Rocket Mass Heater Design - Please Help!

Hi Permies long time listener, first time caller.

Currently I'm building myself a fine woodworking workshop on an off-grid mountain-top community outside of Melbourne, Australia, and I plan to heat it with a masonry bell style RMH (hooray!), and I have some questions. Any help and input greatly appreciated.

First, some context/constraints:

- Workshop is ~50m2, walls and roof insulated with batts, concrete floor insulated with XPS sheets, which according to calculations by folks here and elsewhere, I believe means I need a 175mm or 200mm core riser.
- Floor real estate for the heater is small, i'm hoping to limit it to approx 900mm x 750mm, but the ceiling is ~2700, so i could go up
- Cash is pretty limited, I'd love to keep build cost down as much as possible (a lot of the build is already with second hand, recycled and/or reclaimed materials)
- I'd also love to keep it simple as possible, so a square riser, cutting as few bricks as possible, rectangular prism shaped bell without a bench, etc
- I want the heater to be a batch box style core housed inside a simple brick box. I'd love to load it and fire it in the morning, and not have to constantly be feeding it. I also love the look and vibe of bricks.

So, some questions:

- Core material: insulated fire brick vs ceramic fibre board (vs something else)? What's the cheapest and easiest? And is longevity/durability the exchange for that?

- Location of the flue: I've seen brick RMH's with flues inside the bell, and I've seen them outside. It seems to me that if it was inside, as soon as the burn starts it would begin heating the flue and starting draw, thus minimising (or eliminating) smoke coming into the room. However, it seems to me that this setup would also create quite a strong draw during the main burns, and would thus be drawing out hot gasses before they had given their heat to the bricks, thus making the system much less efficient. Conversely, the flue on the outside means that only gasses cool enough to reach the bottom would ever make it to the outlet, which would mean much more efficiency in terms of heat given to the thermal battery, but how does the draw start in that set up? I've been thinking about a Tee piece at the outlet, and putting in some wood shavings at the start of a burn to heat up the chimney directly? And how does the draw continue? As I understand, any flue situation relies on the temperature differential between the pipe and the surroundings, and in the scenario where the pipe is outside of the heater, and the outlet of the heater is at the bottom because that's where the coolest gasses end up after giving their heat to the brick and thus making the thermal battery a thermal battery, how does the draw maintain?

- Secondary air inlet: Does this piece of square tubing have to just be a replacement part, since it will sit directly underneath the primary burn chamber and be much hotter than steel is designed to get? I've heard of fancy expensive metals that can be used to make tubes out of, but I think in Australia it's hard to get. Love to know what folks use for this.

- Sizing the Bell: If i've read the tables correctly, if i go with a 200mm riser, I need a Bell with a 9.4m2 internal total surface area. I've read about people adding columns inside the bell to add surface area, which sounds like a good idea to me to keep overall size down. I've also been hoping to do it as a single skin for the same reason, but is that a bad idea?

- Bell lid material: What do I make the lid of the bell out of?? I've got the business end of a rocket stove pointed directly at this thing, and I can't find info about what it should be made of... I don't know much about concrete, but it seems like it would just crack? And what does it sit on for support across the span? Another reason I like the flue on the outside is a simpler construction of the lid, so there's that too...

- Bricks and mortar: What bricks for the actual Bell? Plain old building bricks? Solid/vs hollow? And what about the mortar, both for the core and for the bell?

Okay, that's all I can think of in this moment. I'm sure there's more, but it already seems like kind of a lot. If anyone has any time to answer any or all of my questions, or ask clarifying questions of their own, I'd so so so appreciate any help. And if there's a different idea it seems like I've missed, I'm open to change where I can.

