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Crafting My Own Core: Designs, Pictures and Questions  RSS feed

 
Troy Fairclough
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i am going to start out with the pictures of the 8" core mold i made today, complete with measurements from the 15 dollar book by ianto evans.

in the following posts i will talk about my procedure and ask a few questions
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Troy Fairclough
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Now bear in mind all of the dimensions provided for the core mold in the pictures above are outside dimensions, not inside dimensions. dimensions for the outside box/case are obviously outside dimension. the idea of making the core from chip board is that when you fire it, the mold burns away leaving you the exact perfect size core. there is some planning and double checking to make sure that you dont make an error, and i was easily able to fabricate the entire core mold and box with 1 sheet of 7/8 inch waferboard. cost me 15.99 in Alaska, surely its quite a bit cheaper elsewhere. i did the entire build with a circular saw, a tape measure, a pencil, a straight edge and some dry wall screws and an electric screw gun.

again make sure that all above measurements for the core are outside of the mold, not inside.

as for the ash cash, i plan to screw a few additional peices to the bottom of my core mold which will allow for an ash pit and will burn away during firing.


now i am preparing to cast this bad boy, and i am fairly uncertain about what is best to use, and what mixtures. i have read over the book and i dont find anything specific to casting J tube cores.

this is my proposed mixture using portland cement, perlite, silica sand and fire clay.

http://www.backyardmetalcasting.com/refractories.html

is this a good idea and will it handle and be durable in the higher temps?

i hope you all like my plans and appreciate any advice i can get about pouring my core.

troy
 
Kevin Riley
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Having just built a great stove out of firebrick, I'm not in a huge hurry, but I'm thinking my next stove will also be cast core. I found your post, Troy, after I watched this video:

https://www.youtube.com/watch?v=7ANMXGrxgnE

I'm hoping to ride on the coattails of your question, soaking up suggestions. The criteria I'm filtering ideas with, are as follows:
1) Very Durable, especially in feed tube area
2) Insulative (keep the heat concentrated in the burn tunnel for optimal combustion)
3) The more eco-conscious and locally available the materials, the better
4) Cost effective

If it doesn't meat those criteria, I would be tempted to stick with firebrick.
 
Troy Fairclough
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Kevin Riley wrote:Having just built a great stove out of firebrick, I'm not in a huge hurry, but I'm thinking my next stove will also be cast core. I found your post, Troy, after I watched this video:

https://www.youtube.com/watch?v=7ANMXGrxgnE

I'm hoping to ride on the coattails of your question, soaking up suggestions. The criteria I'm filtering ideas with, are as follows:
1) Very Durable, especially in feed tube area
2) Insulative (keep the heat concentrated in the burn tunnel for optimal combustion)
3) The more eco-conscious and locally available the materials, the better
4) Cost effective

If it doesn't meat those criteria, I would be tempted to stick with firebrick.


thansk for responding i have been waiting patiently for any feedback!

ive done a lot of research. and i feel very confident that i have a good recipe to pour the core.

in addition to the above mix, i plan to add at least a gallon of furnace cement to harden things up as much as possible, this mixed with the clay and portland cement is going to harden things up as much as possible. in doing research on the feed tube area, ive found that if there is wear on abrasion points, you can use furnace cement, or just a plain cob mixture to patch any wearing areas. from the research i've done there is no wear from heat, just in your feed tube area with sticks and the like hitting the surfaces.

perlite is the best substance ive found in my research to insulate against heat, and ill be using plenty of it in my mix.

you might be able to find more materials naturally than i can. i live in alaska, we have peat moss, not clay. but it seems that a lot of people were able to harvest the clay, sand and concrete should be pretty inexpensive.
 
Troy Fairclough
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Kevin Riley wrote:Having just built a great stove out of firebrick, I'm not in a huge hurry, but I'm thinking my next stove will also be cast core. I found your post, Troy, after I watched this video:

https://www.youtube.com/watch?v=7ANMXGrxgnE

I'm hoping to ride on the coattails of your question, soaking up suggestions. The criteria I'm filtering ideas with, are as follows:
1) Very Durable, especially in feed tube area
2) Insulative (keep the heat concentrated in the burn tunnel for optimal combustion)
3) The more eco-conscious and locally available the materials, the better
4) Cost effective

If it doesn't meat those criteria, I would be tempted to stick with firebrick.


this is the mixture i am using

http://www.backyardmetalcasting.com/refractories.html

very similiar to the one posted in your video which i have also watched, but i think it will be harder and more durable.

