First rough test rocket bell heater

So I bought 100lbs of fire clay and 4 feet of perlite. I have 2 cardboard sonotubes. An 8 inch that I'll turn into a 7inch and a 12 inch for the outer form giving me a 2.5 inch wall thickness. I just now saw that you say 50 is not too tall IF I have a barrel on the outside. I planned on just having clay riser alone with no outer shell. Are you recommending against no outer shell?

Dave; My riser has the 14" dia outer barrel, it worked all last season with no problems. This spring I rebuilt my core unit and simply lifted the riser off , placed it aside during the rebuild, then set it back in place and its working perfectly again ! This method worked very well for me and I recommend it to anybody. Will your method work... probably, but the outer sonotube will produce more ash then you want as it burns off, possibly plugging up the horizontal piping , if you can get a barrel it will make a better outer form than sonotube and will last a good long time. Also you can use sheet metal & tie wire to create an outer shell if a barrel is hard to come by. So for what its worth my opinion is yes, you want an outer shell .

I was planning on removing the cardboard outer shell as well after its cured so I don't think the ash would be a problem. Also just wondering, why did you have to rebuild your unit?

Sorry for the bombardment of questions but I just realized something else. In this website

http://www.mha-net.org/visit-with-peter-van-den-berg/

The batch box dimensions say the depth of firebox can be 4 to 5.5 times base.

Is that still accurate or is it just 4 times base at the maximum? I could definitely go for the 5.5!

dave marth wrote:The batch box dimensions say the depth of firebox can be 4 to 5.5 times base. Is that still accurate or is it just 4 times base at the maximum? I could definitely go for the 5.5!

Still accurate, could also be shorter though. The thing will act as a tunnel, so the length isn't critical.

That's great because I am coming from a wood stove that could hold 30" logs and I have about 6 chords already precut to 30 and I think I can get a 30" deep box if I use 5.5 times B. Thank you!

Dave; My first core was a cast core using the same mix as the riser. I had wear issues in the feed tube and the burn tunnel roof started to fail . My rmh is in our greenhouse and it gets run a lot more than one in a home would so I choose to rebuild with heavy firebrick that will take the abuse of putting more wood thru it .

Thomas, your just talking about using the heavy brick on the core only, not the riser also?

Correct , riser is still the same cast from last season.

I was getting so far ahead of myself with designing my bell heater and realized I don't even know how to glue my firebricks together for the batch box and the brick bell. I bought furnace cement but believe that isn't the right stuff to use. I search for refractory mortar and don't really know what I'm looking at and can't find it in stores. Will fireclay alone work as a glue? Is there a reliable diy recipe for high temp refractory mortar? How solid does clay get when used to bond firebricks together?

Dave; Fireclay is what you want to use ,to (glue ) your bricks together. Unlike regular brickwork your clay slip is applied thin. Its purpose is to seal imperfections in the brick and stick the firebricks together . So a 1/16 -1/8 " layer of fireclay is all you need . Lately I have heard of quickly dipping your fire brick into a tub of fireclay slurry and stacking . I have not tried that , I mixed a thicker batch and applied by hand . Either method works, just keep the mortar layer as thin as possible. Nice thing about working with clay is being able to work barehanded unlike concrete which requires p.p.e.

Dave M. : Old masons tell us that the job of the mortar is to keep the bricks apart, (allowing for a level and plumb build ) and not to hold them closely together
where their imperfections would make the job harder !

I believe that the total monolithic mass of a Rocket Mass Heater lends itself to wetting your fire brick in water, and then dipping them in a Clay Slurry as you stack
your bricks up one course at a time ! ( this should be repeatedly practiced with dry stacking before final assembly )

I do recommend using a clay slip for your 1st firing out of doors, it helps seal the bricks imperfections and then can be knocked apart, with any additional cleaning
needed done by rubbing the bricks against each other, i do not have a teardown on the RMH that was sealed with clay slip alone, so consider this an experimental
technique for the final build

If you will go with a Cob mortar, remember to dip your bricks in water this keeps the bricks from sucking water out of the mortar even before it has a good set !

