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Bricks for the box of my cob bench

 
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Thomas, oh I got it, you don't have a small manifold opening, you have half of your barrel open into your own build manifold which also include an ash-pit, right? How do you build the top of that manifold?
 
Diana Lee
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Thomas,

1) Your burn tunnel was the length of two bricks, so it is 16 inches, right? Do we need it long? Mine is only the width of two bricks which is 9 inches. Now in my rebuild, I just want to make everything right, should I elongate my burn tunnel too? Would it make the stove more effective?

2) Are the bricks you use for your burn chamber firebrick or regular bricks?

3) What is the side of your burn tunnel and your heat riser please? What is the dimensions of those bricks

 
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The burn chamber should be as short as practical.
Thomas mentioned he thought he made this one too long, and it is one of the things he identified as what he will change (back) on his next build.
 
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the more you are asking, the more I am learning to appreciate the short book!
pleas continue.
 
Diana Lee
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Then I will leave mine at 9"

Shilo, read book! read book!
 
shilo kinarty
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Diana Lee
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I am done reading the book and I even have cheat sheet. Here it is for you Shilo:

Cheat Sheet from Rocket Stove Book
1) Soft common bricks are as good as firebricks and can be used in burn chamber
2) Marson sand and good fireclay and #1 sand proportion: 1 to 4
3) Cob bench could be any sand
4) Use the flue bigger than you think you need
5) Feed tube should be smaller than flue. 8” flue should have 7 square or 8 by 6)
6) Keep feed tube short
7) The width of the burn tunnel should be the smallest in all meausrements, and should be smaller than flue, smaller than heat riser cross section. Otherwise will cause smoke back up into room or slow combusion
Heat riser should be from 35-50
9) Heat riser’s cross sectional should be bigger than burn tunnel’s width
10) The gap between heat riser and top/lid sould be 2-3 for 8” flue, and for 6” flue, it should be 1.5 to 2
11) Dimension from heat riser to barrel should be 1.5” (For real? Mine is way different)
12) One clean out at the transition and one at the bottom of verical pipe (can use this hole at bottom of verticle pipe as primer to burn paper
13) Straw (chops in 2 inches) need on surface layer of cob bench to prevent crack (avoid straw near flue)
14) When start the file, try to use only two pices of newspaper, don’t use more, as the clay in papers don’t burn and rapidy build up as ashes. (minimal the burn of paper as it build up ashes and suffocate the burn)
15) Feed verical and load a full feed tube or we will have smoke


 
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Bacon, Shilo has a few builds Under his belt.

About your cheat sheet:

Normaly the cross sectional area (CSA) of a rocket should remain the same all over the place.

You can reduce a smidge the burn tunnel CSA.

The transition should be about 3 times CSA. But the chimney and horizontal flues should be as close to sca as possible. Exept if your delve into bells.

You can reduce te feeed with a brick when realy hot or at startup, so it doesn't smoke back. Actualy, it acts as a venturi, and speeds the gases entering the rocket's feed tube.
 
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Bacon; Burn tunnel roof is 18" , I will be shortening it by 2 bricks (5") making it 13" long, stay with 9" long on your build. Bricks are all firebrick ,4 !/2 x 9 x 2 1/2, a few are the soft highly insinuative firebrick. Feed tube , burn tunnel and the start of the riser is all firebrick mortared with fireclay. My heat riser is a home built casting using matt walkers recipe of fireclay & perlite with a 16 gal barrel as the outer form and a piece of 8" sono tube (cardboard concrete form )as an inner form. I can't say enough good about this heat riser... ITS AWESOME ! When I rebuilt, I simply lifted the riser off & moved it out of the way! No issues with it at all, its on its second season and I expect to use it for many / many years in the future. As far as my manifold , brick walls support a large flat rock on two sides, on the barrel side I used 1/4 " hardware cloth to form around the barrel and same thing on the back side. Cut your metal cloth to size , form it where you want your cob to be and slowly cover it with cob. While I had my cleanout door open I would reach inside and smooth the cob where it seeps thru the form. Inside your manifold all the cob should be as smooth as possible with sweeping curves that funnel your hot exhaust gasses into your horizontal piping, and an ash pit to collect your ash.
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Diana Lee
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Shilo, you trick me to thinking that you are on the way just like me and trying to get tips and short book from every where, so I post the cheat sheet for you. But I make one error in the cheat sheet so I have to repost in case someone is learning the process and reads it.