 
Troy Fairclough
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also i have decided that the end of my outside box are just not wide enough and will be increasing my outside box size by 3/4 or 1 inch before pouring so it should measure 31.5 or so after i make the modification

i think three inches on each side and 4 on the bottom is enough size to lend strength and heat resistance.

 
Matt Walker
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Troy Fairclough wrote:
this is the mixture i am using

http://www.backyardmetalcasting.com/refractories.html

very similiar to the one posted in your video which i have also watched, but i think it will be harder and more durable.



Hi guys, that's my cast core video you linked to. Just a quick note, I've tried the backyard metal casting mix exactly as written there. It turns to powder after not very long as the heat breaks the bond in the portland. I'd strongly recommend leaving Portland out of any of the real hot areas.
 
Troy Fairclough
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Matt Walker wrote:
Troy Fairclough wrote:
this is the mixture i am using

http://www.backyardmetalcasting.com/refractories.html

very similiar to the one posted in your video which i have also watched, but i think it will be harder and more durable.



Hi guys, that's my cast core video you linked to. Just a quick note, I've tried the backyard metal casting mix exactly as written there. It turns to powder after not very long as the heat breaks the bond in the portland. I'd strongly recommend leaving Portland out of any of the real hot areas.


what cement would you suggest?
 
Matt Walker
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Troy, I describe my mix in the video linked above. Furnace cement as an additive is all you'll need.

 
Kevin Prata
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Hi Matt -

What ratio of furnace cement are you using? Just curious. I was (am) planning on a vermiculite + refractory mix at a 4:1 ratio.

Regards

K.
 
Troy Fairclough
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Kevin Prata wrote:Hi Matt -

What ratio of furnace cement are you using? Just curious. I was (am) planning on a vermiculite + refractory mix at a 4:1 ratio.

Regards

K.


i think he says 14 parts clay 14 parts perlite and 1 part fire cement.

seems like you should be able to do more for the sturdiness than just adding such a small amount of furnace cement.

now i am in hiatus again until i do more research. i really want this to be perfect and well built and the only way to allow that is to take your time and make sure each part is right

fired my 55 gallon drum last night and drank beer till 430 am. it was a lot of fun.

all the paint except for the very bottom burned off. ima take it to a wheel grinder maybe on monday

enjoy your weekends all

 
jacob green
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Matt Walker wrote:Troy, I describe my mix in the video linked above. Furnace cement as an additive is all you'll need.



Hi matt. Excellent videos.

I had two questions.

1. Is your heat riser a different formula than your core? I did not notice you add any fiberglass to the heat riser but you did to your core.

2. Your heat riser mix looked like it had more perlite, so this has MORE insulative properties than firebrick correct? I thought the goal with the heat riser was to use material that got hot, not reflected heat, to help create chimney effect and keep air moving through the system, and more importantly to burn more of the leftover smoke to increase efficiency and reduce smoke out the chimney cap.

I am a little confused that your heat riser mix seems to be even more insulative than your core mix.

I notice however that you don't need a separate insulating layer for your heat riser which keeps construction less complicated. Have you simply struck a balance between insulative and heat sink qualities that works ok for a heat riser? If so, do you think there is any, even slight advantage with hotter heat risers made from firebrick core wrapped in insulation, that heat the air in the riser to higher temperature than your design?
 
Matt Walker
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Glad you found them helpful Jacob.

As for mixes in the core and riser, in theory they are the same for my process, but in practice I do change them up a bit depending on how it's all going together. I tend to go for a harder mix in the feed, so I'll use a bit less Perlite and if possible up the furnace cement ratio. Risers never see any abrasion, and using my method they are always contained in the outer canister(the small drum) so I will sometimes eliminate the furnace cement altogether, although if you have it, use it.