Many thanks in advance,
Aeron

Hi Aeron;
I'll try to give you some answers.
First, in my opinion, do not build a full-size first-generation Batchbox with CFB.
Wood abrasion will happen no matter how careful you are.  
Use heavy firebricks; although sometimes they might crack, they rarely fail.
I recommend an external chimney using a bypass from near the top of the bell.
Internal chimneys require insulation to avoid stalling, not runaway heat.
Adding a bypass is foolproof for starting during shoulder seasons; adding a tee and building a stick fire to start the draft will quickly turn into a royal pain in the ...
The draw maintains once your bricks absorb heat, that heat has to rise. With the super-hot air coming out of the riser, the air stratifies and moves toward the bottom, where it exits the bell.

OK, the secondary air tube sit in a gap of the firebrick floor, as it has outside air rushing through it , and a layer of ash on top it is insulated from extreme heat.  The portion of that air tube that sits in front of the port has spauling issues.  A standard piece of carbon steel Schedule 40 pipe will wear out and need replacement. if you copy my quick change secondary design, changing that stub is a matter of a few minutes.  With the early design, the stub needed to be welded in place, and then the entire tube was removed, and the stub was cut off to replace it.
With light use, a plain steel stub should last you at least two seasons before needing to be replaced.
The special metal pipe you hear about is RA 330, or if you talk to me, RA253MA. Here in the US, I sell them for $60.  

Bells) Single skin bells get hotter to the touch than a double skin, but will cool faster.
Roofs are either built with firebricks sitting on T-bar angle iron, and then covered with a second layer of plain clay bricks.
Or you can use an insulation layer (Morgan super wool) and then use cement board, which also has T-bar support.
The bell is constructed with plain brick below the top of the riser; beyond the riser, the bell is constructed with firebrick.
Solid clay bricks or bricks with holes can be used.  The holes need to be filled with clay mud, or clay mortar, but not sand.
All mortar is made with one part fireclay to three parts graded sharp sand (NO ROCKS)
Using dry product (bagged clay, and bagged sand) is the easiest way to properly mix your mortar.

Here is my website https://dragontechrmh.com/. Go take a look at quick-change secondary tubes; you or a local welder can easily build your own.
Check out my studio dragon build; it could easily have been made as a single skin bell.
Take a look at my Shorty core build, it has smaller requirements in bell size and in the number of firebricks needed.

Have no fear of cutting bricks, it is quite easy.

Tom did a great job with some helpful advice,

From my past builds, I have changed my mind a bit but would like to add pertinince to just a few things.

No matter how good of IFB  ( insulated fire brick ) you can get, I would put my money on full fledged hard fire brick, and insulate that if you must, on the exterior of the brick. NOT hard to do, and you gain 10 fold in the toughness of the brick.  YES there are some really good IFB out there, but you get the wrong ones, The rebuild is generally not so fun.

As top stated, the by pass is simply a must, even if you never use it,  Again, hardly cost a thing, easy to put in,  And on the chilly but not cold days, simply a perfect solution, to get things going.

Posting your planned shape and design to this forum, will reward you with thumbs up, or gentle suggestions from those that have gone before.  Life is to short for redo's.   As they saying goes, if you don't have time to do it right, when will you have time to do it over?    Am pretty confindent that with a thumbs up plan, this will work well for you first time out of the gate.

I would say insulating firebrick for the riser of a core would be fine, getting hot faster than hard firebrick, but concur that hard firebrick is the best material for the firebox.

Hi Aeron, a question: is the workshop in use each and every day of the week? In case it's 4 days or less, you have to start all over again in the morning as soon as you come in. A quick response heater is usually the way forward in such a situation. All barrels or a single skin brick bell.

I'll try to go through all the questions and try to provide answers, starting down here.

Aeron Ironbark wrote:- Core material: insulated fire brick vs ceramic fibre board (vs something else)? What's the cheapest and easiest? And is longevity/durability the exchange for that?

Hard firebrick, no question about that. You can insulate the core or not, although I've seen larger mass heaters without core insulation.