Thomas, I wish I know how to build the inner cord like you did, but I don't think I can do that, so I have to stick with firebricks or soft common bricks.

Satamax: I try to have the same dimension every where but my burn chamber is made of bricks (square and rectangle) and flue is circle. So I plan my dimension as follow:

I have 6" flue. I think I will make my feed tube and heat riser same dimension and is 5.25 by 5.25, my heat tunnel will be 5.25 by 4.5, flue as I said is 6". Please see if I am ok

With 6" flue, the area for flue is 28.27
heat riser and feed tube be 5.25 * 5.25, area = 27.56
burn tunnel area is 5.25*4.5 = 27.
I think this is the closest I can get to area of flue without go over the area of the flue.

Here is the cheat sheet repost, not for Shilo, but in case someone is in the process of learning, I don't want they get the wrong information and do it wrong.

Cheat Sheet from Rocket Stove Book
1) Soft common bricks are as good as firebricks and can be used in burn chamber
2) Marson sand and good fireclay, proportion: 1 to 4
3) Cob bench could be any sand
4) Use the flue bigger than you think you need
5) Feed tube’s area should be smaller than flue’s. So, do calculate the areas. 8” flue should have 7 square or 8 by 6)
6) Keep feed tube short
7) The cross sectional area of the burn tunnel should be the smallest of that from other intestins or ducts or heat riser. Otherwise will cause smoke back up into room or slow combusion
Heat riser should be from 25-50
9) Heat riser’s cross sectional’s area should be bigger than that burn tunnel
10) The gap between heat riser and top/lid sould be 2-3 for 8” flue, and for 6” flue, it should be 1.5 to 2
11) Dimension from heat riser to barrel should be 1.5” (For real? Mine is way different)
12) One clean out at the transition and one at the bottom of verical pipe (can use this hole at bottom of verticle pipe as primer to burn paper
13) Straw (chops in 2 inches) need on surface layer of cob bench to prevent crack (avoid straw near flue)
14) When start the file, try to use only two pices of newspaper, don’t use more, as the clay in papers don’t burn and rapidy build up as ashes. (minimal the burn of paper as it build up ashes and suffocate the burn)
15) Feed verical and load a full feed tube or we will have smoke
16) Floor protection would be 2-3 inches of clay and perlite.



 
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"11) Dimension from heat riser to barrel should be 1.5” (For real? Mine is way different) "

This is a minimum. A larger gap will be fine, and make it easier to transition to the manifold for the horizontal duct.
 
Satamax Antone
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Bacon, top gap and side gaps are often giving trouble. Myself, i always advise to make the top gap at least three, as well as the sides. This way, there's no problems. A closer top gap is good for harder cooking.

One thing, i haven't read much your dimensions. When it's not in metric i'm struggling. That's because i'm aging.

But, burn tunnels which are wide and low; are known to give more trouble, incomplete burn etc! This is due to more heat being in contact with the bricks which form a kind of heatsink at the ceiling of the burn tunnel.
 
Diana Lee
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Thanks that's good to hear, my side gap is many inches

Now the dimension of the burn chamber, I will make them all 5.25 then. But thinking about cutting the bricks, wow, it's a lot of bricks to cut.

I think I have to either change my heat riser to cast ion to easily makes 6", or I have to change to 8" duct so I don't have to cut the bricks. Either way it will cost me more money. What do you think? Do you have any solution for my case?

 
Erik Weaver
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Bacon Lee wrote:
With 6" flue, the area for flue is 28.27
heat riser and feed tube be 5.25 * 5.25, area = 27.56
burn tunnel area is 5.25*4.5 = 27.
I think this is the closest I can get to area of flue without go over the area of the flue.



Looks good.
Although, 5.25*4.5 = 23.625. Still, that is the burn chamber, and we want that to be the smallest CSA, and it is.