In my opinion the whole combustion unit, core and riser, should be as insulative as possible. Contrary to what you've written, mass in the burn area actually cools the gasses as the heat is drawn in to heat the high mass of the bricks. In this way any mass in the riser will actually rob heat from the burn for a while to achieve that hot effect you are describing, so I prefer to focus as much heat back on the gasses as possible through the burn areas. In this way you can create a smokeless burn almost immediately and use the majority of the heat downstream to heat the space via the radiator or the mass. I'm a huge proponent of highly insulated core/riser combos. I've built both, and find a large performance improvement in insulated builds. Not to say firebrick builds don't work, they do, but if you listen to a recent podcast you'll find that Paul and Ernie state they felt they had 80% better performance when using insulated materials in the core and riser. I agree with them, so I try to avoid heavy firebrick in any of the burn areas. That said, you'll be able to make a dang good heater using any of these proven methods, so don't get too hung up on the mythical "perfect" heater.

I recently purchased a flue gas analyzer and have started to get some numbers on my home heater. If anyone has or is building a heater in the PNW and wants to get together to see how it performs, I'd be happy to try to make that happen. Here's a snapshot of what you can expect if you build a heater following my cast core design.

 
jacob green
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Matt Walker wrote:

mass in the burn area actually cools the gasses as the heat is drawn in to heat the high mass of the bricks. In this way any mass in the riser will actually rob heat from the burn for a while to achieve that hot effect you are describing,


One of the ideas for mini RMH to use in small space I was toying with is to do a single pour in to a small metal tub. In this case I was thinking of using the tub the core sits in as some amount of thermal mass. This would involve using a material that attracts heat, and radiates it back out. For a small space this would be effective because no external thermal mass would be necessary for the exhaust run, thus keeping the footprint as small as possible and still having at least SOME amount of thermal mass that radiates after the fire goes out.

If using this method, the burn chamber and mass surrounding it would absorb heat from the burn, not repel it.

In this scenario, would an efficient burn at least be possible AFTER the unit warmed up? I realize it would not come up to efficiency nearly as fast as what you are doing, but if I could get it up to efficiency after say a half hour, I would be fine with that.

Also I understand the purpose of insulating the riser, but I thought the CORE of the riser was ideally firebrick (dense material that absorbs/radiates heat ), then wrapped by an insulating layer which in turn is contained by metal shell, so that in essence the entire riser is insulated from the external barrel chamber, but that the CORE of the riser itself did not refract heat, but rather GOT HOT, to enhance chimney effect, and burn gasses?

I thought I had read Ericka and others said using the dense firebrick in the core of the riser (heat absorbing) then sandwiched by insulating material to insulate the riser assembly from the external barrel airspace was ideal.

I thought the purpose of YOUR riser design was simply to split the difference between insulative and heat absorbing, so that the riser would not need to be constructed of multiple sandwiched layers, thus making the design and build simpler.

Just to clarify, your riser design is simply ONE homogenous material enclosed by the 17 gallon drum to hold it together, is this correct? Then you simply place your external heat barrel over that assembly to complete the stove, correct?

Also if I use material in the metal tub that works as a heat sink, would it then be NOT a good idea to use your riser mixture which repels heat? And instead it would be good to stick with a riser core that gets real hot, sandwiched by insulation as per old designs?

My concern would be if I create a thermal heatsink in the tub that holds the burn chamber, eventually it would get hot, and if the core material in the riser does not also get hot, but instead repels heat, ( as per your design) that the burn chamber would be hotter than the riser, thus impeding and or cancelling the chimney effect.

Any thoughts on this?

 
Matt Walker
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Just to clarify, your riser design is simply ONE homogenous material enclosed by the 17 gallon drum to hold it together, is this correct? Then you simply place your external heat barrel over that assembly to complete the stove, correct?


Yes.

I thought the CORE of the riser was ideally firebrick (dense material that absorbs/radiates heat ), then wrapped by an insulating layer which in turn is contained by metal shell, so that in essence the entire riser is insulated from the external barrel chamber, but that the CORE of the riser itself did not refract heat, but rather GOT HOT, to enhance chimney effect, and burn gasses?


No, in my opinion this is less than ideal, although it still makes a good heater. I describe why above, but basically anything in the burn area that gets hot cools the gasses, not the other way around.

