J-tube cast core with firebrick floor/walls

Glenn Herbert wrote:For an 8" J-tube, I think the 24" overall would be good, depending on your build setup. A barrel over the riser will not have much thickness and you will be fine with an 8" burn tunnel roof, or adjust the core length to get a 9" roof for brick dimensions if you are using bricks there. I think castable will be a good choice for the roof; mine is still in good shape in the seventh heating season while the walls have started to spall some. The feed tube should be deep enough that your standard firewood can fit all the way in and have the top covered if necessary. Deeper than that is not useful. So measuring along the outer edges of the form, 16" feed, 24" burn tunnel floor, and the cast part of the riser equal to the feed or less if you have good insulating riser material and want to maximize that. My castable core is 1 1/2" thick on all faces including the burn tunnel roof, and the roof is fine.

Fox James wrote:

I have cast tons of refractory cement as I build pizza ovens, I hope I am up to speed with the latest methods and  techniques.

Fox James wrote:Scott, I live on an island between France and England, I dont know where you live so my source may not be applicable  to you.
I normally use ready mix refractory in 25 kg bags I buy straight from the manufacturer in bulk, normally 30 bags to a pallet.

https://donkey32.proboards.com/thread/355/small-scale-development

Tim Comer wrote:For the third week in a row the local masonry supply house still doesn't have any castable refractory cement.  They say they normally stock it.  I don't know if this is a supply chain problem like many other things are today.  So I guess a 120 mile round trip to our "local" Menards is in order.  If that store doesn't actually have what the web page inventory says they have it'll be another 200+ mile round trip to another store to pick up what I need.  Did I mention that I live in the middle of nowhere?  I love it but it can be frustrating when looking for material.

And I promise... no mixing of perlite with the castable refractory!

Tim Comer wrote:Thanks Thomas.  I might look into that later.

I did find the castable refractory (dry) at a Menards that is about an hour away.  Three other Menards I stopped at have the premix (wet) refractory on the shelf where the castable is supposed to be.  I told each store's CS dept that they have the wrong product on the shelf.  Not sure the good folks at those three stores care enough to actually report it.  So, be careful what you're buying.

I'm assuming the premix refractory and the dry castable refractory are a completely different product.  If Menards starts their 11% rebate deal soon I will probably go get two pails and the fire brick I need.

Tim

Tim Comer wrote:

I just want to thank all those that gave advice to me about casting and J-tube sizes.  I have spent the majority of the last year thinking about how to approach this build and I'm stuck.  I'm tossing the casting idea out with the plywood mold I created.  I am now at square one.  I tossed around building a brick bell RMH and then bought a plan from Matt Walker (SuperHot J Ceramic Fiber
Rocket Stove Core and his Brick Rocket Mass Bench, a J-tube design) before finding out ceramic fiberboard is going to cost me an arm and a leg; mostly due to shipping charges.  And I really don't want a bench so that plan is not going to be very useful.  I'm sure it's a good plan but not suitable for my space.  I may be able to adapt the gist of the heater plan without a bench.  I'm sure I can build the J-tube and adapt to a brick bell to fit around it.

I've read the Wisner's book, The Rocket Mass Heater Builder's Guide (2016), and decided to use their "recipe" for a 6" J-tube combustion chamber found on page 98.  About nine months ago I mistakingly picked up 30 firebrick splits for a burn chamber and some Morgan Super Wool for a "five minute riser".  USing Wisner's book as a guide I just picked up 36 full firebricks at a local supplier and started to dry fit the full bricks into a burn chamber.  To my surprise and utter bewilderment I now realize the drawing by the Wisners (again page 98) is nowhere near scale using standard firebrick.

So now I am researching what dimensions I should be shooting for because the Wisner's dimensions are confusing.  I have a masonry tile saw and have no problem cutting any or all firebrick to the dimensions needed.  Wisner's book shows a 12.5 cm (4 3/4") wide by 53 cm (20 7/8") long burn tunnel.  It shows using ten full bricks flat on the base.  That's two brick more than needed for those finished dimensions, which has totally thrown a monkey wrench into the process.  I thought the ideal dimensions were a 12" high feed, 24" long burn tunnel and ~48" high riser.  And I thought 5 1/2" to 6" square J-tube dimensions are suggested.  Not 4 3/4".

So back to the drawing board.  I'm hoping someone can give me the latest J-tube interior dimensions that will work with a 6" chimney.  I reread the previous posts and the impression I get is it really is a wide range of sizes suggested.  And then closing down the feed tube with a brick removable cover seems to blow the idea there is an "Ideal" feed tube dimension.

I know, I know... it is an evolving technology and many builders are experimenting.  Batch box RMH seem to be the latest shiney thing but that adds major expense (door) to the project.  I'll stick with J-tube and 6" chimney.  If you can help an old fart make up his mind it will be appreciated.  

Scott Weinberg wrote:

I appreciate your input.  Sorry for not being clear.  I'll try to clear up some of the post...

1) Ok, the dry castable refractory, is just that. But certainly different than any mixture containing portland And if you have investigated enough, you will or should know, that anything like what your talking about on the HOT side of the J tube, no matter who is telling you, NOT EVEN WORTH trying.  it is my feeling that those that suggest it, have not used it for hard everyday burns.

I was looking for Castable refractory.  Menards had pre-mixed refractory mortar.  Their inventory level was wrong.  They showed two buckets of castable and zero refractory pre-mix.  I was trying to explain to the Menards CS person that the inventory was mixed up.  I don't think I was mixed up on the intended use of those products, though I have never used either.

