Rocket stoves in Greenhouses , our own forum topic

thomas rubino wrote:Hi Dina;  Possibly the book you are referring to is the original Rocket mass heater book by Ianto Evans  & Linda Jackson. For years this was the RMH book of choice. Its still a good choice for backround.
There is now a newer option called the RMH Builders guide by Ernie & Erica Wisner.  This book is now the RMH go to book for accomplished builders and first timers.
My green house in northern montana has an 8" J tube and uses less than 5 cords all winter to keep it 40-70 all day and all night with NO FIRE from 10 pm -6 am .... pretty neat huh.
If you are just learning about RMH's be prepared to get the bug ... you'll be up half the night reading ... and soon .... you'll want to become a rocket scientist yourself!
ENJOY !

Awesome! Thank you! I would LOVE a visit to see how you have things set up and operating! There is so much to read and so many different ways to 'skin this proverbial cat' depending on materials on hand and location on the planet and all that changes with that pinpoint that my head doesn't seem to stop spinning!

Sounds to me Dina, that you have caught the BUG!!!  OH NO, No hope for you ....  Now you will have to build a RMH!!  
Looks like your down in the Salmon area.  To get up to where we are, its a long haul up US 93 . You would be passing numerous RMH's some with greenhouses , including the Wheaton ranch. We are 2 hrs NW of Missoula.
Our greenhouse is actually an artists studio now with nothing edible growing (unless you count the coffee & lemon tree... err bushes.)  
We did start out as a working greenhouse but my wife is now more interested in her art than a year round garden.
Visitor's who want to talk rocket science are always welcome.  

I haven't really paid much attention to the rocket options for greenhouses.  But now that I'm building a greenhouse, I should keep it in mind if my initial heat source fails me (compost).  

Is there a mass heater for a green house that you can load a bunch of wood into once or maybe twice a day and that's it?  I don't want to be tending a little fire for an hour+, I want to get a larger firebox burning and let it convert that wood into heat that lasts all night (and day).  

Hi Mike;
Your asking for a lot... In an insulated home 2 fires a day is plenty , in an uninsulated greenhouse... that really won't cut it in winter.  RMH Jtubes burn 45-60 minutes , batchbox rmh they say burn 60 minutes but at a hotter rate.   If you want a mass in a greenhouse to stay warm overnight in a northern climate it has to burn the better part of the day.  Its really not that hard to set a timer for 45 minutes and pop over to stuff more wood in the dragon.  

I know, I like to ask for a lot.  Luckily my greenhouse will be insulated and passive solar with nighttime deploying insulation for the glazing.  I think it will be able to keep itself warm during the day with the sun, I'm just trying to get it through the night.

So "batch box" sounds like what I'm talking about right?  A batch of wood?  Versus little splits and twigs?

According to my crude thermodynamics, my greenhouse will theoretically lose 345KBTU per day maintaining a 60 degree difference to the outside.  On an "average" day in December it will receive 483KBTU of solar energy.  So if I have a day with half the gain of average, I'd be short 100KBTU.  If wood is 7KBTU per lb, and if I could convert 14 lbs of wood perfectly into heat, I'd be all set.  So a batch that holds 25 lbs of wood could do the trick.  Unless I'm way off on the math or overly optimistic (which I often am).

Always good to be optimistic !  I like the sound of your greenhouse. Is that automatic deploying or manual on the glazing insulation ? I hope you post photo's ,Would be nice to see your build.
Yes, a batch box holds its wood horizontal,uses split,but larger wood and more of it than a J tube. It is also a closed unit with no sparks able to get out.
Peter van den Berg has been making some really nice units.  There seem to be new innovations in design every few months.
A batch box with  brick bells  would have a smaller footprint than a long low mass. Might be something to investigate.
Seems like northern Wisconsin would get mighty cold in December. Also seems your predicted solar gain might be a smidgen overly optimistic for that time of year ??? :) We have clouds and snow or snow and clouds at that time of year.
We love our rmh in the studio / greenhouse , it is evenly warm thru the whole building, not the typical hot by the stove and freezing by the wall wood stove effect.  Of course going from 12 plus cords to less than 5 sure helps.