I thought the purpose of YOUR riser design was simply to split the difference between insulative and heat absorbing, so that the riser would not need to be constructed of multiple sandwiched layers, thus making the design and build simpler.


No, the purpose of my riser is an attempt to improve performance over a firebrick riser. I think a firebrick/insulation riser is a simpler build/design.

It's not a hot chimney that makes the draft, it's the hot gasses in the chimney. They are hotter if they don't lose their heat to mass, like fire brick, in the burn areas.

Again, don't over think it, if you want to build with brick/insulation, it will work great. If you look at pictures from Paul's "shippable core" workshops last October, you'll see that the fancy risers they use are formed ceramic fiber, which are low mass/highly insulating risers. That's ideal, in my opinion, and those are my favorite risers. My cast risers are a decent way to DIY a riser with pretty good properties. Firebrick/insulation strikes a decent compromise between function, durability, availability, ease of construction, and so on. Those are my opinions, btw, I'm sure there are lots of others. Again, don't worry about it too much, they all work well, but remember that the guiding principle of any rocket stove is insulate as much as you can anywhere in the burn area, focus the heat on the gasses to burn them completely, then harvest the heat downstream. That's really what sets these burners apart from all the things that have come before.

All that said, you can build a dang good heater with heavy stuff, and people do all the time. Here's an old heater I built that was poorly insulated and worked just great. And, it's in an old metal tub, just like you are discussing.

http://www.permsteading.com/viewtopic.php?f=6&t=8
 
jacob green
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Matt Walker wrote: Here's an old heater I built that was poorly insulated and worked just great. And, it's in an old metal tub, just like you are discussing.

http://www.permsteading.com/viewtopic.php?f=6&t=8


Cool thanks. That is exactly what I had in mind.

What did you make the heat riser from? and what was the diameter of the riser, burn tunnel, and exhaust?

I am trying to make a 4 inch system, and from everything I have read, the smaller you go, the more insulated and efficient things need to work to get proper drafting.

Also, what about using super insulated burn chamber and riser, but building mass around the OUTSIDE of the external barrel?

Would cooling the outside of the external barrrel mess up the functioning of the stove in any way? I was thinking of just leaving the top of a 17 gallon barrel exposed for immediate heat, and building a layer of cob or equivilant AROUND the barrel itself.

The reason for this is I am trying to keep the footprint very small, and eliminate te long external run through a cob mass.

Also, without the external exhaust run, I am hoping this will help the smaller diameter system draft more effectively, and save on exhaust pipe costs.

One more REALLY IMPORTANT question.

In your single more you mentioned lots of cracks, but said that would be ok because it will be covered with cob.

If those cracks were left exposed and not covered with cob, would that present a carbon monoxide hazard? or would the cracks in the burn core suck air rather than emit it?

I am assuming most C0 problems come from the exhaust portion of the stove leaking back in to the environment. Also the stove inlet is a HUGE hole by comparison to the cracks so if carbon monoxide wanted to escape it could just go through there during times when burn was winding down.

Thanks for all your help, your comments and contributions have been excellent!

: )
 
Matt Walker
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Dang Jacob, that's a lot of questions. You would also do well to read as much as you can over at donkey32.proboards.com, you'll answer most of your own questions in no time. I'll answer a couple here.

What did you make the heat riser from? and what was the diameter of the riser, burn tunnel, and exhaust?


That was a 6" system, with sacrificial flue pipe for the inner and a sheet of metal wrapped into a tube for the outer canister. I insulated with found clay/perlite combo.



If those cracks were left exposed and not covered with cob, would that present a carbon monoxide hazard? or would the cracks in the burn core suck air rather than emit it?


There's a lot to your question here, but I'll give you the short answer. If cracks in the core are open to air, the concern isn't CO coming out but rather that any air it can suck in through those is that much less draft in the feed, which can cause creep.

You will need to wrap that core in something, as much for durability as for concerns of leakage.