Aeron Ironbark wrote:- Location of the flue: I've seen brick RMH's with flues inside the bell, and I've seen them outside. It seems to me that if it was inside, as soon as the burn starts it would begin heating the flue and starting draw, thus minimising (or eliminating) smoke coming into the room. However, it seems to me that this setup would also create quite a strong draw during the main burns, and would thus be drawing out hot gasses before they had given their heat to the bricks, thus making the system much less efficient. Conversely, the flue on the outside means that only gasses cool enough to reach the bottom would ever make it to the outlet, which would mean much more efficiency in terms of heat given to the thermal battery, but how does the draw start in that set up? I've been thinking about a Tee piece at the outlet, and putting in some wood shavings at the start of a burn to heat up the chimney directly? And how does the draw continue? As I understand, any flue situation relies on the temperature differential between the pipe and the surroundings, and in the scenario where the pipe is outside of the heater, and the outlet of the heater is at the bottom because that's where the coolest gasses end up after giving their heat to the brick and thus making the thermal battery a thermal battery, how does the draw maintain?

The batchrocket system has been built for decades now, on every continent, Antarctica being the exception. There are calculations for every system size you might choose, designed in such a way there 's always enough heat transported to the chimney to keep it going. In short: not all the heat is extracted from the gases. Have the chimney external to the bell, with a bypass to help it going in case the heater is stone cold.
The first generation batchrocket, with its long vertical riser blows the hot gases to top, thereby pushing the colder gases at the bottom out. Built exactly to specs and with a good chimney, this one doesn't need a bypass. That said, as a rule of thumb, within 20 minutes of lighting a cold heater the exit temperature of the bell need to be at least 60 ºC.  If this isn't the case, water vapor is big time condensing inside the chimney and all smoke comes back into the house.

Aeron Ironbark wrote:- Secondary air inlet: Does this piece of square tubing have to just be a replacement part, since it will sit directly underneath the primary burn chamber and be much hotter than steel is designed to get? I've heard of fancy expensive metals that can be used to make tubes out of, but I think in Australia it's hard to get. Love to know what folks use for this.

Thomas answered this very comprehensively. Suffice to say: use the latest Shorty core, that one doesn't need a secondary air provision.

Aeron Ironbark wrote:- Sizing the Bell: If i've read the tables correctly, if i go with a 200mm riser, I need a Bell with a 9.4m2 internal total surface area. I've read about people adding columns inside the bell to add surface area, which sounds like a good idea to me to keep overall size down. I've also been hoping to do it as a single skin for the same reason, but is that a bad idea?

A 200 mm system seems overkill to me, it's a 50 m² workshop, moderately insulated. You might get away with a 150 mm one, but scaling up can be done witout steps so a 175 mm system and ditto chimney might be plenty large enough. This system scales up very fast!

Aeron Ironbark wrote:- Bell lid material: What do I make the lid of the bell out of?? I've got the business end of a rocket stove pointed directly at this thing, and I can't find info about what it should be made of... I don't know much about concrete, but it seems like it would just crack? And what does it sit on for support across the span? Another reason I like the flue on the outside is a simpler construction of the lid, so there's that too...

Given enough distance between ceiling and riser, steel t-bars and firebricks would do the trick. Also used multiple times worldwide, including my own red bell heater since two years. OR you could use a Shorty core, much less firebricks in the walls and ceiling possible, because it's tuned down a bit and blows horizontally back to the front instead of vertically up. Bell would also a bit smaller, not to mention cheaper.

Aeron Ironbark wrote:- Bricks and mortar: What bricks for the actual Bell? Plain old building bricks? Solid/vs hollow? And what about the mortar, both for the core and for the bell?

This isn't one question, but a handful instead.
Normal, solid building bricks are perfectly adequate, the denser the better. But anything could be used, including concrete paving bricks, commonly called clinkers. Have a look at a large heater like that:  The Sculpter's Shop heater!
Refractory mortar for the core, clay/sand OR sand/lime/portland for the bell.