Bacon Lee wrote:
Cheat Sheet from Rocket Stove Book
1) Soft common bricks are as good as firebricks and can be used in burn chamber



Others have found this is not always true.
If I recall correctly, Evans was speaking of recycled brick (he scavenged nearly everything) and it happened to be really old, clay brick.
I do not think this is the same thing as saying all soft common brick performs as well as fire brick.
Given fire brick is rated as a refractory material, I would go so far as to say this is a misleading claim.
I also have noticed, a number of folks using something other than a refractory material in the fire box, end up rebuilding it.
2000-3000 F is a lot of heat, and the changes in temperature are relatively rapid, which causes stress on the material.


Bacon Lee wrote:
Heat riser should be from 25-50



See page 28 of the Evans book, which offers a nice summary of the critical relationships.
For a "rule of thumb" list, I'd offer ratios (geometry) rather than hard numbers.
Heat riser should be at least three times as tall as the feed tube, and at least twice as tall as the horizontal burn chamber is long.
Where, specifically, to measure these points seems to vary according to who is saying, so I take these to be good as guidelines, and not meant as super hard numbers. If you are off 1/2-inch or an inch in the riser height it should still work just fine. But cutting it short by a foot or two, is an entirely different matter.

Bacon Lee wrote:
15) Feed verical and load a full feed tube or we will have smoke



And keep moving your covering bricks to stay close to the wood as it burns down. If that gap at the opening of the feed tube gets too large, it has a greater tendency to back-smoke into the room. The same may be said for making the volume inside the feed tube too large; smoke tends to collect in the eddies, but so long as it is covered, snugged up against the wood, this seems to remain a minor issue. That said, any time smoke is caught in an eddy like that, it may flare up when suddenly exposed to air (like when picking up the bricks to look inside the feed tube; this is a bigger problem when completely covering the feed tube, and why that is not recommended).

Bacon Lee wrote:
16) Floor protection would be 2-3 inches of clay and perlite.



I still don't feel that is sufficient, unless you do not care how hot your floor gets.
So you have to modify this statement, at a minimum, to state that it depends upon the construction of the floor.
For a wooden floor, I think this would present a fire hazard.
For a concrete floor, I think it risks heat damage (cracking and pitting).
For an earth floor, I do not see any risk at all, 2-inches should be fine.

Another dependent variable is the length of winter (and how well the earth sinks heat immediately below the fire box).
If you only have to burn once a day for an hour for a couple months of the winter, and only every other 2nd or 3rd day the other two months of the heating season, that is radically different from someone living in a much colder area, where they may have to burn two or more times a day for three or four months.

As I observed at some length previously, if the heat cannot easily escape (air gap) it will slowly build, slowly and steadily raising it's lowest-low temperature. If that reaches the region of 170 F is posses a risk to wood structures (and Ernie Wisner is even more cautious, and says do not exceed 125 F). Masonry will take higher temps (what I forget off the top of my head).
 
Erik Weaver
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Bacon Lee wrote:
Thomas, I wish I know how to build the inner cord like you did, but I don't think I can do that, so I have to stick with firebricks or soft common bricks.



Actually, you already do

It may be filled with a perlite-clay mixture. You just get two tubes, of the sizes that fit your system needs, and set one evenly inside the other, and then fill between them with perlite-clay. I would suggest making the bottom more on the clayish, to be sure it will hold together when dry. I thought I had finally gotten my mix right by the time I got to the top of filling in my tubes. But I discovered I was wrong. As I said before, mixing the clay-perlite (and cob) is a touchy-feely thing. You cannot just follow a recipe, because there are too many variables in the raw materials.

Anyway, when I inspected my perlite-clay insulated tube, I discovered the top part was very crumbly, and you could easily knock it apart with your finger tip. OK so long as it is not disturbed. But had that been in the bottom of the fire riser tube, when the metal liner burns out, it would have begun to fall into the burn chamber. Not good.

It took me three or four batches to fill the tube, maybe more, and each time I used more perlite, trying to get it perfect. So I know the bottom has more clay in it, and therefore is more stable, and will be more likely to bake hard from the heat, so by the time the metal burns away, it ought to be able to stand up on its own.

I used 7-inch metal ducting as my inner lining, and a cardboard concrete pier form as my outer shell. Now, *if* the cardboard all burns away (and it may not) I'll find out where my perlite-clay mix because stable, because that part will stand up, while the rest will fall/pour into the bottom of my barrel (the top of the metal duct should last a very long time, so if the insulating mix or perlite-clay spills out anywhere, it is far more likely to be through the cardboard side, not the metal side).