You can wrap a large part of the barrel and it will work, probably, depending on the rest of your system. However, if you don't have any mass downstream of the barrel, you'll just end up blowing a ton of heat out the chimney. If you don't have mass I recommend letting the barrel radiate as much heat into the room as it can, at least you'll get some heat from the wood. So, I'd say don't cover it. Without mass it will be a quick response/short burn cycle heater, so embrace that. You won't get long heating cycles without mass, so go for getting the most heat into the room from the fuel. You might even go for a double stack of barrels if you have the ceiling height. Same riser height, just another barrel above it to radiate as much heat as possible into the space. Good luck on your build.

 
jacob green
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Thanks again for everything, I am ready to build now. I am definately going to incorporate some of the things I learned from you in to my designs.

Incidentally have you seen this guy, he has come up with an impressively simple way to build small rocket heater. Only drawback is horizontal feed which doesn't allow putting really long vertical sticks in to reduce fire tending. But he got amazing results using portland, and no insulation on the heat riser. He has 140 degrees coming out the exhaust to outside with no visible smoke.

His design started as a rocket cook stove, and morphed in to a rocket heater. The build is insanely simple, and doesnt require a manifold:

here he starts as a cook stove:






and here when he turns it in to an indoor heater:





I think I will build a similar design, except I will use your fireclay mixture, and your insulated heat riser. Or possibly I will use vermiculite board or thermal blanket with hardener since you inspired me to go insulated to get the air as hot as possible. Also, one HUGE flaw in his design is that the build itself is rather delicate. If it got heat stressed, or weight stressed, or knocked into with a heavy solid object, it could easily collapse/crack through the entire shell of the burn core, and all that heat and fire spilling in to the living space would be a huge problem. Pouring in a metal tub would solve this as would several other methods.

If you have any cautions about the build and vent configuration I provided a link to as regards to C0, or woodgas explosions, let me know. Other than that I dont need you to answer any more questions as I fell ready now to build.

I don't mind if I build an experiment that doesnt work, I just dont want to build an experiment that endangers me or someone else.

I will be using a C0 moniter btw at all times indoors, I was just trying to get any helpful info to avoid a pointless build if I could.

Thanks again for everything, you have been really helpful.

: )

Oh, as pertaining to this comment:

"You won't get long heating cycles without mass"

Check out my recent topic regarding "device to retrofit standard j tube to burn pellets resulting in less fire tending"

I am working on the problem of short burn cycles with no thermal mass. : )

Someone really needs to figure out how to burn 6-8 foot fuel in the feed tubes to reduce the amount of refills during the day. I present several ideas in that thread. Maybe you will think of something to add.

thanks again.
 
Troy Fairclough
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jacob green wrote:
Someone really needs to figure out how to burn 6-8 foot fuel in the feed tubes to reduce the amount of refills during the day. I present several ideas in that thread. Maybe you will think of something to add.

thanks again.


maybe you could try using some formed chicken wire above the feed tub to harness longer sticks. however they would probably get caught in it and not lower into the feed tube. just my first thought.
 
John McDoodle
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troy
how did your ianto evans cast core turn out? did you finish this build? im about to cast a 32x16x20" core for my 2nd 6" rocket with massive core and im curious regarding longevity and such
 
Glenn Herbert
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Troy hasn't posted in over two years, so you are probably not going to get a response from him.
 
F Styles
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he probably upgraded the feed chamber to a larger size.
 
Tobias Ber
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hey ... i am still struggling in my head with the planning of a cast core. i m thinking to go for a 6" system and add a mass later, when the interior of the tiny house is a bit more finished.


are the dimensions in the photo above ok? how thick should the outer walls be? 2 inches? 3?

for the feed tunnel and first part of the burn tunnel i d like to make a first layer (0,5 to 1 inch) of clay-sand mix. does it make any sense to add rock wool fibres there?

he put the inner form in and outer box as mold. will that box be taken off, or would it stay? i think it would be good to make a permanent outer casing/mold from sheet-metal, but i am not sure if that s neccessary.
 
Glenn Herbert
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A permanent outer casing would make sense if you plan to move the core repeatedly, but would not be of any use for something built in. You would want to surround it with rigid insulation and an outer shell - cob, masonry, plaster...

Cast refractory cement is recommended to be no less than about 1 1/2" thick, and again, if it is not going to be moved around, there is not much reason to make it thicker. Just surround it with cheap rigid backup like perlite-clay.