Big AL

I've searched clay slip and cob mortar. There's many recipes, some with sand and perlite also. If I just dip bricks in the straight clay slip (pancake batter consistency) that would be good for being permanent? Just worried if I can pop the bricks apart after a test run, is it sting enough to hold together strong for years to come?

dave marth wrote:I've searched clay slip and cob mortar. There's many recipes, some with sand and perlite also. If I just dip bricks in the straight clay slip (pancake batter consistency) that would be good for being permanent? Just worried if I can pop the bricks apart after a test run, is it sting enough to hold together strong for years to come?

It's not the clay slip or else any mortar which holds the bricks, it's their weight. The mortar is just filling the gaps.

Thanks for clarifiing! So is my only option for the top of the bell to make a casting of a high temp refractory concrete?

You could make a vault too. Or a stepped cone Or use a slab of stone, protected by rockwool. A metal plate with kaowool joints, and bricks, brick slabs, stones or else. There's many options. RSJ with rests, so they can expand and contract, a layer of sheet metal and whatever on top. Plenty of options.

Just saw another problem. My dimensions call for a 10inch wide burn box in my batch box. My fire bricks are only 9". How can I still use firebricks to span the sides for the top of the box?

dave marth wrote:Just saw another problem. My dimensions call for a 10inch wide burn box in my batch box. My fire bricks are only 9". How can I still use firebricks to span the sides for the top of the box?

Hold theses with rolled steel T-bar. https://www.google.fr/images?q=T+bar+rolled+steel&rlz=1T4SAVJ_enFR550FR551&hl=fr&oq=&gs_l=


Or buy cheap pizza stones

Just one more question before I start cutting and stacking bricks bricks... Do you think I am making a mistake using a 7 inch heat riser with a 6inch insulated 18' chimney with 1 90 degrees bend upon entering the wall. I'm hoping the bell will cool everything off enough to squeeze it into a 6 inch chimney.

dave marth wrote:Just one more question before I start cutting and stacking bricks bricks... Do you think I am making a mistake using a 7 inch heat riser with a 6inch insulated 18' chimney with 1 90 degrees bend upon entering the wall. I'm hoping the bell will cool everything off enough to squeeze it into a 6 inch chimney.

Dave, i'm using a 6 incher, with way too much pipe, uninsulated, with a 1.7m² metal bell. Going through the wall with a 4 1/4 inch pipe, and going into a brick chimney, which exits against a wall. No chimney pot whatsoever. I have to prime it with a fan, but it works!

http://donkey32.proboards.com/thread/849/green-machine-inch-tube-help

So even if your thing doesn't work, you could always fan it!

I've begun. I poured my perlite clay furnace cement mix into 2 cardboard sonotubes 7 and 12 inches. So 2.5inch thick riser. Hoping it solidifies in a day or so to test run everything outside soon. I still have to cut all the bricks to size and build the firebox outside. I'm getting a little more nervous as time goes on about my 6 inch flue and 7 inch system batch box. Guess time will tell.

If this stove were 1 inch too big for the 6" existing chimney pipe is there any fix like reducing primary air or just using less fuel? Or would a complete rebuild be necessary.

Dave M : Way back in the days when Ernie Wisner was training with Ianto Evans, an 8'' system was necked down to a 7'' late in the horizontal pipe
for the Thermal Mass and in that one example the build was immediately successful, as reported. I never heard any update to this story, and it is
just one example!

Generally we spend 90% of your time trying to get our new fellow Members to build their 1st rocket as conventionally as possible.

The problem happens when after a Rocket built with several modifications has troubles, which part do we blame, and how do we test our hypotheses
with out even being there ! Seriously, the very worst outcome for the future of The Rocket Mass Heater is the single guy who says ''I tried, and the
damned things don't work'' ! For the good of the craft ! Big AL

I gotcha. Ive already poured my riser into the casting. I'm going for it I suppose.

Anyway I just checked my casting and it is rather damp and not nearly feeling tough or solid. Does the perlite fireclay furnace cement mix need to be heat cured? I have the cardboard forms inside and out I could burn out the inner one but seems one it will just crumble. It seemed like I got the perfect mix last night that I could squeeze it into a tight ball the pop it apart.

