house, I think it has potential, if the large potential hazard is properly addressed: you need to be absolutely sure there is no way now or in the future you will be pumping exhaust from the rocket into your house!

To give an example of one way this is addressed in the commercial market that I see: older gas furnaces had natural draft chimneys and the fan on the household ducted air is before the heat exchanger and creates positive pressure in the heat exchanger, so if/when it rusts out or leaks in some way the household air leaks out through the chimney, and not the other way around. Newer furnaces have fan driven chimneys, but that fan is placed after the heat exchanger so it gives negative pressure to the exhaust side, which increases the pressure difference between the two sides of the heat exchanger.

In addition to putting your household blower before the rocket heater so if there's a leak you blow air out instead of sucking exhaust in, I would recommend separating your exhaust and ventilation pipes in the mass (rather then running ventilation pipes through a stratification chamber, or doing a tube in tube exchanger), even just by a few centimeters, so you can surround all the ducting with a layer of clay, cement, or similar, to minimize air cross contamination long into the future, even if/when the ducting rusts through; similar to what you would want to do to all the piping if the rocket mass heater were inside your house.

A few other potential downsides I see, and ways to address them:
-I would suggest adding some kind of temperature sensor in the ducting or the mass, so that you can keep the indoor blower from running if the mass is cooler then inside your house, to prevent sucking heat out of the house to warm the mass.
-Since the mass is going to be outside the insulation envelope of your house I would recommend insulating the mass, to minimize heat loss to the outside.
-Your heat will be dependent on electricity to run the indoor blower, but that is a minimal power draw compared to electric heat or a heat pump and could be set up to run off a battery with inverter or generator.
-You'll have to go outside into the cold to start/tend the fire. I don't have any suggestions of how to address this other than good winter clothing, but you can at least take warmth in the mess of the firewood staying outside and heating your house for only pennies! :)

I can't speak to the design of the rocket heater, having not built one myself yet, but the more I think about your design the more potential I see; with the heater outside you also eliminate the issue of a backdraft down the chimney or a cold plug/hard start dumping smoke into your house.

Looking forward to seeing the finished product in action!

Miles Teg wrote:I'd already be a buildin'
and explodin' and a killin'
if I only had a brain.

Sounds like one helluva project you have going there...

Miles,

The finer aggregate you use, the higher total surface area the particles will have and you will have to use higher clay ratio to make it stick. The general rule is to use particle size to be no more than half the joint height, so for example if your joint is 10 mm then grains up to 5 mm are acceptable. When laying firebricks, small joint of 2 or even 1 mm is desired and in this case the particles should not be larger than 0.5 mm which would be equivalent of US mesh 35. When I prepare the mortar for adobe bricks, or rooftiles I use the ratio of 2.8 (coarse sand):1 (clay), but the same clay when mixed with mesh 18 sand has to go in ratio 2:1 and for mesh 35 the best strength is when I mix 1.5 (sand):1 (clay).
Fine silicas from the ceramic supply store are used to prepare glazes or engobes, coarser ones are used as grog. The finer silica will help to develop lustrous glazes and the coarser will not quickly melt and will help to develop matte effect. For glazes I purchase mesh 325 - it's like wheat flour.
For cob it would be most efficient if you just dug some soil (without top organic matter) on your property and amend it with clay if needed. Some samples would have to be done first to determine the best ratios.

Regarding the idea of blowing air in pipes through the mass I doubt it will work. Air has 4 times lower heat capacity than water, but 1 m3 of air weighs only 1.2 kg, so the same volume of air has 3400 less heat capacity than water. Forced air heating systems also use air, but it goes through heat exchanger and multiple torches. It could potentially work if the exchanger was closer to the fire, but at the same time it would lower the efficiency of the wood combustion, which is not a problem with gas torches. Nonetheless I believe that it somehow could be developed.

Coydon Wallham wrote:

Miles Teg wrote:I'd already be a buildin'
and explodin' and a killin'
if I only had a brain.

Sounds like one helluva project you have going there...

I heard that some have had their burn chamber literally explode due to water left in the refractory cement when they did their first full burn...

If I only had a brain, I may have made that mistake!

thomas rubino wrote:Hiya Miles;
I was looking at your drawing again... maybe I shouldn't do that, eh?
Take a look at this thread
https://permies.com/t/270559/Stratification-chambers-Bells-explained

You really do not need the European design with serpentine channels.
You and your buddy can whip up a strat chamber in no time!

Is the door you salvaged today for the pizza oven?

I was thinking about that, though the ups and downs give more thermal mass since the outer layer of this thing is only 1 or 2 layers of brick thick, would the extra mass be a better idea, or do you think the strat chamber would be better for me? I'm still pondering that topic, the way I went was only because more mass packed into the area I have.