My plan is for manually deploying insulation at first so I can "feel" for issues as it goes up and down.  Then once I get sick of doing it manually I'll put a motor on it.  I've been documenting the design and build in this thread: Mike's Passive Solar Greenhouse Design/Build  The thread is rather long but now I'm building and there are pictures to look at.  Design chatter is mainly on the first page but I've evolved the shape a bit over the last year.

Yup, it gets rather cold here in December.  Jan/Feb is worse but it's sunnier then.  Mid Nov through Mid Jan is my challenge due to cloudiness.  My calculated solar gain is from a few online sources but the data is hard to really understand.  They list things like "average" and "average minimum".  I really want to know what the worst day is, the 5th percentile, etc so I can predict better.  Oh well...

Thanks for the batch box info and the brick bell option.  Hopefully my compost will work but if not, now I know where to begin my RMH research.  Thanks!

thomas rubino wrote:Sounds to me Dina, that you have caught the BUG!!!  OH NO, No hope for you ....  Now you will have to build a RMH!!  
Looks like your down in the Salmon area.  To get up to where we are, its a long haul up US 93 . You would be passing numerous RMH's some with greenhouses , including the Wheaton ranch. We are 2 hrs NW of Missoula.
Our greenhouse is actually an artists studio now with nothing edible growing (unless you count the coffee & lemon tree... err bushes.)  
We did start out as a working greenhouse but my wife is now more interested in her art than a year round garden.
Visitor's who want to talk rocket science are always welcome.  

I would love a road trip with many stops along the way! I am located in Arco Idaho...

I just got back from a dog rescue run to Dillon Montana on Monday and had a lovely experience with a flat tire on a back mountain road on my way home. I was able to see a lot of elk bedded down while finding my 'center' and strength to break the lug nuts loose and change the tire to an old donut and drive ever so slowly BACK to Dillon (I only managed 'slow' on the gravel but once the pavement was under the rubber, desperation kicked in to get to a tire shop before they closed) I was unfamiliar with that area between Grant Montana and Leadore Idaho... I am ready to take that road again with my new tires! Maybe I can get a critter sitter for a long weekend so the husband can tag along, maybe mid October. We need to get our chicken coop winter ready and our high tunnel finished before it cools off too much...


Would ya happen to know anybody in my area willing to help out with building the end walls and draping the double plastic over the 24Wx88L ribs??

Hi Rocketeers,

short but important question about the draft of a Rocket in a Huge !!! Greenhouse

I have already build 16 RMH before I build my 2 last ones I have the question on. I also have books and DVD´s and so on but no Answer so far on the following question:
We have a greenhouse  10 meter wide and 50 meter long. In the middle I have build 2RMH


The Rockets are build in the middle so each pipe in the ground is approx 30....32 meter long including the chimney outside. The dimensions of the tummel, heat riser, .... is the same like always.
But I get NO draft to the Rocket.  Sometime the Smoke sucked in, some time the flame comes out  of the feeding mouth, some times the fire went even out because of a lot of smoke
The barrel becomes warm but not hot.
It seems because of the big cold system i can get no draft in, even after some hours of heating up the system.
Is this system to long (30 m horizontal pipe in the ground)??  

Thanks for help in advance

Hi Thomas; Your trying to push almost twice the distance an 8" rocket can push.    
Max run for an 8" is 50' or 15 meters.

Not to mention the horizontal pipes in the soil, when there's no insulation beneath it the ground below acts as a giant heat sink.

Hey folks!

You guys are awesome!

I have a little different perspective maybe as a 20-something interested in commercial aquaponic greenhouse production using zipgrow towers. For those who aren't familiar that means my plants won't be in the soil or what you all seem to be using for your mass. Instead they'll be hanging in fiber media towers with water from the sump tank trickling through them.

I am in the planning stage of my system and I am planning to start small and scale quickly.

For context I am in zone 4 south central Missouri with an altitude of approx 1750ft. I obviously won't need the stove year round but would like to keep up production through the winter. I may even eventually choose to supplement light for some plants like basil.

I tried to read this thread front to back and just couldn't get through it all so I apologize if I have missed the answers to some of my questions.