2) it takes good molds to make good castings, and vibrating bed is of great benefit.   All of that cost money/time and head scratching.  

I won't be casting anything.  I have moved away from that idea and will use firebrick and Morgan Super wool for my burn chamber and riser.


3) You say "wet premix"  Very doubtful, a castable mixture, is wet.  The wet premix, is fire brick mortar.  Just excellent stuff. The two are not the same, nor could you justify the wet to make any kind of casting.  it is intended to be used very thinly.  I set all my fire bricks with it, from 1/16 to 1/8" MAX joint bond.

Agree!  I may have come across as saying the pre-mix was castable but I understand it isn't and didn't intend to use it as such.  I was merely describing my frustration with Menards which I will avoid in the future as it just causes confusion.  I'll try to be more concise in my posts.


So to me, it seems they do not have the wrong product, but perhaps you have the wrong intention for each product.  Not 100% but it would seem so.

See above...

knowing what those items cost in Menards.
Keep in mind, there is certainly many types of Castable as well as cost.  With 50# for $40 being in the market.  
there is also many grades of fire brick, and for the money, I have always felt the Menards bricks are near the bottom for quality vs cost.
If you have a brick supplier or fireplace store or anything similar, you may find a better deal.

Agree!  Last spring I posted somewhere in the RMH forum a question about US Stove Works firebrick when I was looking for splits.  I don't recall any negative answers but I totally understand now and that was my mistake.  The local masonry supplier, where I recently bought full firebricks, has splits and full at a much better price than anywhere on line or Menards.  I will return what I can to Menards.

These are just thoughts.

Scott Weinberg wrote:

Thanks, I'll take a shot at clearing this all up : )

1st paragraph) You can easily spend a lot on fire brick mortar, bricks, Insulated fire bricks, super wool, K wool and the list goes on.  While I have not sat down and compared the two, I bet Matt's design is not all that different in cost, if you through in labor, even closer yet in cost.  Time is money to some degree.

1st paragraph b) it looks like your set on a bell, and I have built a pretty large bell (7") as a batch box.  EVERY dimension is called out for such as a batch box
on Donkey32 site   https://donkey32.proboards.com/    having had both, you simply get more heat for less time spent with the stove, mostly because it holds a lot more wood. Per full burn.  I don't mind to be corrected, but this seems to ring true. And certainly won't take up any more space, perhaps a bit more planning, but you got to do that anyway.

I'll take a look but I'm still not liking $300 and up for a door.  That's my biggest con of building a batch box.  And yes, I've seen Matt's crock pot door.  Very clever.

2nd paragraph) I am not sure, where the "miss" developed on Wisner's plans, but without having them, in front of me, I am guessing it is not there. And you don't mistakenly  pick up fire brick, there is always a use for them, my fault with splits is that around here they cost about the same as full ones, maybe that is what you meant.

I can't argue one way or another on the Wisner book plan for a six inch J-tube.  I don't think it is a distorted drawing since they show ten firebrick as level two and the dimensions will be way larger than their plan shows.  Basically, two firebricks will be doing nothing to add dimension to the burn chamber if you follow their burn chamber length in the plan.  Eight firebrick on that level gets closer to the plan dimensions but I still don't understand why they are giving a 4 3/4" wide burn tunnel, feed tube and bottom of the riser.

2nd paragraph b)  unless plans are CAD drawn, and in a certain View, ( ISO and so on) they can be distorted, and thus, do as they say, not as you see. This has been true of sketches for years.

3rd paragraph)  Your very own description, of what you think is called out, and how you worded it, leaves out dimensions.   i.e, "10 full bricks, flat on the base" ?    This makes it hard to know what is intended, if things are not explained, in detail.

A failure of the Wisners to identify the layout orientation in the plan on page 98 leaves me unable to address this.  I've spent a lot of time staring at the drawings and attempting to discern the orientation of the firebrick but other than the number it is very difficult to make an educated guess.  This is where I hope someone with experience and access to that book could verify what I'm looking at.  Although, I think I have moved on from seeking that answer and will play around with the firebrick until I come up with a layout that works.  My first task will be to try to find the desirable interior dimensions.  The Wisner's book did what it was supposed to do.  It encouraged me to do a RMH.  I just carried it too far and tried to follow their drawings which leave something to be desired.

4th paragraph) While I am not so sure about 6" J-tube dimensions being variable, it has been fairly rock solid about 6" batch box dimensions that are KNOWN to work, In a bell, to boot.  As the interior sq foot surface area (ISA) has also been calculated out, this really leaves little to calculate as the bell height should be as high above the riser as your room allows, yet still be in the "good zone" of the ISA for your 6" stove.   Often not mentioned, but it never hurts to put in a bypass, as high as you can, just in case.  That is a whole separate subject, but worth investigating.

Gotcha.  I believe a batch box would be better in many regards but I don't want a fire lasting much more than a half hour after my last feed.  I will not be leaving the stove unattended for more than a trip to the pot or refrigerator.

I hope this doesn't come across as a "it has to be this way: but really the numbers are out there.  And with a little bit of time can be adapted to.  Lastly, you mention your wet saw, I have found this has been a fantastic help, but still a good thing to do the preplanning for the least amount of cutting. In the end, this will cost you less.

Yes, I appreciate your help.  I still need to get some numbers for interior dimensions of a 6" J-tube.  I'll get there.  There's a lot of winter left and I hope to get this thing done about the time I don't need it anymore this winter.

Best of success