Oh, and yes, the fireplace door is for the oven... but now I have to figure out where to put one as shortening the J tube doesn't leave me much room in the front of the unit over the heat box. I may end up leaving it out, but I do like a pizza and I was even looking into growing some Roma tomatos next year for homemade pizza sauce, I used to work at a pizza place so I reckon I could recreate a pretty good pizza shop pie.

Andrew Welser wrote:

Miles Teg wrote:
I haven't heard of anyone running air duct work through one of these things before though, which makes me wonder why.... I have either thought of something new, or something someone else has tried and it didn't work, that is the biggest worry at the moment.

While I haven't looked to see what other people have tried, as someone currently working in the HVAC field my initial thought is that, while it might be less efficient then a rocket mass heater with the mass inside the house, I think it has potential, if the large potential hazard is properly addressed: you need to be absolutely sure there is no way now or in the future you will be pumping exhaust from the rocket into your house!

To give an example of one way this is addressed in the commercial market that I see: older gas furnaces had natural draft chimneys and the fan on the household ducted air is before the heat exchanger and creates positive pressure in the heat exchanger, so if/when it rusts out or leaks in some way the household air leaks out through the chimney, and not the other way around. Newer furnaces have fan driven chimneys, but that fan is placed after the heat exchanger so it gives negative pressure to the exhaust side, which increases the pressure difference between the two sides of the heat exchanger.

In addition to putting your household blower before the rocket heater so if there's a leak you blow air out instead of sucking exhaust in, I would recommend separating your exhaust and ventilation pipes in the mass (rather then running ventilation pipes through a stratification chamber, or doing a tube in tube exchanger), even just by a few centimeters, so you can surround all the ducting with a layer of clay, cement, or similar, to minimize air cross contamination long into the future, even if/when the ducting rusts through; similar to what you would want to do to all the piping if the rocket mass heater were inside your house.

A few other potential downsides I see, and ways to address them:
-I would suggest adding some kind of temperature sensor in the ducting or the mass, so that you can keep the indoor blower from running if the mass is cooler then inside your house, to prevent sucking heat out of the house to warm the mass.
-Since the mass is going to be outside the insulation envelope of your house I would recommend insulating the mass, to minimize heat loss to the outside.
-Your heat will be dependent on electricity to run the indoor blower, but that is a minimal power draw compared to electric heat or a heat pump and could be set up to run off a battery with inverter or generator.
-You'll have to go outside into the cold to start/tend the fire. I don't have any suggestions of how to address this other than good winter clothing, but you can at least take warmth in the mess of the firewood staying outside and heating your house for only pennies! :)

I can't speak to the design of the rocket heater, having not built one myself yet, but the more I think about your design the more potential I see; with the heater outside you also eliminate the issue of a backdraft down the chimney or a cold plug/hard start dumping smoke into your house.

Looking forward to seeing the finished product in action!

Thank you for the great tips, and I think I can address some of the points already:

The plan was always to bury the air ducting inside the mass at the top of the heater, so, yes, I plan to encase the whole duct in brick and concrete basically, there will be very little chance of a leak with a few inches of masonry between them (I hope!)

But, in the off chance that this does happen, I also had already planned to push air FROM the house back into the heater, mostly because it will save me having a hot fan running all day, but now that you mention it, this does make the most sense for multiple reasons.

Also, I have an iron door that just arrived for fitting over the top of the burn chamber, plan is to shut it after the burn to stop the draft and keep the bricks hot as long as possible.

And the unit is literally just at my back door at the end of my decking, so lighting is as easy as I can make it, I just need to keep the sticks dry! (currently looking into a couple of storage places either side of the burn chamber but they may be too hot to store wood, so I might look at other options too)

Thanks again, hopefully that clears some stuff up, and please chime in with anything else you can think of, always happy to take advice, this is my first build, I need all the help I can get!

Cristobal Cristo wrote:Miles,

The finer aggregate you use, the higher total surface area the particles will have and you will have to use higher clay ratio to make it stick. The general rule is to use particle size to be no more than half the joint height, so for example if your joint is 10 mm then grains up to 5 mm are acceptable. When laying firebricks, small joint of 2 or even 1 mm is desired and in this case the particles should not be larger than 0.5 mm which would be equivalent of US mesh 35. When I prepare the mortar for adobe bricks, or rooftiles I use the ratio of 2.8 (coarse sand):1 (clay), but the same clay when mixed with mesh 18 sand has to go in ratio 2:1 and for mesh 35 the best strength is when I mix 1.5 (sand):1 (clay).
Fine silicas from the ceramic supply store are used to prepare glazes or engobes, coarser ones are used as grog. The finer silica will help to develop lustrous glazes and the coarser will not quickly melt and will help to develop matte effect. For glazes I purchase mesh 325 - it's like wheat flour.
For cob it would be most efficient if you just dug some soil (without top organic matter) on your property and amend it with clay if needed. Some samples would have to be done first to determine the best ratios.