Firstly I'm not sure if anyone answered the humidity problem for the cob? I was thinking you'd have to fire the thing up and dry it out before you started growing and building up humidity in the greenhouse, then once dry paint the cob with whatever that seal is they use for pottery.

Now here's where the eyes will start to roll because I will need to heat the air. My plants will be hanging in it. I was actually looking into wood burning boilers before I stumbled upon what this community here has been up to with the rocket stoves.
Here's a quick look at how those wood burning boilers work with heat exchanges to warm the air: https://youtu.be/SRuCzKal0o8?t=108
Note the water does not actually reach boiling temp in these systems.
So that would be great but its such a huge upfront cost obviously.
I know I'm hitting some touchy ground here but I have to ask if anyone has figured a way to make essentially a rocket boiler (that doesn't actually boil).

I saw a video where Earnie was talking about the temperature differences in different parts of the system as related to the distance from the top of the barrel down to the top of the shaft coming off of the burn chamber. Could that distance be increased so as to keep the top of the barrel at a mere 200 degrees so that another barrel of equal size could be set on it filled with water that never actually boils but circulates to heat exchanges? I was thinking that instead of a bench to each side of the burn chamber you could have more barrels where heat is pooling and heating water barrels above them. On these secondary barrels over heating should be less of an issue and maybe they could be insulated on the sides so that their heat is more efficiently directed upward into the water.

I know feeding the rocket stove is also an issue, and I have seen Rob's pellet feeder. I love that Idea but I also have to wonder if rocket stoves can be scaled up to accommodate larger loads that burn for longer.

Many of you maybe thinking at this point that I need a different solution. I have to say the thing that draws me most to the rocket stove is that it burns away the smoke! With the addition of a dehumidifier you could just let the exhaust come out inside of the greenhouse and you would be supplementing CO2 for your plants! (not to mention you're wasting none of the heat you are producing) That is one less limit on your plants growth and because the plants are getting a more efficient "breath" they will open up their "mouths" less and therefor slow the transmission of plant diseases.
here's a far more educated guy than me going on about it:  https://youtu.be/xgLGCH9ErVE

I think that covers my questions and ideas on the subject. I know that was a lot, feel free to pick it apart. I appreciate your input!

Exhausting carbon monoxide into the greenhouse makes for a short life.

Exhausting carbon dioxide into the greenhouse is futile unless you first provide adequate light levels as it only works when carbon dioxide is the limiting growth factor, eg. In summer.

Hi Jay;
I hardly know where to start.    Graham already answered about venting exhaust into your green house, lets just say ,not a good idea.

Next lets talk about boiling water...  https://www.youtube.com/watch?v=1oDpmmsqHwQ This is a safe way to make hot water with a rmh.  https://permaculturenews.org/2012/11/23/rocket-stove-hot-water/

Giving  a larger gap to the riser would lower the barrel top temperature yes,  but trying to set another barrel on top would steal to much heat and cause an incomplete burn.
You could try hanging a barrel above your hot barrel but not touching. Would certainly warm up, without taking to much heat away from the burn.

Cob and humidity.   Yes, you must allow your cob to dry, and heat up. This can take 4-6 weeks or more. Thing is , depending on what style you build, you may not have huge amounts of cob to dry.  Using a brick containment or  building a Matt Walker 1/2 barrel bench in a containment , Masonry bells , there are options other than an all  cob seating bench.

Longer burn times .   A warm  8" J tube needs wood apx every 45 minutes.  an 8" batchbox might go hour and 15 maybe 1.5 but no more.  You can build larger ones they say.

We heat a solex covered greenhouse in northern Montana with an 8" J tube. Fire is burning all day until 8-9 pm then no fire all night long. commonly +10 - 20 outside and 40 - 50  inside next morning

Hi Grahm, Hi Thomas,

I was under the impression all the carbon monoxide burns off. I guess I should have assumed the science on that isn't an exact one. Has anyone held a carbon monoxide detector up near one of these things to test what's going on there? I do hope one day to supplement light when necessary and that is when carbon will become the limiter.

