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rocket mass heater riser - the five minute riser

 
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Mud and I are working on a book.  This is the part about five minute risers.  We strong encourage everybody to share their experiences.  We hope that this thread will grow to ten pages of information about five minute risers.



High temp wool shaped into a bit of a tube that is propped up on the outside with some sort of metal cage. The outside of the riser is exposed to temperatures in the range of 400 to 700 degrees, so it is safe from melting or spalling.

Because the interior of the riser has more texture, the inner diameter of the riser must compensate by being a little larger.  If you are building an 8 inch rocket mass heater, your riser would normally have an inner diameter of 8 inches, but with the 5 minute riser, it should be 9 inches.   And a 6 inch system would need a 5 minute riser with an inner diameter of 7 inches.

I don’t have long term experience with the 5 minute riser, but Mud does and he says “they start to get a bit wobbly after three years of regular use.”

Total cost is about $30 to $60.  






 
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Mud: The metal exoskeleton of the 5 minute riser can be hardware cloth or steel remesh (generally used to reinforce concrete) or my favorite is to use a piece of the next larger stovepipe. (Caution, hardware cloth is galvanized so it could offgas zinc at you). Life span has been about 3 years with the hardware cloth before needing a rebuild, longer with the stovepipe surround.
IMG_4254.JPG
The 5 Minute Riser in Mud's tinyhouse CottageRocket design
The 5 Minute Riser in Mud's tinyhouse CottageRocket design
 
Chris McClellan
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Thomas Rubino’s 5 Minute Riser made from the new less toxic Morgan Superwool he carries on his website
https://dragontechrmh.com/morgan-superwool-plus-non-ceramic-fiber-blanket/
morganwool-5-minute-riser.jpg
Thomas Rubino's 5 Minute Riser made from Morgan Superwool
Thomas Rubino's 5 Minute Riser made from Morgan Superwool
 
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Mud's remark about them "getting wobbly" over time makes me wonder if I could prolong the lifespan by putting a skim coat of fireclay slip on the inner (and maybe outer) surface from time to time. Gonna try it and see. I replaced a clay and perlite riser last year with one of these and I really like the performance improvement.

As I think about it, the coating would also very likely improve the laminar flow of the superheated combustion gases in the riser by reducing the roughness of the wool. There's another motivation for trying it.
 
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What about using a SS insulated chimney pipe to hold the high temp wool?
 
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Some images of 5 minute risers from the rmhj!
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Rmh-5-minute-riser-8
 
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Mike Haasl wrote:What about using a SS insulated chimney pipe to hold the high temp wool?

I'm using such a chimney pipe as the riser inside a grease drum (barrel) in a trial RMH in my greenhouse. It certainly works and hadn't shown damage after a month of use -- surprisingly. It's a 6" system.
 
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Mike Haasl wrote:What about using a SS insulated chimney pipe to hold the high temp wool?


The wool already provides the insulating qualities so wouldn't insulation in the pipe be redundant/extra expense?
 
Coydon Wallham
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Chris McClellan wrote:Thomas Rubino’s 5 Minute Riser made from the new less toxic Morgan Superwool he carries on his website
https://dragontechrmh.com/morgan-superwool-plus-non-ceramic-fiber-blanket/


Do we have data yet on if rubino's super wool holds up longer or shorter than the regular?
 
Coydon Wallham
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Andrés Bernal wrote:Some images of 5 minute risers from the rmhj!


What was that material called again, extra heavy duty hardware cloth? It was ungalvinized, right?
 
Mike Haasl
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Coydon Wallham wrote:

Mike Haasl wrote:What about using a SS insulated chimney pipe to hold the high temp wool?


The wool already provides the insulating qualities so wouldn't insulation in the pipe be redundant/extra expense?


Yes, but it's a metal cylinder that might be able to handle the temps in there.  Don't know if it would work and you'd probably need an 8 or 10" pipe for a 6" riser.
 
Coydon Wallham
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Mike Haasl wrote:Yes, but it's a metal cylinder that might be able to handle the temps in there.  Don't know if it would work and you'd probably need an 8 or 10" pipe for a 6" riser.


