Extreme Burning Batch box roof material.

Hi all;
As many of you know, I have 2 Batch box RMHs.
Both are in uninsulated buildings, both burn nonstop all day.  
The 6" in the studio burns every day, the 7" shop stove burns when I work from home.

The first roof material tried was ceramic fiberboard.
In my opinion, with an extreme burning stove... It Sucks!
Wood abrasion ate the soft CFB in no time. I will not be using it as a roof ever again.
However, it works great as an insulator outside the firebox behind the brick.

I have two different materials now, that I am trying.
The 6" studio Dragon received a very expensive 12x24x2" heavy firebrick tile.  This cost me $160.
The 7" Shop dragon received a large heavy cast iron griddle as its roof. It cost me $75
I have/had great hopes for cast iron. It cherries at 1800F and melts at 2100F
I installed it with no insulation on top, simply a 1" air space and then a 12 gauge steel plate overall for brick to be dry stacked on top.
My hope was the cast iron would shed its heat before warping.
In that aspect, it has performed as desired.
However... With extreme burning, I have a spot almost directly over the secondary stub that is sagging almost an inch...
Recently it also has developed a permanent red spot at that location. (changing molecular structure?)  I am slightly concerned that it might not last.
I may be worried about nothing,  only time will tell.
I do believe in an insulated home, cast iron would last indefinitely.  I recently received a report on that exact situation.
For three seasons an 8" batch has been burning with a griddle roof covered with several inches of cob mortar.
It has no sagging or discoloration at all.

My  Studio Dragon with the high-dollar heavy firebrick roof is as solid as the day it was installed!
Coffee table chat between myself and other rocket scientists had us thinking the brick would crack... (it still could I guess) and the cast iron would be the winner!
We will see by spring which roof is in better condition.

I have located a new source of heavy firebrick.
I can get a 24x9x2.5 firebrick tile delivered for around $100
Cutting that brick in half gives me two pieces to combine for a 18x12x2.5" roof...
Might be if the cast is not satisfactory my Shop Dragon will receive a Firebrick roof next season!

Thank you for this report!
I have question about roof temperatures in the batch box.
I know the class room batch box that lives at the Lab had a glass top and pulled decent numbers.
Similarly, the glass and steel cook tops used with Matts riserless designs are serviceable.
I'm starting to think this a part of the batch box that be cooled down without hurting overall performance.
I  was surprised  when I found out that a batch box didn't need insulation around the burn chamber, now I'm wondering if the roof of the burn chamber could be actively cooled, cooled by convection or allowed to radiate freely?

In your build ,  you could remove the bricks and 12 gauge plate.
You could leave it like that, put a stock pot of water there, or maybe a pile of scrap steel.
If you left the bricks and steel in place, a creating couple of strategically placed openings could make the 1" air gap into a convection cooling passageway.

I've also seen that the roof of the burn chamber can be the bottom of an oven.
This makes me want to add an "oven" that has a small opening into the burn chamber, fill it with biomass and make bunch of charcoal with every burn.

Thomas, what about using kiln shelves? (no idea about available sizes) Your comment about leaving the air space above the iron, and using steel to support the bricks above, made me think "yeah, that doesn't need to be structural" and reminds me of "relieving chambers" in the Egyptian pyramids (they hadn't invented arches yet...) Then also, why not an arched top? there are ready made arch firebricks...

Hey William;
Indeed Batch boxes do need insulation.
My 6" in the Studio has 1" of #8 superwool outside the firebricks.
Previously the 7" was completely wrapped as well.
Stove performance is not nearly as good if you send heat out of the box rather than up the riser and use it for complete combustion.
The Cast iron roof 7" in the shop was allowed to remain as is in the hope it would not warp.
It has not warped, but I understand I am losing efficiency by leaving it uncovered.
As far as trying to cook on that cast... forget it, cooking on a stovetop over 1500F would not be recommended.
Putting an oven on top would be a struggle to moderate the temperature for baking.

Matt Walker's cores are all fully insulated.
His stovetops are not when used for cooking.
He has mentioned, covering that stovetop to send higher heat to his bench.

Hi Kenneth;
Yes, I neglected to include kiln shelves as a possible rooftop.
I did consider them before, buying the cast iron griddle.
We have a Rocket Scientist in Canada who is trying a kiln shelf roof this winter.
I am waiting to hear how it holds up over the winter.
The cost of the shelves is more than I paid for the cast iron but less than the heavy firebrick I purchased.
Peter has successfully used Kiln shelves on his builds.  However, his builds are not normally used as extremely long-burning batches.

Arches are cool but...
Peter's Batch boxes are designed with a flat roof.
I believe that is a necessary part of his design for clean burning.

"Arches are cool but...
Peter's Batch boxes are designed with a flat roof.
I believe that is a necessary part of his design for clean burning."

Now, you know I just have to ask "Why?"

Thomas Tipton wrote:"Arches are cool but...
Peter's Batch boxes are designed with a flat roof.
I believe that is a necessary part of his design for clean burning."
Now, you know I just have to ask "Why?"

Yes, my developments were all done with a flat roof. Due to the simple fact I had cast slabs lying around at the time. So, all my test runs were done with that configuration detail. Whether or not it's an essential detail I simply don't know, as such an arched roof is an untested configuration.

But... Lasse Holmes original idea was done with an arched roof. Even the post on Donkey's that triggered my curiousity showed an arched firebox.
https://donkey32.proboards.com/thread/511/adventures-horizontal-feed
There's the need for a tension frame to hold it all together and the top of the port need to be lower than the roof above it. The one pictured is one of Lasse's first heaters.

Plus... the Münster build, being a 25 cm (10") system, did sport a completely round firebox that worked right at first fire. The box' volume was calculated conform the accepted standard, the space above the port as well.