This coming year I'm starting so small I will just utilize the warmer months then hope to move into a bigger greenhouse with adjoined fish house and the works next winter.  I'm sort of feeling out the feasibility of the RMH for that project and how I can grow from there with it. I'm thinking feeding the thing all day I could handle one day.. after I quit my day job and build a WOFATI!

I hadn't thought of the heat being stolen and messing up the burn. I'll definitely get to those links on heating water here in a couple hours and keep thinking about it.

Thanks for your time guys. I know I'm kind of the lunatic late to the party already drunk here.

Jay Gore wrote:For context I am in zone 4 south central Missouri with an altitude of approx 1750ft.

 I don't want to take the thread off track but I just wanted to check this detail.  I think south central Missouri is closer to a zone 6.

Also, along a tangent...  How are you designing the greenhouse?  Passive solar designs and insulating the foundation can make a huge difference in how much heat you'll need to create.  May be worth another thread so that it doesn't take this one too far away from RMHs.

Hey Mike!

You are right! I just did a quick image search for zone maps and whatever I was looking at was not the USDA system because it was going the opposite direction! I'm right at the border of 6a and 6b.

I'm going to be in crumby little tents for a while. I'm thinking if I just keep the water livable for the fish, grow something crazy hearty like tatsoi and not worry about air temp during the winter I might be able to keep it going year round in these smaller stages. When I scale up serious I'm leaning towards a walipini! That should make the RMH far more feasible! I've been up all night researching! SO MUCH COFFEE! I was working on this earlier... er.. sometime:
https://www.instagram.com/p/Br-UXVTlNn7/

I think a water tank on top of the regular barrel would not affect the burn in the riser at all, but would cool the exhaust gases more than a plain barrel, increasing the downdraft effect if anything. I would give a good gap from the riser top, several inches at least, but all combustion should be done once the gases exit the riser if the core is well built with a tall enough riser.

CO should be very low in a well-functioning RMH, but at start and coaling stage at the end of a burn it will be higher, and if conditions are not good it might become an issue at other times. Best not to take a chance.

Re climate zones, I knew that the USDA growing zones and the heating zones went in opposite directions, but when I looked, I found three different heating zone systems in a quick glance, so my part of NY would be zone 5, 6, or 1, depending on which one you use, while I am in USDA zone 5b.
https://www.inchcalculator.com/calculate-many-btus-needed-heat-home/
https://basc.pnnl.gov/images/iecc-climate-zone-map
https://www.eia.gov/consumption/residential/maps.php

https://planthardiness.ars.usda.gov/PHZMWeb/

Hi all,

First time to the group and hoping someone can help me wade through the mass of posts in this thread and elsewhere to ferret out the gems - my eyes have gone buggy reading over the past two days. I have Ernie and Erica Wisner's RMH design information, which it seems is the most current design information (??), but that doesn't cover greenhouse issues.

I am researching options to add heat to a walipini design planned for Nova Scotia Canada (Zone 6a - can hit -10 F / -25 C) rough image of the planned cross-section attached. The footprint is only 10' N-S by 16' E-W (i.e. windows face almost due south). The design _should_ stay warm enough for cold-season veggies without any active heating, but we are hoping to push it a bit more and I would like a contingency plan.

A RMH looks like a nice option, but there were also a lot of concerns posted about cob and humidity - maybe those are dead now? I want to be careful to design it to not overheat the small space while allowing for a slow release overnight. I will only be able to tend a fire in the evenings and maybe a short burn in the AM. I don't want to give up too much growing space - hopefully not asking for too much.

My first thought was to put the long pipe and cob "bench" under a raised bed at the back, but it looks like moisture might be an issue. I like the idea of using water for heat storage - are there any gems for a good manifold design to support this or could you just put the water jugs on the bench?

I'm happy to read and research, but I'm hoping that someone can give me some quick pointers to the most relevant content. Thanks in advance.

Hi my name is Shiloh. I have a few questions. We want to build a thermal mass rocket stove for our aquaponic greenhouse. It is 72x30 ft and has media beds that run 58ft long. Our desire in to put the chimney/ thermal mass bench  under the media beds. The trick is there is not a lot of room under there. We have chosen to use 4 inch double insulated pipe because of space limitations and can only put 3-4 inches of cob on top of the channel. The channel is made of cinder blocks and filled with gravel, then the cob. This will leave about 3 inches before reaching the bottom of the media beds. My question is  Is that enough space and mass to not melt my beds. Another concern is am I going to need a S formation for the exsaust coming from the barrel to cool down the air flow before it goes under the beds.
thanks for any help.