Mud mentioned using a size larger pipe previously, but regular works for that and is more common than insulated.
 
Mike Haasl
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Sweet, that would be better then.  I guess I was thinking about how you can't use metal but I guess that's inside the riser.  
 
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Phil Stevens wrote:Mud's remark about them "getting wobbly" over time makes me wonder if I could prolong the lifespan by putting a skim coat of fireclay slip on the inner (and maybe outer) surface from time to time. Gonna try it and see. I replaced a clay and perlite riser last year with one of these and I really like the performance improvement.

As I think about it, the coating would also very likely improve the laminar flow of the superheated combustion gases in the riser by reducing the roughness of the wool. There's another motivation for trying it.



For what it's worth, I have this kind of setup in my main RMH. It's situated in the "sunroom", making it effectively "outdoors" along side the camper-trailer, so it gets run hot and heavy during our winters. A normal burn is 4 to 6 hours with frequent burns running 12 to 14 hours on the really cold nights.

I've had to replace the riser a few times on that 8" beast over the years - first try was a perlite/clay mix that crumbled after 2 seasons of use. That was replaced with ceramic fiber blanket between two sizes of hvac ducting - the interior ducting melted out and collapsed on itself after 1 season. I repeated this failure again due to time constraints and the "make do" mentality, knowing it would fail.

Realizing the conundrum I faced with such high temps for prolonged periods, I did the skim coat (about 1/8" layer of clay slip which quickly absorbed into the wool) on the interior, then rolled that around more hvac ducting as a form. After firing for a bit, the hvac ducting predictably collapsed and was pulled out leaving just the fiber blanket with slip coating. Haven't had trouble with it in 3 seasons so far. Still looks sturdy as I come out of that third burn season :)

Maybe rename it the 10 minute riser, as it takes a couple minutes to coat the wool, then a few more to pull the failed hvac ducting "form" out when it gives up. 10 minutes for 3 seasons of heavy use certainly ain't bad in my book!
 
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This is a great report Tristan!
I'd thought to use a parge of refractory cement, but this just works.
Considering clay with perlite failed, but clay over fiber did not, perhaps the fiber is making the difference.
 
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Chris McClellan wrote:Mud: The metal exoskeleton of the 5 minute riser can be hardware cloth or steel remesh (generally used to reinforce concrete) or my favorite is to use a piece of the next larger stovepipe. (Caution, hardware cloth is galvanized so it could offgas zinc at you). Life span has been about 3 years with the hardware cloth before needing a rebuild, longer with the stovepipe surround.



It appears from the picture that you use 2 layers of 1" superwool. Is that correct  and if so, would you start with an 8" stove pipe to build a 6" riser? Would stainless steel stove pipe work for the outer layer of the core?
 
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What I am reading here has me thinking.

I like the way the "re-mesh" was used.  Looks very heavy duty.

Observation seems to be indicating that the clay slip smeared over the ceramic fiber wool is protecting it.

What if we combine these two innovations and form a 5 minute riser with a "re-mesh" or hardware cloth cylinder with ceramic fiber on the inside and then put a wrap of ceramic fiber on the outside of the cylinder.  Fasten it however you will with wire so that the outside layer of wool is bound to the inside layer.  Then coat the entire thing inside and out with the clay or refractory cement slip.

In this way the fibers are all protected from the erosion caused by gas flow and the metal skeleton of the riser is protected from oxidation by being sealed inside the ceramic fiber.

I'd be willing to bet this configuration will make a riser that will last for ages.
 
Tristan Vitali
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William Bronson wrote: This is a great report Tristan!
I'd thought to use a parge of refractory cement, but this just works.
Considering clay with perlite failed, but clay over fiber did not, perhaps the fiber is making the difference.



The fiber definitely holds up well once coated - stiffens the surface and smooths it out for gas flow. I'm sure there's some loss of insulating with the slip coating filling up air spaces in the wool, so in future riser builds, I plan to increase the thickness with an extra 1/4 or 1/2 inch of wool to compensate.