Regarding the idea of blowing air in pipes through the mass I doubt it will work. Air has 4 times lower heat capacity than water, but 1 m of air weighs only 1.2 kg, so the same volume of air has 3400 less heat capacity than water. Forced air heating systems also use air, but it goes through heat exchanger and multiple torches. It could potentially work if the exchanger was closer to the fire, but at the same time it would lower the efficiency of the wood combustion, which is not a problem with gas torches. Nonetheless I believe that it somehow could be developed.

Thanks for the grain info, that does help with selecting the size now that I know the basics.

As for the air movement, it was my best idea to use the existing ducting that came with the house, but if there is a better way I am all ears, maybe water running through the mass with copper tubing, running into a radiator to blow the hot air into the system? I'm not sure that would be more or less efficient, however.

This is going to be an ever changing design until the build happens, and I want it to be as good as I can get it, so please let me know if that would be a better idea.

EDIT: My other option to heat my house this winter is a reverse cycle aircon, I don't mind running it in bursts to cool the house in summer, but running it all day in winter is a little cost and pollution prohibitive, so anything better than that is fine by me. Just for context of what I'm trying to do better than, a heat pump.

Thanks.

Miles, with respect, you might consider following a proven, basic design that has worked well for a decade or more. Then a later attempt could be for quirky,  esoteric ideas that pop into your head.  Folks here are trying to help ensure a quality heater for you, but cannot build it for you.   For the sake of your safety and your happiness long term, please consider this idea and ignore various Youtubers and fly-by-night TMH builders.   Experts on Permies are legit and generous.  

In the name of Prometheus,
Rico

Rico Loma wrote:Miles, with respect, you might consider following a proven, basic design that has worked well for a decade or more. Then a later attempt could be for quirky,  esoteric ideas that pop into your head.  Folks here are trying to help ensure a quality heater for you, but cannot build it for you.   For the sake of your safety and your happiness long term, please consider this idea and ignore various Youtubers and fly-by-night TMH builders.   Experts on Permies are legit and generous.  

In the name of Prometheus,
Rico

Thanks, taken with the good will it was intended to have.

A little on my thought processes before coming here:

I bought a house, realized I have ducting in every room and no ducted heating system because someone blocked it all up with renovations, and my only heating/cooling is the reverse cycle aircon in the loungeroom which does nothing for my bedroom and the rest of the house, as well as costing quite a bit to run as a heater for as long as I want the house warm.

So, I looked up other options, found rocket mass heaters and started watching all of the videos on the subject... and I mean all of them, I think I've actually watched 90% of what is available on youtube now, and I assume that some of the fine folk here are also some of the ones making a few of those videos I watched, but for one thing, everyone has their own special way to do the little things, and also I cannot find anything that is exactly fit for my purpose (having the mass outside and running the air through to utilize the existing ducting in the house) in any of those videos, or anywhere else. The closest I could find was the batch oven mass heating devices from russia, but here is my problem:

I have a backyard full of trees that I will be pruning every year, leaving a large amount of small sticks not really suitable for a batch heater as they would burn a little to quickly, at least going by the people mostly throwing a couple of logs at it and leaving it like a normal fire box. I don't really want to spend money or days cutting and lugging firewood, so I figured the J tube would give me the best bang for my existing fuel buck.

But no J tube mass heater that will fit in the footprint I have for this project, so the shape of the thing has to be more like the batch heaters, though I figure as long as the airflow is still there, it will work the same way, and this whole place was built on people tinkering, so I am here in that spirit, trying to make the best of the constraints I have, and I don't want to spend the world on this either, I have a budget and a footprint, and the fuel ready to go, I just have to make something that fits.

If anyone has a design that matches those specs, I am more than happy to copy it, but so far I have come up short in that regard, so this is my only option, and this is the best place I can get advice so that my tinkering is not just random stabs in the dark, which is why I am very open to suggestions from the crew here.

I hope that clears it up a bit? All due respect to yourself as well.

EDIT Also the decision to use brick comes from the fact that I know a mason who as built some very nice brick and even stone pizza ovens, and I'd rather have something that lasts 10 seasons than replace a barrel 5 times in that same period, I even purchased a barrel to use but decided it was going to be too much and I don't want the radiated heat, I want it directed into the air in the ducts if that is possible.

Cheers.