I'm no expert.
I think single walled duct is better for heating a bench.
Double walled would be right for the vertical chimney.
Using all cob will save money and deal the duct better.
If you are going to stick with gravel,  uninsulated  single wall stove ipe is recommended.

One single 58' stretch seems to be push the boundaries of what an 8" j rocket is capable of.

We are holding a learning, hands-on workshop on Nov 16 starting at 10:00 in North East Texas.
We will be building a huge batch box rocket mass water heater. The expectation is in excess of 250,000 BTU into the water.
The cost will be $45, lunch is provided.

Here is the video with the details:




Link to signup for the workshop
https://app.barn2door.com/e/Q8pGQ/all/3JYBV

Event on the East Texas Aquaponics website
https://www.easttexasaquaponics.com/new-events/rocketmassheaterclass2019

Link to videos of the current Rocket Mass Water heater build
https://www.youtube.com/playlist?list=PLgT0Z9NIZwSnhAy9_byeeTI0FRtdRbuBh

Ok, I'm back to this thread.  Yes, that means I'm likely giving up on using compost to heat my greenhouse  But since I've been through 1.5 winters already with this structure, I have a decent idea of how much heat I need to achieve my goals.  The question is if I can do it with a RMH system.

For background, my greenhouse is 20x40 with a 17 foot peak.  R20 insulated N, W and E walls with insulated foundation.  Double poly glazing and moveable insulation that seems to help at night.  Last winter the coldest it got outside was -29F and that morning it was +20F inside.  In the cold part of the winter it hovers in the 25-35 degree range inside at sun rise.  If it's sunny it heats right up to 80-100.  I have more thermal mass in there now so we'll see how it compares this year.  So far, if the outside temp stays above 10F, the greenhouse stays above 35.  So I'm dreaming of a solution that lets me start a fire on nights where it will be below 10 and the heat will carry it through till morning.  I don't want to feed splits into a J tube for hours...

I'm assuming a batch box would be the technology?  I'll look at Peter and Donkey's sites again to see what's happening there.  If anyone has a thread or design to point me at that sounds perfect for this application, please let me know.

Hi Mike;
Your greenhouse is doing amazing... as is!
What size RMH footprint were you thinking ? A long bench or a taller brick bell ?
I would suggest a straight forward simple B.B. Any of Peter's early designs would work well.
You could skip the metal barrel (unless you want the instant radiant heat) and go with  all brick around the riser.

For a bench  I would think Matt Walkers 1/2 barrel system would heat up quickly. You can side them with brick and have a fair amount of cob over top.
Or an all brick bell and mass would hold your heat, longer and have a smaller/ taller footprint. But take longer to fully heat up.

Thanks Thomas!  I was thinking something with a smaller footprint and maybe no bench if I don't need it.  Anything under 8' in diameter is an improvement over my compost bin .  And I can run the chimney out the ceiling with an 18' height, unless that's too much height/draft?  

It seems like the standard Peter designs go up to 8".  I'm assuming bigger is better and I'm assuming you generally burn one batch of wood per session?

I like the idea of the all brick bell.  From the start of a fire to the first reasonable bit of radiant heat, how long are we talking?  An hour?  My assumption is that I'd build a fire at 3-5PM (dusk) on cold nights and want that heat to bleed out of the mass over the course of 16 hours.  Is that hoping for too much?

I saw mention somewhere that in the US they use a double wall bell and in Europe they do a single layer.  Did I take something out of context?

Do you (or anyone else) know of a build thread, website, youtube, blog, etc of a build that might be similar to what I'm after?  

Thanks!!!