I'm sure refractory cement would behave much the same way as clay slip, especially if you dillute and thin the cement into a more slip like consistency. Depending on the quality of the clay you have available, refractory cement might end up performing much better. My clay, straight from the ground, fires to a soft and crumbly pink (likely part of why the perlite riser crumbled so quickly). I can imagine a better quality clay or refractory cement would be an improvement for the laminar flow inside the heat riser, providing a much smoother / cleaner surface and would enhance the durability even more.

Tim Osborn wrote:...

It appears from the picture that you use 2 layers of 1" superwool. Is that correct  and if so, would you start with an 8" stove pipe to build a 6" riser? Would stainless steel stove pipe work for the outer layer of the core?



That's roughly what I have around the outside of my slipped version - 10 inch round galvanized hvac ducting around the outside of the riser while an 8 inch galvanized duct was used as a form on the inside. The interior of the riser is the metal killer, where pretty much nothing would stand up to the temperatures, but the outer wrap of the riser doesn't get nearly as hot. The Only "wear" I've noticed on the 10 inch ducting is the 3 or 4 inches at the top where some of it has become brittle and cracked over the many years of use (7 seasons already!). A good hot burn with dry beech and sugar maple will occasionally cause the top of the barrel to glow red, so I'm not surprised the gasses are hot enough to damage even the outer wrap a bit, but it hasn't seemed to interfere with its ability to hold things up and in place thus far :)  

I'd imagine that stainless steel, though, is financial overkill for that application with such cheaper alternatives available that would do the job just as well. Even wrapping with thick wire would probably suffice as long as it's on the outside of the riser and you account for the extreme temps likely near the top.
 
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The problem with coating ceramic fibre inside a heat riser is finding a suitable product to do the job!
If the stove has been built carefully to proven specification then there are very few materials that will stand the heat, especially    near the bottom of the riser!
Water glass is something that is cheap and might work for some folk but from my own experiments it cracks very badly then leaves a lot of fibers exposed.
However there is a product called Zircon that has recently come to my notice and is for sale from my refractory supplier.
I have not tried it although I will be ordering some ….
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Thomas Tipton
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"there are very few materials that will stand the heat, especially    near the bottom of the riser!"

This is true.  Perhaps we need to look towards a craft that works with even higher temperatures.  What if one were to tile the susceptible area at the bottom of the riser with these?  

Graphite tiles.
 
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What if one were to tile the susceptible area at the bottom of the riser with these?  Graphite tiles.

You will get a very hot burn, once. Graphit burns like coal in the presence of oxygen.
 
Thomas Tipton
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Thomas Cleman wrote:What if one were to tile the susceptible area at the bottom of the riser with these?  Graphite tiles.

You will get a very hot burn, once. Graphit burns like coal in the presence of oxygen.



That is unfortunate.  My thinking was, as graphite is commonly the material used in metal foundry crucibles, it should be able to handle the environment found at the base of a RMH riser.  
 
William Bronson
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I was just thinking, "Maybe basalt fiber cloth would work as a liner for the rocket stove" .
Nope.
It's rated up to1292°F, no where closes to the expected temperatures.
It's manufactured at  around 2,730 °F , which is at and about the expected temperatures...


 
Thomas Tipton
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I'm very much interested in pushing the boundaries of what can be accomplished here without breaking the bank, or introducing anything that might be found to be toxic down the road.  Perhaps there is an argument for how a vortex inducer at the entrance of the riser can offer protection to the back wall of the riser by taking the edge off the fire stream?  I have seen examples from other builders how a vortex generator can cause the fire to turn abruptly toward the center of the riser tube.  Maybe this is enough to more evenly distribute the abuse coming from the port and help the riser wear more evenly?  

I also read some more on coatings and some say rigidized (Sodium Silicate) ceramic fiber with a coating of an Alumina slurry can be used to achieve higher temps.