Hi everyone,

Thanks so much for your detailed answers. Apologies for not getting back sooner, truth be told, I'm finding this whole idea pretty overwhelming. But I'm not ready to get off the horse yet!

It seems like a single skin bell with hard fire brick is the way to go. I'm curious if anyone knows where to source hard fire bricks in Melbourne at a decent price. My first look the other day turned up AUD$14 each..! I think that's where my overwhelm went into overdrive and I put the laptop away.

I'll do some sketches in SketchUp and post them here to see what everyone thinks.

Some follow ups below:

thomas rubino wrote:
I recommend an external chimney using a bypass from near the top of the bell.

Multiple people recommending bypass, which sounds good, only i've never done any of this, and am trying to keep it simple. Adding a bypass sounds like a lot more work than adding a tee piece at the base of the flue. Plus won't i need a clean out down there anyway?  

thomas rubino wrote:  if you talk to me, RA253MA. Here in the US, I sell them for $60.  

This I will definitely be getting from you once I start collecting materials, thanks!

thomas rubino wrote:
...you can use an insulation layer (Morgan super wool) and then use cement board, which also has T-bar support.

just so i'm clear, the first thing in the path of the business end of the riser, 30mm above, is this super wool? must be hardy stuff! And the t-bar supports are made of plain mild steel? are they insulated themselves somehow? and then by cement board, do you mean just a plain concrete pad made from store bought concrete, that I can just pour in a form? That sounds easy enough...

thomas rubino wrote: All mortar is made with one part fireclay to three parts graded sharp sand (NO ROCKS)
Using dry product (bagged clay, and bagged sand) is the easiest way to properly mix your mortar.

I don't suppose you know where to source this stuff in Australia...?

Peter van den Berg wrote: is the workshop in use each and every day of the week?

It'll be in use between 3 and 4 days a week I reckon, so a single skin bell is what I'll go with, I think. I prefer the look of brick to the all barrel heater anyway, but am i right in thinking that your all-barrel systems don't have a thermal mass?  

Peter van den Berg wrote:Suffice to say: use the latest Shorty core  

I've tried to find info on this, and can't find any detail that might help me. Please keep in mind that while I'm definitely handy, I'm a total beginner at bricklaying...

Peter van den Berg wrote: scaling up can be done witout steps ... This system scales up very fast!

what do you mean by this?

Peter van den Berg wrote: Given enough distance between ceiling and riser, steel t-bars and firebricks would do the trick.

What kind of distance are we talking about? I do have plenty of room to go up, but the higher I build, the more costly my brick bill.

Peter van den Berg wrote: you could use a Shorty core, much less firebricks in the walls and ceiling possible, because it's tuned down a bit and blows horizontally back to the front instead of vertically up. Bell would also a bit smaller, not to mention cheaper.

Cheaper and smaller both sound good! Love to understand what the shorty core actually entails and how to build it 🙏🏽


Another thing I;ve realised yesterday is that it would be a lot easier to send the flue out the wall instead of the roof. I understand this would create friction in the system, but i've seen plenty of fires with flues out walls, and i'm curious to get any thoughts on this.

Thanks again everyone. Really appreciate the support.

Best,
Aeron

Lots of points to address. I will start with a few.

There is no need to build the whole bell from firebrick; most of it can be ordinary clay brick, with just the part above the riser top level firebrick. If you build a shorty core, it appears that you only need to have firebrick on the part of bell wall that the (horizontal) exhaust hits. The whole combustion core needs to be built of firebrick, with a possible exception of the riser which can be made from ceramic fiber blanket or Morgan superwool (safer to handle before firing).

The shorty core does not use a floor channel with its spalling risk, but it does require a specially welded door frame for controlled air supply.

The shorty core development is described in the Development of a Compact Batchrocket Core thread. There are links to construction drawings scattered through the thread.