Hi Mike;
Lets start easy. Your chimney can go straight up single wall pipe until the roof. Then class A insulated to go outside. This is ideal ! You keep that heat indoors and your chimney is always warm.
Next) 6" or 8" is the standard on all RMH's .Bigger can be built but I really don't suggest  it as a first build.
As far as one batch per session. For you... maybe....your building is doing great currently with compost!  In an insulated house  at 70 F, yes, once in the morn and again in the evening.
Your next question about how long before you feel heat.   My brick bell is a hybred. I have an 8" J tube with a barrel and then go into the brick.  So I get radiant heat instantly and then the brick warms.
So I'm not burning as hot as a batch nor as long.  For me at 1 hr the bricks on the sides are warming. At 3 hrs or so I've got lots of heat... Inside your bell you will have towers of standing brick (extra mass)
If I work in my shop every day (I don't) that mass stays warm even though the external brick temps are 40 F or so.  Then I get much faster heat on the sides.
Next)  In the US, a certified Masonry stove is required to be double skin. This is to be absolutely sure no exhaust can leak in the home.  I believe in Europe this is not a requirement, but I'm not sure.
As this is in a greenhouse with tall ceilings I would go single skin. Purely personal choice for you as this is not in a living area.
Is your floor a slab?  Insulated ? I'm guessing it must be. If not, under the rmh needs to be.

As far as plans...   the BB itself, Peter gives the ideal dimensions. After the core unit, its up to the builder how large and what shape your bell is. It really doesn't matter as long as you stay under Peter's max ISA (internal surface area) for the size core burner you are using.

As far as building materials,  I used fireclay for the whole build (keeps cost down) but using masonry cement (high temp) your bell would be bullet proof. A clay build can be broken if crashed into.
Building with brick is fun (I love how the old brick looks) However properly done, everything must be straight, square and level. Any of Peters builds are this way.
 
Myself... I winged it, Straight , square, level ??? whats that... give me another brick and lets build a brick box...  I certainly don't recommend my methods but it gives you an idea how much leeway you have.

Thanks Thomas, that's very helpful!!!  

My floor is uninsulated dirt.  The perimeter of the building is insulated though.  So I'd probably need to make a flat thin pad of cement for this to sit on.  Would insulation under that be recommended to keep the heat going into the bell instead of the ground?

You said you built yours with fireclay but it sure looks like there are bricks in there.  Is the fireclay just the mortar part?  Or is that for the batch box part?  I like the idea of a brick heater because I think it would look cool.  Would any bricks work for the bell and general exterior or do they have to be a special type?  I'm thinking the core needs to be full firebricks?  And there is probably some magic stuff to put around the riser.  But I have time to round that up.

New line of related thinking...  I love the idea of having a little sauna in the greenhouse.  If I built a small sauna next to the RMH that shares a side with the heater, might that work?  Or if I inset a steel plate into the side of the bell for some fast radiant exposure into the sauna?  Or am I trying to stack too many functions?

Do people ever take an old woodstove with a glass door and make that the door to the batch box?  Basically cut the front face off of the stove and magically attach it to the core of the batch box?  Seems like a possibly slick way to get a window and a durable hinge/latch arrangement.

Hey Mike;
Yes, 2" EPS foam board and a small slab over it will keep your heat rising.
Next) Those indeed are real bricks:)  Fire clay is used as mortar to seal the bricks.  As I mentioned, clay mortar pops right apart with an impact. Makes for easy rebuilding. However an accidental hit can break them apart.
Cost on a #50 sack of fireclay ranges from $7- $20. I think I used 6 bags of FC and that many or more of building sand. (sand is used to fill, if your bricks have holes)
When you use clay as a mortar it is applied very thin. Its meant to seal imperfections not to provide a thick layer "cementing" the bricks together.
Refractory cement is a high temp cement. It is applied with the traditional 1/4" layer to "cement" the bricks. Unfortunately it is harder to locate and is much more expensive.  Expect $50-$100 per #50 bag.
It is very sturdy , could take an accidental slam with no damage. Will last a lifetime.
Next) In the core you will want Split firebricks, maybe insulated ones as well.
In the surrounding bell, standard red clay bricks are used.
The newest risers are called a five minute riser, made with Morgan superwool ceramic blanket and stove pipe.
I'm not sure where on batch boxes but ceramic boards will be used there as well.
Next)  Yup, your stacking functions.  Give up the attached sauna idea. I mean your hoping for one burn a day, You would not reach sauna temps before it was cooling off.
Now, build a small J tube in a little room next to the bell and you got a sauna!
Next) Doors... The high heat of a batch I think makes a standard wood stove door warp. (I could be wrong) I think Matt Walker is fabbing doors with windows. I don't know if he sells them or just builds them for his stoves.
No Matter which , Matt is a super awesome guy .Who like Peter, shares all his innovation's freely .