Alumina
 
William Bronson
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Funny thing, alumina is aluminum oxide right?
If so, we might be able to obtain it by melting aluminum cans in a rocket stove powered founders.
The aluminum oxide is in the slag, so it's a  "waste product"
It's very hard, grinding it would be a real challenge.
 
Thomas Tipton
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Interesting idea William.

But probably much easier to just buy it.  

I like the idea of a rocket powered foundry.

I've read that aluminum cans make for the worst kind of casting stock.  But when alloyed with other metals it works just fine.
 
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Tristan Vitali wrote:

Phil Stevens wrote:Mud's remark about them "getting wobbly" over time makes me wonder if I could prolong the lifespan by putting a skim coat of fireclay slip on the inner (and maybe outer) surface from time to time. Gonna try it and see.........
Maybe rename it the 10 minute riser, as it takes a couple minutes to coat the wool, then a few more to pull the failed hvac ducting "form" out when it gives up. 10 minutes for 3 seasons of heavy use certainly ain't bad in my book!



loved it, did it last week, could not! finish yet new version (4th in 6 ys! same stove) of rsmh, will try to post info on how it works. planned movable cooking plate to monitor this too. not the original motive.
thanks a lot, carlos at casa verde montevideo, uruguay

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How are you guys connecting the 5 minute riser to, say, firebrick?
 
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Nico Poeta wrote:How are you guys connecting the 5 minute riser to, say, firebrick?


I can’t answer how Carlos did from his pictures, but a 5 minute riser generally has a flat bottom that can just sit upright on the firebrick (or other) levelled off surface at the top back end of the burn tunnel. A little sand/clay ring can help seal the gap and also help support the riser to keep it from shifting over time.
 
Tristan Vitali
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Gerry Parent wrote:

Nico Poeta wrote:How are you guys connecting the 5 minute riser to, say, firebrick?


I can’t answer how Carlos did from his pictures, but a 5 minute riser generally has a flat bottom that can just sit upright on the firebrick (or other) levelled off surface at the top back end of the burn tunnel. A little sand/clay ring can help seal the gap and also help support the riser to keep it from shifting over time.



Yep - that's exactly what I do as well. The riser sits flat on brick without issue, then I put a ring of "thermal cob" around it to seal and further stabilize everything. I've only used standard clay brick, no fire brick yet since that's an expensive upgrade, but same concept. The fly ash tends to collect on the ring at the base of the riser which evidently helps insulate it from the really hot burns when winter's in full swing - the cob seems to hold up nicely with minimal "crumbling" like I've seen in other exposed cob areas that see the more extreme temperatures.
 
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Good morning permies. I built my riser for the rmhf by parging the ceramic fiber blanket. I used sakrete high temperature mortar. I ended up mixing it thin and spraying it on with a cheap harbor freight gravity feed spray gun. I  would spray it on then use a foam brush to smooth it and spread it. I used two  fifty pound bags and it took a week because I had to wait for each coat to dry before applying the next. I had to spay it because trowling it on would pull fiber from blanket and wouldn't stick. It  ended up being extremely heavy. Did it work. I think so far  so good. I can only inspect it with my phone camera from the top or through the burn tunnel. I burnt it extremely hard last winter. So now I'm experimenting with water glass. When it drys it makes a relatively hard surface and no dust. We'll see how it holds up very soon. Here's some pics of my riser.
Screenshot_20220913-074726_Google.jpg
Sprayer.
Sprayer.
 
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Does anyone recall the instructions we were sent home with from the PTJ on the 5 minute risers we made as part of the shippable cores? My recollection is that the Morgan Superwool in the riser should protrude an inch or so from the bottom, and resting it on that should provide enough of a seal...
 
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Hi Eric, that was a good idea to seal the ceramic fibre, Zircon seems to be a good option.  https://shop.vitcas.com/zircon-paint-coating-dry-powder.html
 
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Phil Stevens wrote:
As I think about it, the coating would also very likely improve the laminar flow of the superheated combustion gases in the riser by reducing the roughness of the wool.