So I tried to cure my hear riser. It ended up just crumbling I think from just plain old moving it on and off the firebox. It dried out a lot but just got crumbly. I will have to build another riser. Is it possible to reuse the material from my first try? The fireclay and perlite and furnace cement mix? Can I just ad water to try again?

My solution, going back to the insulated firebricks for the heat riser. I was thinking is it a good idea to put just one course of dense heavy firebrick on the top of the heat riser? That way maybe it will hold the heat and stabilize the draft longer than just all insulated firebrick.

Also will the perlite clay mix stay bonded to the burn chamber with expansion ad contraction? Or will it separate and come off?

I made a batch stove core outside. It's a 6 inch system. I did make the firebox 30" deep (peterberg stated depth of chamber isn't too critical since it acts as a tunnel) then behind that is the base of the box that the heat riser made of insulated firebricks sits on. I did put in a p channel although I don't think it's wide enough since it's only about 2 inches wide and the port is about 2.75. I only had a 2 inch steel pipe I flattened to use for the p channel.


So I burned it for a while and it didn't smoke much when it got hot but it did smoke still. I tried all combination of different amounts of air and it always smoked. I don't have the firebox insulated right now. It's built from dense firebrick with fireclay joints. I was gonna sit the core in a box of perlite when I build it indoors and hopefully that will allow eye box to get hit enough for better combustion.

I have thought of 2 reasons it was smoking. Either it's because the firebox couldn't get hot enough since it was uninsulated. Or... Maybe it was because the firebox is too deep. I thought maybe it's too deep to concentrate the heat toward the back of the firebox. Also I know my p channel should be corrected. I will try to make a better one tonight.
Also I realize there will be smoke upon starting up the fire. With smoke there is creosote. It seems like with a batch box it will create more creosote than a standard j tube because there's more mass to a batch box. Am I correct or misunderstanding?

I am wondering if the creosote will make it to my chimney or just condense in my brick bells that I build. Does anyone ever get creosote with these systems still if burned properly? My wood was dried and seasoned as well.

I'll also try to get some pictures soon if that helps.

Thanks guys

Well, i'm affraid that 30 long, is too long. It seem to act the same as when overloaded heightwise. Except that it's overloaded lenghtwise. If you have all the 30 inches lengh burning, that ough to be too much i'd think.

Lack of proper P channel, big load, discutable heat riser insulation. Put a layer of batt insulation, like 4 inches all around your riser. And see if that helps.

Check this video, does your P channel draws like this one?

http://donkey32.proboards.com/post/11505/thread

As well, did you close the front with a door or pile of brick? Left 20% of the CSA as air opening? It often doesn't work for me, i have to go more for 25 or 30%

I think it's time for pics!

dave marth wrote:I have thought of 2 reasons it was smoking. Either it's because the firebox couldn't get hot enough since it was uninsulated. Or... Maybe it was because the firebox is too deep. I thought maybe it's too deep to concentrate the heat toward the back of the firebox. Also I know my p channel should be corrected. I will try to make a better one tonight.

Ive only once tried a 50% longer firebox on a fully fledged system with a good chimney behind it. I'd suggest to build it as close to the tried and tested dimensions as possible. Also, the whole thing is more finicky as the J-tube style and very vulnerable to leaks in the entire thing. Dry stacking isn't an option here. A correct p-channel will help when running flat out.

dave marth wrote:Also I realize there will be smoke upon starting up the fire. With smoke there is creosote. It seems like with a batch box it will create more creosote than a standard j tube because there's more mass to a batch box. Am I correct or misunderstanding?

No, you are not correct. There isn't necessarily more creosote produced by a batch box, normal time from cold startup to smokeless operation is 5 minutes, when hot about 3 minutes. Creosote build up should be absolutely minimal.
So there ought to be something not right, check the dimensions, make a proper p-channel and ensure there are no leaks.
See http://donkey32.proboards.com/board/6/reference-library for the dimensions spreadsheet.