From here on it's guess work, albeit an educated one so to speak. Keep the proportions the same as the accepted standard and do an arched roof. Weld yourself a tension frame and go for it. Chances are that it would work just like that. Leave at least an open space of 2" above the fuel. The workings of this rocket thingy is due to laws of physics, so when the volumes and openings are correct the thingy will obey those specific laws without as much of a flinch.

thomas rubino wrote:
The 6" studio Dragon received a very expensive 12x24x2" heavy firebrick tile.  This cost me $160.

Did you have an aversion to casting your own refractory piece for the top?

For a 6 1/2" batch box I am building for my best friend, I added corbels on each side to support the firebrick central span. The total width is about 9 1/2", so I cut the corners off of projecting bricks to narrow the span at the top without seriously altering the configuration.

Even a slight angle for the roof would allow fire brick to be used but you would need to restrain the bricks and still allow for expansion movement.
One way would be like this ….or you could cast a self supporting piece from refractory cement.

@Fox James, something like what you suggest has been done in the woodshop at Wheaton labs. It's a 7" batchrocket, feeding three barrels.

I think your picture reflects what was done on the woodshop's 7" batchbox .
https://permies.com/t/174418/a/193594/thumb-IMG_20221031_132219.jpg

Edit: I see Peter beat me to it!

Peter, since you are commenting here,  can you tell me what your current views on insulating the box of a batchbox?

In the past you have described it as optimal but optional, and the 8" that was built in in the classroom used hard firebrick without insulation and a glass top.
I was thinking of this, and the durability of the barrels when I suggested the top of the box could be made from a radiating plate of iron.
Does this batch box still exist in its original form?

Related to this is the behavior of a core when it is built into a bell.
As hot as the gas inside the bell is, it is relatively cool compared to the core.
The sides, bottom and top of the batchbox are often left un-insulated in such an arrangement.

@William, as far as I know the original 8" batch box rocket in the classroom is still there. I have seen some pictures of it lately and it looked battered, some bricks around the port seemed to be dislocated. It was built in 2014 as a demonstration project, this is its 9th winter. I would guess it's due for an overhaul. And yes, the firebox wasn't insulated but the riser is, it's a vacuum formed riser tube. Low mass, highly insulative and heat resistant.

Insulating the firebox of a batchrocket is still optional. I'd suggest to insulate the riser very well and build it out of split firebricks. The top of the firebox will get very hot, a plate of heavy gauge iron might or might not be up to this. My choice would be a refractory slab anyway.

thomas rubino wrote:
As far as trying to cook on that cast... forget it, cooking on a stovetop over 1500F would not be recommended.
Putting an oven on top would be a struggle to moderate the temperature for baking.

Peter addresses the need to insulate the burn chamber above.

I'll need to look at Matt's designs again.
I thought the glass top formed one side of the flame path.
If covering the glass sends heat back into the core, that would seem to support that.

This thread on Donkeys site is what made me think of an oven over the batch box ceiling.

https://donkey32.proboards.com/user/1957/recent

It mentions how that location is liable to be too hot, burning food instead of cooking it.
This would not be problem for material we want to turn into charcoal.

The process of pyrolysis could use up heat in a place where excess heat is proving to be a challenge, and also contribute fuel to the burn chamber.

I have always been drawn to the batch box design that uses a floor channel, but I wonder could  the secondary air provide cooling for the ceiling of the batch box?
The air space between the cast iron and the other steel could form a plenum, cooling the cast iron as it heats the incoming air.

This topic is of interest to me at the moment also.

My current plan is to build a 10" batchbox per Peter's designs, but finding a suitable roof is a concern.
My primary goal in this is durability, of course functionality is crucial as well. I really want something that will last as long as possible to prolong the need for any rebuilds.
Peter mentioned above that his choice would be a refractory slab.

Two questions about that...

1. What do we know about the longevity of a roof like this?
2. Can anyone speak to the difference in quality between a purchased refractory product and something that is home cast?

Hi Miguel;
A factory-made large firebrick tile is the longest-lasting batch box roof available.
Casting your own works but it is harder than it sounds, cracking is a definite possibility.
Cast iron works well if you are not extreme burning.

thomas rubino wrote:Hi Miguel;
A factory-made large firebrick tile is the longest-lasting batch box roof available.
Casting your own works but it is harder than it sounds, cracking is a definite possibility.
Cast iron works well if you are not extreme burning.

Thanks for the reply, Thomas.
I've been admiring your site and products for some time now as I plan my build and will likely be ordering some components in the future.
What would you consider 'extreme burning'?

Hey Miguel;
I consider extreme burning to be back-to-back burns for hours on end.
We burn like that in our uninsulated greenhouse and it remains above freezing all night with no fire.
My shop stove is the same way, a large poorly insulated building.
I burn nonstop when working out there in cold weather.
I used a cast iron roof on the shop stove and although it is still in place and usable it now has a large sag at the hottest part.
In contrast, the factory-made Firebrick tile I have in the greenhouse is as good as the day I installed it.
For longevity, I have found the premade tiles to be the best.

thomas rubino wrote:Hey Miguel;
I consider extreme burning to be back-to-back burns for hours on end.
We burn like that in our uninsulated greenhouse and it remains above freezing all night with no fire.
My shop stove is the same way, a large poorly insulated building.
I burn nonstop when working out there in cold weather.
I used a cast iron roof on the shop stove and although it is still in place and usable it now has a large sag at the hottest part.
In contrast, the factory-made Firebrick tile I have in the greenhouse is as good as the day I installed it.
For longevity, I have found the premade tiles to be the best.

I won't be burning to that degree, but based on your input I definitely think a factory made firebrick tile is the option for me, regardless of cost.