Is laminar flow important? I recall Peter van den Berg writing somewhere that a more chaotic flame pattern is likely better than the typical ram's horn pattern in the burn chamber. So maybe laminar flow doesn't have any special value.
 
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Tristan Vitali wrote:

Maybe rename it the 10 minute riser, as it takes a couple minutes to coat the wool, then a few more to pull the failed hvac ducting "form" out when it gives up. 10 minutes for 3 seasons of heavy use certainly ain't bad in my book!




How about using an 8 inch carboard tube (concrete form or mailing tube) as the form and just let it burn off?
 
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Eric Lyle wrote:

Tristan Vitali wrote:

Maybe rename it the 10 minute riser, as it takes a couple minutes to coat the wool, then a few more to pull the failed hvac ducting "form" out when it gives up. 10 minutes for 3 seasons of heavy use certainly ain't bad in my book!




How about using an 8 inch carboard tube (concrete form or mailing tube) as the form and just let it burn off?



Haven't tried it myself but the theory is fine - give it a try and let us know :)  Only issue might be that it, itself, would be flammable, and so might change the behavior during the initial burns. No idea, but might be cheaper (or "free-er") when compared to sacrificial hvac for many people
 
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@David - I'm going on intuition here. As I understand it, and also observe in working examples, the places where the direction of the combustion gases change abruptly are the points where mixing and turbulence are introduced. In a typical J-tube design, these are the right-angle bend of the burn tunnel to riser junction, and the top of the riser to barrel or plate. Along the horizontal run of the tunnel and the vertical portion of the riser, gases are still expanding and moving fast, so these segments are straight and smooth-sided. The system is a balance, and the transition areas add enough turbulence to ensure complete combustion while the straight lengths keep things flowing and encourage a strong draft.

I run a 4" J-tube in my glasshouse and it is very sensitive to any impedance in the burn tunnel or riser thanks to the low ratio of cross section to surface area in the system. Larger rigs are more forgiving.
 
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Phil Stevens wrote:
I run a 4" J-tube in my glasshouse and it is very sensitive to any impedance in the burn tunnel or riser thanks to the low ratio of cross section to surface area in the system. Larger rigs are more forgiving.


I started my greenhouse RMH experiments in Ottawa (east central Canada) with a 6" J-tube using galvanized ducting cast in aircrete and a 24" section of stainless steel insulated chimney, because it was the simplest way to go. It worked, but I quickly tired of the frequent feeding and sawed it into a batchbox form.

Obviously the SS riser is smooth, but I've read that it will burn out, which is why I'm interested in ways to replace it. I'm hoping that Thomas Rubino's insulating blanket inside an 8" stovepipe will work well without having to coat it with a layer of refractory material.
 
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You dont need to coat a 5 minute riser with refractory to get it working well, in fact coating it will quite likely take away some of the insulating  benefits (only by a tiny amount) but coating ceramic fibre is a very sensible thing to do for a different reason!

You have to be carful and cautious when handling any form of ceramic fibre as the individual fibre can very easily become air born partials, always use a mask and protective clothing but there are more dangerous  issues once the product becomes super heated as the fibers then become very brittle and take on a different form.

Anyone who has handled ‘super heater ceramic fiber’  will know how it is only too willing to pollute the air and become a super fine dust, this dust is potentially very dangerous and can enter your body where they can then travel around the blood stream and cause serious health problems!

I am not sure if the fibers could escape from a sealed rocket mass heater but any form of cleaning out from the inside should be taken very seriously.

This is not to say you should not use the product, just be very aware of the potential dangers.
 
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paul wheaton wrote:
Because the interior of the riser has more texture, the inner diameter of the riser must compensate by being a little larger.  If you are building an 8 inch rocket mass heater, your riser would normally have an inner diameter of 8 inches, but with the 5 minute riser, it should be 9 inches.   And a 6 inch system would need a 5 minute riser with an inner diameter of 7 inches.


Is this new information? The riser supplied by mud with his burn block at the PTJ this summer appears to be a ten inch sleeve with two wraps of half inch superwool in it to make an eight inch chimney.
 
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