Which is to say, if you want to make an insulated fire riser, using perlite-clay, you can do so with relative ease.
 
Diana Lee
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Erik: You mean we have a brick sitting on our burning wood? I thought the brick should be cover the feed tube and sitting on the feed tube?

Also, the book mean minimum, yes, as the side gap should be minimum of 1.5 (Per Glen explaination). I was terrify when mine side gap was several inches. The same thing for the floor protection I think, the books mean minimum is 2-3 inches.

Please everyone thinking about my heat rise, is there any easier solution for me?
 
Diana Lee
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The cardboard concrete form I can buy it at homedepot? Does it need tools to work with. I don't think I can do that if this job require tools or a lot of heavy work. Mixing perlite and clay and stuff them in the empty space, I can do that.

I am checking the triple wall stainless steel pipe. If I have this one, I don't need any tool, because they already in the cylinder shape with empty space, I just stuff the mixture in it, right? It is costly, but I can't back out now. I will not eat this month to save enough for that, but is triple wall stainless is ready to use then I should go for it.
 
Erik Weaver
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Bacon Lee wrote:
Erik: You mean we have a brick sitting on our burning wood? I thought the brick should be cover the feed tube and sitting on the feed tube?



No, no. Not at all. Like you say, the brick sits on top of the feed tube, but as I thought you had also observed, you move these bricks during the burn. You keep them relatively snug to the wood - assuming some of it is sticking above the top of the feed tube opening. If the wood is all shorter than the feed tube depth, the brick would not be snug against the wood itself.

When you get your stove burning again, get it going on a good burn, adjusting the cover bricks as you should to encourage the draft and discourage smoke-back. Then take all the bricks off, and observe. You will eventually see smoke coming into the room. Maybe right away. Then cover the feed tube opening with the bricks again, and see the smoke follow the draft into the burn chamber again. That's what I was talking about.

Which raises an interesting question I may investigate if I remember to try it. If *all* the wood inside the feed tube is short than the height of the feed tube, can you just set the bricks to make a one-inch gap, and be done with it?
 
Erik Weaver
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Bacon Lee wrote:
The cardboard concrete form I can buy it at homedepot? Does it need tools to work with. I don't think I can do that if this job require tools or a lot of heavy work. Mixing perlite and clay and stuff them in the empty space, I can do that.

I am checking the triple wall stainless steel pipe. If I have this one, I don't need any tool, because they already in the cylinder shape with empty space, I just stuff the mixture in it, right? It is costly, but I can't back out now. I will not eat this month to save enough for that, but is triple wall stainless is ready to use then I should go for it.




Home Depot, Lowe's, both have the cardboard forms. They are used as forms when pouring concrete piers. There are several sizes. I used the 12" diameter, 48" tall form for my outside. I then picked up two pieces of 7" diameter, 24" metal ducting, and put them together to form the inside form; they ended up being about 46" tall (you loose an inch or so at each connection of the ductwork). It is really easy, after you get the stupid duct work to snap together. But you should be used to that by now, you have laid all that ductwork in your bench.

Very easy. And insulated, when filled with the perlite-clay mix.

Or you can build it with fire brick, and wrap that with the rockwool. Either way will work. Personally, I find the brick easier to work with, and it is faster for me. But in my case, I am on a wooden floor over a basement, so keeping weight low is critical. Otherwise, I'd've used fire brick and insulated around that. I'm too cheap to buy insulating fire brick (or kiln brick).

Triple-wall stainless steel

Do not use that for the fire riser. It is far too expensive, and will not work nearly as well as either of the other two ideas mentioned above. It is however, really, really good for the chimney exhaust sticking through and out of the roof. That is where it is best used.
 