All-barrel systems do not have thermal mass. Single-skin bells have decent mass, but will take some time to start heating the room (maybe a half hour). I would advise a hybrid bell with either a barrel or a good-sized steel panel built into the bell, for some instant heat delivery. The more often there will be gaps between occupation days and the more often occupation will be short like just a few hours at a time, the more I would go toward metal in the bell.

"Steps" refers to jumps in system size; these systems can be built in any size from small to huge and still work, using the same proportions. A minor jump in size gives a significant increase in power. Going from 6" to 8" might double the power (just a guess). 8" is a very powerful core, and 10" would be a monster suitable for extreme circumstances.

Thanks so much Glenn. Yes, I've been planning to build the main body of the bell with ordinary brick. Since posting earlier, I've found a reasonably prices source of both hard and soft fire bricks, and my current thinking is to use hard bricks for the core, and soft insulating ones for the riser, as well as the roof.

I think the shorty core is above my pay grade for this one. I don't weld, and don't know anyone up here who does. Keeping it simple as much as I can i think...

Hearing you about the lag time in the heater getting to temp. I'm going to buy an off-the-shelf metal door, which unfortunately doesn't give me the visual vibe of the fire, but it does throw off heat relatively quickly while I wait for the whole thing to heat up. The workshop is also a 90 second walk from my house, so it's entirely feasible to go over in the morning, light the fire, go back and have breakfast, and then come back to a warm space.

Sizing the system is still TBD I think, but having it a little bigger feels prudent. Sometimes it snows here on the mountain, and I can always have a shorter burn in a bigger heater, but can't have a bigger burn in a smaller one

Aeron, you wrote this,

"Multiple people recommending bypass, which sounds good, only i've never done any of this, and am trying to keep it simple. Adding a bypass sounds like a lot more work than adding a tee piece at the base of the flue. Plus won't i need a clean out down there anyway? "

When all the folks that have built RMH's of nearly all designs, suggest this "bypass" thing, there is a good reason.  All are wishing you to have good success with your build.   By the way, a T-piece at the base of the flue, in no way, would be considered a  easy work around bypass.  I am not saying this to be rude, but rather stating it is simply not the same thing.

I think your letting the "idea of doing it" causing thoughts of a great obstical.  It is not.

Let me simplify this, by saying, on the build And this includes all of the designs!  you have a exit for the exhaust flue.   No getting around that. And the flue will always go UP.  if you feel you can do this, your only a few steps away from adding one more exit hole and slide gate and having this connect to your rising flue pipe.   if you feel you can do the bottom one, you certainly can do the upper bypass one.  I don't know about others in exact detail but this bypass does not have to be huge, mine is only 4"  so about 1oo mm or so--  it is simply to get things going the right direction easily and for maybe 10 minutes.  When you need it, you need it. This won't be often if  your stove is warm, as things are primed to get things going in the right direction.

Don't let this little thing stop you from building.

While your at it, you mention that you "will need a clean out anyway"  Not sure what your envisioning here, but a simple inspection hole across from the exit hole for the flue will be fine.  If your in any kind of Grain harvest country, there are very simple auger tubing rings that work extremely well. Build in, plug and your good to go. Again a very minor thing in the sckeme of things.  If you wnat pictures of the clean out, just post a note, and I or someone will post a few phots of this.

cheers
Scott

Hey Aeron, since this is a shop heater rather than a home heater, you might find my shop build interesting.
A unique hybrid build: I use double barrels for instant heat and a brick bell to store it.
When I built this, I was told it would not work...  they were wrong, it works great in a poorly insulated shop in Northern Montana.
https://permies.com/t/279929/Rebuilding-remodeling-Shop-Dragon. This is a rebuild thread.
Here is my original thread from many years ago,
https://permies.com/t/94980/Brick-Bell-Shop-Heater
You might find this one interesting as well
https://permies.com/t/140568/Dragon-flames