The photos of the 8" core mold are not the recommended ideal dimensions. if a core is not square in section, it is said to work slightly better if it is taller than wide. That one is distinctly wider than tall. A 7" square or 7" wide x 7 1/2" tall cross section is the standard, and a 26" long burn tunnel is a bit longer than needed but not excessive. Scale all the dimensions proportionally for a 6" system; 6" or 5 1/2" square should work just fine. (A square flow channel is about equal in capacity to a round channel of the same diameter, as the corners introduce drag and do not count towards useful flow area.)

Using the recommended 1:2:4 proportions, I would start with the height of the barrel plus cast interior and see what you get. Adjust all the lengths until you arrive at a good compromise between fitting the barrel and riser in your space, and having a tall enough feed tube to hold your wood inside so you can slide a brick over the opening for air control. You can make the burn tunnel shorter as long as there is enough space for the barrel and feed clearances, but making the burn tunnel much longer risks having all the fire in the burn tunnel and not much in the riser, potentially reducing the draft.
 
John McDoodle
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troy used exact dimensions from the grandfather of rocket stoves- Ianto Evans - so it depends on who you ask lol
 
John McDoodle
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Troy Fairclough wrote:pictures of the 8" core mold i made today, complete with measurements from the 15 dollar book by ianto evans.
 
Glenn Herbert
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The book mentions 7" square or 6" x 8", but doesn't distinguish between taller or wider. However, Ianto almost always used bricks, which have a maximum length of 8" (common) or 9" (firebrick), so a narrower width seems like what he would have used (and not even have considered the possibility of an 8" wide burn tunnel).

Lots of other researchers have experimented with the RMH and have useful advice, some in areas that Ianto didn't get into much or didn't publish about.
 
John McDoodle
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thats true
ive heard more modern day rocketeers like Ernie say "short as possible" burn tunnel is better for riser pull and cook top heat also.
 
Anthony Donner
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a question on fireclay, does anyone know if this would be a good fire clay to use in the mixture for making the cast? http://alsey.com/assets/pdf/product_data_sheets/DMFC_20_Mesh.pdf
thanks
 
Glenn Herbert
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Pretty much any commercial fireclay should work for this application.
 
Anthony Donner
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thanks,
I was just wondering because the product sheet on it says it needs high temps to set up in ...like 2400 F ?,
another question , what would be the longest Horizontal exhaust run for a 6" unit that wouldn't be using mass heat storage, would a horizontal run of 24' then vertical run of 24' be to much? anybody know the limits?
thanks
 
John McDoodle
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Matt Walker wrote:
Just to clarify, your riser design is simply ONE homogenous material enclosed by the 17 gallon drum to hold it together, is this correct? Then you simply place your external heat barrel over that assembly to complete the stove, correct?


Yes.

I thought the CORE of the riser was ideally firebrick (dense material that absorbs/radiates heat ), then wrapped by an insulating layer which in turn is contained by metal shell, so that in essence the entire riser is insulated from the external barrel chamber, but that the CORE of the riser itself did not refract heat, but rather GOT HOT, to enhance chimney effect, and burn gasses?


No, in my opinion this is less than ideal, although it still makes a good heater. I describe why above, but basically anything in the burn area that gets hot cools the gasses, not the other way around.

I thought the purpose of YOUR riser design was simply to split the difference between insulative and heat absorbing, so that the riser would not need to be constructed of multiple sandwiched layers, thus making the design and build simpler.


No, the purpose of my riser is an attempt to improve performance over a firebrick riser. I think a firebrick/insulation riser is a simpler build/design.

All that said, you can build a dang good heater with heavy stuff, and people do all the time. Here's an old heater I built that was poorly insulated and worked just great. And, it's in an old metal tub, just like you are discussing.

http://www.permsteading.com/viewtopic.php?f=6&t=8



thats a cool stove Matt- im currently working on something similar
 
Glenn Herbert
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The difference between a heavy firebrick riser liner and a light insulating liner is not that one gets hot and the other does not, but that the inner surface of the insulating liner can get hot so much faster and not keep absorbing heat from the combustion gases.
 
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