Diana Lee
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Erik

So i buy the same pipe as the duct, i don't have to buy the steel pipe? why don't you buy the one 48", why would you buy 2 of 24" ? I think this is easier for me since I don't have to cut bricks. Stainless steel is real crazy, forget about it. I thought 150 for 3 feet, but no, 150 for one feet, unbelievable
 
thomas rubino
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Erik; I cover my feed tube either with two bricks ,one on each side , leaving apx. 3"- 5" or so gap. I have also placed one brick (flat) across the center. Watching the smoke swirl it seems that it draws down the sides much more evenly that way and no smoke back, down side is your relying on friction to keep that brick sitting there. I use a metal P channel on my feed tube and I may weld a small ledge on that to hold a brick across the center.
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thomas rubino
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Bacon; The hardest part of building a heat riser like mine is finding a 16 gal barrel. If you are located in the U.S. then any quick lube shop or a truck repair shop, even a local farmer with equipment should have a 16 gallon "grease" barrel that they may give you free or at a minimal charge. Cut appropriate size hole in the bottom, use a round concrete form as the inner liner (burns up with first fire) and the barrel as an outer form. Pack perlite/fireclay mix between. At the top, after you are above the barrel use more clay than perlite in your mix and taper slightly in. Build this slightly taller than you need and after your 55 gal barrel is in place use a straight edge and a tape measure to get your top gap and use a wood chisel (or any metal tool ) to shave down your riser to the perfect height. If you use a cardboard outer form then you are relying on your mix to be self supporting, the method I used is more forgiving on your mix being just so.
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Erik Weaver
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Bacon Lee wrote:
Erik

So i buy the same pipe as the duct, i don't have to buy the steel pipe? why don't you buy the one 48", why would you buy 2 of 24" ? I think this is easier for me since I don't have to cut bricks.




I'm using the same metal as the duct work. It will burn out no matter what I use, so it might as well be cheap

I couldn't find 48-inch duct where I live. And, yes, if I could have found that, I would have used it. But it was either use two 24" pieces or a 5-foot piece that I'd have to cut. I opted for the two short pieces to avoid the hassle of cutting.

As to cutting the fire brick when making the riser, I don't know that is really a requirement. It will look better, and be easier to insulate around if using a wrap like rockwool. And I suppose it may reduce the insulation factor by the degree the ends of the fire brick that stick out, and are fire brick in stead of that space being insulation.

But how much difference does that really make? Enough to notice? I don't know. I have my doubts that it makes a big difference, but then again I haven't tried it, so maybe I'm mistaken on that point.


 
Erik Weaver
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Bacon Lee wrote:
Here, please see if this is ok, tripple wall, ready to use. Is it good idea?

https://www.google.com/shopping/product/14802291840450353435?q=triple+wall+stove+pipe&biw=1517&bih=714&sqi=2&bav=on.2,or.r_cp.r_qf.&bvm=bv.84349003,d.eXY&tch=1&ech=1&psi=vWTFVLXGK8SlNtuagegL.1422222541129.3&prds=paur:ClkAsKraX5HAEAg-tVJZ8cE0kCmD4ALcIfvn4StbED-RuRFr15kSp2dpoW6RlOM1br-eh2pDtWdx562b015PWQLmZUBdPkPIOjhdQWXqLDyv0Q-IaihJngiTqBIZAFPVH72PykZNXpo9fNLGoWDO_Tsrg21Ssg&ei=yWTFVL36KcGkgwTy-IGwDA&ved=0CHoQpiswAA



I do not recall anyone using it as the fire riser. And I already offered my opinion, above:


"Triple-wall stainless steel
Do not use that for the fire riser. It is far too expensive, and will not work nearly as well as either of the other two ideas mentioned above. It is however, really, really good for the chimney exhaust sticking through and out of the roof. That is where it is best used."


Perhaps others will have a different opinion.
 
Diana Lee
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Ok, then I go for the duct work because I can cut duct better than bricks. So I will buy one with 6" for inner cord, and one with 8", and stuff the mixture of clay and perlite in the middle, right? I wonder how many years this last huh? Did you ask around how long would it last. It also available at homedepot I can go buy it and start tomorrow.

So with this heat riser, my burn openning of 5.25 by 5.25 will fit in with this heat riser just fine, right?

I wish I can do what Thomas did with 16 Gallon barrel to last longer, but it's very hard to find barrel that side and no paint. I am still having hard time to fine 55 Gallon barrel with no paint.
 
thomas rubino
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To remove paint from any size barrel, fill full of wood and burn it. Nice hot fire and 95% of your paint is gone . If you live rural then you can do this in your yard, if you live in the city then go camping and have a bonfire ... inside your barrel. If you are going to use concrete tubes for inner & outer forms then buy a 6" inner and a 10" outer too get a 2" thick riser, a 8" will only get you a 1" thick riser... not thick enough.
 