Thanks again Thomas!  More questions...

I've seen in Erica/Ernie videos how they use clay slip for the fire bricks and I'm assuming that is what's done on the inside of these puppies.  For your heater with the red bricks, it looks like you have mortar between them.  Or is it just a think layer of fireclay?  If so, how thick is it?  1/16", 1/8"?  The slip that I've seen used seems to be a few thousandths of an inch thick...  I'm probably just getting my terminology and my memory confused.

I thought a batch would burn cooler (at the front by the door) than a standard wood stove.  I assumed that because of the air feed back by the riser creating the blast furnace effect way back there.  But I could be totally wrong.  I did see a video of Matt building a door with what I seem to remember being aluminum and some fibrous white stuff.  Of course, the aluminum was on the cold side of the insulation.

Now I just need a batch box heater builder in the midwest to want to come up this summer to build one for me.

I would say it ends up 1/16th  to 1/8th thick. Thinner is better.  
Its not a slip but a gooey mix of fire clay and sand.
I spread it on by hand and use a rubber mallet to seat it. Scrape off the extra, throw it back in the bucket and grab a new brick.
One benefit is your hands love it! As long as you don't use too much sand!

The Fibous stuff Matt used was no doubt Ceramic Blanket. Probably Morgan super wool.  
I know he has used salvaged oven glass in the past. Door glass and  top glass.
I do not know what he uses or if he uses them on a batch box.

Iv'e not built a batchbox yet , so there are some things related to them, that I have no direct experience with yet...
I know Donkey used a casserole door, on that sweet arched bell bb they built at Pauls.
Myself I would want  something like Matt built, with hinges.

Ok, that makes more sense now regarding the fireclay.

I used the arched top batch heater in the red cabin with the casserole dish this past summer.  It was neat but there were some design issues. It didn't seat perfectly so if you ever had smoke at the top of the firebox it would leak out the gaps of the door.  I'm sure a better draw would have probably made that much better.  Or having the top of the door lower than the exit point of the hot gasses.

Mike Haasl wrote:Do people ever take an old woodstove with a glass door and make that the door to the batch box?  Basically cut the front face off of the stove and magically attach it to the core of the batch box?  Seems like a possibly slick way to get a window and a durable hinge/latch arrangement.

Yes, it has been done before, to a degree of succes. From a US Stove or Vogelzang barrel stove kit up to a complete stove conversion like by the French permaculture group in the video below. They used an old large stove to build the batchrocket inside, bar the riser. Just ignore the French text, skip through the video to see how it's done.

Peter van den Berg wrote:Yes, it has been done before, to a degree of succes.

Thanks Peter!  Great video, wish I could speak French.  So when you say "to a degree of success", does that mean it's a compromise that doesn't usually turn out as good as other methods?  Or it's the best thing ever but it's a bit more work?  Or neutral?

I was mainly concerned with the interface between the steel of the modified wood stove and the masonry of the heater core.  How do you make that connection air tight when the materials expand and contract at different rates...

If I could cut the face off of a wood stove and bolt it to the front of a batch box, I can maybe see how I could use some wood stove rope gasket material to seal between them.

Oh, one more question that kept me up last night.  Does an 8" system need an 8" chimney?  It seems like if the air inlet is 10 square inches (cold air) and the riser is ~50 square inches (very hot air), might the chimney just need to be 4-6" in diameter since it's transporting warm air?

Hi Mike:  Rule of thumb. Chimney is the same as the core.  Not sure where you found 10 Square inches for the air inlet?   If we were talking 8" J tubes, your feed tube would be 7.5 x 7.5"

Oh, maybe I got my terminology wrong.  I got that off the chart on Peter's site.  It was called Primary Air and it was 10.0625"