Glenn Herbert
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An 8" outside form would only give you 1" of perlite/clay thickness, which is skimpy. 2" or more is better. You can get two pieces of 6" snap-together metal duct, unbend them a bit, and snap them together to make a single 12" diameter tube and 3" of insulation space. For a 6" system, if you can get 6" and 4" duct that will fit each other, you can put those together to make a 10" diameter form, giving 2" of insulation. A 2" space between forms will let you use a short piece of 2x2 or 2x4 to tamp the perlite/clay as you fill it, giving a superior result.
 
Diana Lee
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Thanks a lot for the tips, I will remember to buy 6" and 10" or combine to make 10".

I saw the burn video to get rid of the paint. From where I live to a bonfire place will be 1 hour drive. I want to try to buy one with no paint first to see if I can find it. If not, then the other option is to go to the bonfire place to burn it

So, that's the plan. I will start do it, don't know how long it will take me, each day I will update with a photo of what I am working on to see if you have any tips for me to make it easier. Tomorrow I have to clean out the firebricks which broke down from the old burn chamber. The next day, then I will build the burn tunnel and feed tube. This time, all those reclaimed bricks will be for the burn tunnel and feed tube. I want to make them all full size (last time I did, the heat riser is half side bricks). So, it is ok I put two half ones together to make a full size, right?
 
Glenn Herbert
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Yes, or if the dimensions work out, you can put the "splits" (half thickness bricks) on edge so they make a thinner wall. THis will make less mass that has to heat up before the combustion gets going properly. Filling around it with perlite/clay will make it about as strong as full bricks.
 
Diana Lee
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Thomas: May I ask why you wouldn't do your heat riser the way others do, which means inside is metal and outside is metal with insulation in the middle. You have your inner is cardboard form so it will be burned away at the first burn, right? What benefit would this be compare to metal as the inner? I am not doing heat riser today, because I want to gather more information and tips. Hope to hear from you before tomorrow so I know what to buy
 
thomas rubino
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Hi bacon; This one is easy. Using a metal inner liner for a perlite/fireclay riser is just not needed. High heat from the first fire burns out the cardboard form and hardens the fireclay/perlite , leaving a smooth path for the burn to take place. If you use a metal form it will quickly deteriorate and fall into the bottom of your burn tunnel. This is not a big problem to remove but...you have to remove it.... why bother if you do not need it. As far as using a cardboard tube for the outer form ... I believe that metal wins hands down ! The "grease " barrel is my choice as it is so durable that it can be moved in the event of a rebuild. However some people have trouble locating that size barrel, in that case any metal is better than a burnable outer form. You can even use sheet metal formed round and held with tie wire, 2 pieces of 6" stove pipe can be put together to make a 12" outer form. Just make sure that you have at least 2" thickness or more ,1" is not enough! Remember bacon , this is just my opinion, many people have built with metal inner liner and cardboard outer and it works for them, so it would work for you also. I believe that the metal outer gives the riser that little extra protection against damage and the fact that it can be moved is a big benefit.
 
Glenn Herbert
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I don't think there is a benefit to making the inner form metal instead of cardboard; but it may be difficult to find a 6" sonotube - I know the big stores around me don't have it. Some place might, but as I have an 8" system, 8" sonotube is common. Essentially, use whatever is cheapest and easiest to get for the inner form.
 
thomas rubino
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Take a tape measure to the store ... not all tubes are the same size, they make them different sizes so that they slip together easily for shipping. Like glen I needed 8" so I didn't even notice if they had 6" but in the ( 8") pile there was 7 1/2" to 8 1/2" so buyer beware... measure first or you might not get what you want.
 
shilo kinarty
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to remove the paint, just make a pocketrocket.
 
Glenn Herbert
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I actually wanted a riser a hair smaller than 8", so I found a piece of sonotube that was 7 3/4" o.d.
 
Diana Lee
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Thanks all,

Now I have to learn what is pocketrocket
 
Diana Lee
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Thomas: How many clean out do you have in that system? Do we really need another one at the foot of the vertical pipe going out the roof
 
Diana Lee
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Does anyone has 2 clean outs or just one?
 
shilo kinarty
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Now I have to learn what is pocketrocket


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