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Rocket Mass Heater First Build questions

 
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Hi everyone

I started with building my first RMH and have gone through the RMH Builder's Guide by Ernie and Erica Wiesner but there are a few things I am not 100% clear about. The 6" pipe system for my RMH is going to be approx. 20 feet long (excluding the chimney pipes), will have 3 elbows, one U-turn near the chimney and a 45 degree angled pipe to connect to the 7 foot pipe system in the chimney. We are based in Central-Italy so the climate is quite moderate here but winters are usually pretty damp but not very cold. Temperatures very rarely sink below the freezing point.

1. I read that I get to use some insulation under the burn tunnel and around the heat riser. I have a couple of hundred refractory bricks I intend to use to build them. Where I live (n Italy) this kind of refractory material is not easy to find at a local construction material shop. What seems a good fit are vermiculite panels, I read that they can withstand heat up to 1100ºC (2010ºF). My question is whether the material is solid enough to put them directly under the refractory bricks. I also intend to put some normal bricks on the stone floor in order to have a solid base, so maybe that takes away the need for insulation under the burn tunnel.

2. Instead of a steel drum, I intend to build a construction with normal bricks around the heat riser. Is it necessary to have the vermiculite insulation material (or something similar) around the heat riser and can I fixate them with iron wire? That leaves me with the question what kind of metal plate I can put on top of the surrounding chamber? I just want to make sure it doesn't melt or starts corroding.

3. I have a 8" pipe in the chimney. I read the system has to have the same diameter but I am not clear whether this also applies to the chimney pipes. Otherwise I could use the existing pipe system in the chimney instead of having to replace it. However, I don't want to loose a good draft of course.

4. I have checked the plans in the book and also watched a lot of videos to find out how I can best build the manifold. Not being an experienced builder, I can't visually see how it needs to be built. However, I know it's a crucial element in the construction so I kinda hoep someone can refer me to a good source for more details on how to design and build it.

Any feedback or help is greatly appreciated.



 
pollinator
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I would of thought that living in Italy would of put you in a good place to buy all the necessary  refractory materials  being the pizza oven capital of the world?
There are some very good threads on this site that might offer you better options than using a piped mass sytem.
Like this one … https://permies.com/t/122458/Advice-RMH-build-Hokkaido-Japan
 
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Hi Bart and BIG Welcome to Permies!

So glad to hear your going to start a RMH build in Italy. Lots to learn and wish you well on your adventure.

Due to your warmish climate, you probably want to concentrate on mass vs instant heat, so eliminating the barrel is probably a good idea.

Its always a good idea to have insulation under the core. This helps to keep the heat directed towards the fire burning hot and clean while also not transfering it down into the earth where it will forever suck it away.
Vermiculite panels could work, although I've never used them so don't know for sure. Another option people have used quite often is perlite mixed with a little clay slip tamped flat as a pad. Elevating the pad and leaving an air space is particularly good as well.

The heat riser must be insulated as this is where a lot of the gasses are burned up to produce a clean exhaust.
A "5 minute riser" in my opinion is a great way to go. An 8" steel pipe lined with morgan superwool.

Lots of options for metal over the heat riser to cap off the top of your brick work: iron skillets or griddle, cut off steel barrel top, high heat ceramic glass even a manhole cover would work. Although its metal, most of the oxygen has been burned off by then so oxidation and spalling is kept to a minimum.

Your larger 8" chimney will work just fine. No need to replace it. If it was the opposite direction, it would have been a constriction and therefore a problem.

Here is a link that will show some examples of the manifold area:
Flue-exhaust-transition-plenum-pictures

 
Bart Leynen
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Gerry Parent wrote:Hi Bart and BIG Welcome to Permies!

So glad to hear your going to start a RMH build in Italy. Lots to learn and wish you well on your adventure.

Due to your warmish climate, you probably want to concentrate on mass vs instant heat, so eliminating the barrel is probably a good idea.

Its always a good idea to have insulation under the core. This helps to keep the heat directed towards the fire burning hot and clean while also not transfering it down into the earth where it will forever suck it away.
Vermiculite panels could work, although I've never used them so don't know for sure. Another option people have used quite often is perlite mixed with a little clay slip tamped flat as a pad. Elevating the pad and leaving an air space is particularly good as well.

The heat riser must be insulated as this is where a lot of the gasses are burned up to produce a clean exhaust.
A "5 minute riser" in my opinion is a great way to go. An 8" steel pipe lined with morgan superwool.

Lots of options for metal over the heat riser to cap off the top of your brick work: iron skillets or griddle, cut off steel barrel top, high heat ceramic glass even a manhole cover would work. Although its metal, most of the oxygen has been burned off by then so oxidation and spalling is kept to a minimum.

Your larger 8" chimney will work just fine. No need to replace it. If it was the opposite direction, it would have been a constriction and therefore a problem.

Here is a link that will show some examples of the manifold area:
Flue-exhaust-transition-plenum-pictures



Thank you so much for your help, Gerry :-)

What an adventure it is turning out to be. Just getting hold of the materials proves to be a hell of a challenge, but I'm making progress. I also watched the first DVD about Mass Rocket Heaters from Paul Wheaton and although it raised some additional questions, it also made a lot of things more clear.

1) So, I found some perlite, I bought a bag of 100 litres but I haven't been able to find the ultra fine powder I have seen the Wisners use in the project from the first DVD. I have informed here because I hoped to find some clay in soil the area where I live, but it is rather rocky as we live near the Appennine mountains. So I had to buy some clay as well but they are rather big chunks. I assumed they would sort of dissolve when mixed with water and sand but the man from the shop where I bought the perlite said it doesn't. They call it 'argilla espansiva', which means "expansive clay". Can I use it in the cobb and in the clay mortar for the firebricks in the heat riser? I attached a picture of both materials to show the size of 'em. I also wonder how one can stabilize the perlite layer that comes under the fire tunnel and heat riser. That wasn't shown in the video. It has to be 100% solid and level for the fire tunnel to be built on top of it.

2) Durablankets, which seem really convenient and were also used in the video on the DVD, are hard to come by but I have found something else which I can order online here in Italy. It is a ceramic fiber with aluminum and silicium and can tolerate temperatures as high as 2000 °F so that already ticks another box. My question regards the thickness of the material. DO I need 1 inch thick, or will 0,5 inch be enough?

3) One more question regards the liquid they dip the firebricks in before they put them on top of each other to make the heat riser? Is that just water mixed with fire clay? That's my mission for tomorrow: find a place where they sell fireclay. So far it seems like they don't have fire clay powder, but there's a local store where they sold me the firebricks, maybe I will have more luck there. They sold me some refractory cement last week but I have read it is far from ideal.

Many thanks for this amazing forum and for sharing your knowledge and experience so generously :-)



20211204_203033.jpg
[Thumbnail for 20211204_203033.jpg]
 
Gerry Parent
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Bart Leynen wrote:
1) So, I found some perlite, I bought a bag of 100 litres but I haven't been able to find the ultra fine powder I have seen the Wisners use in the project from the first DVD.


Actually, what you show in your picture of the perlite looks just fine. You don't want the ultra fine powder (if there even is such a thing).

Bart Leynen wrote: They call it 'argilla espansiva', which means "expansive clay". Can I use it in the cobb and in the clay mortar for the firebricks in the heat riser?


If you can't find any natural sourced clay in the hills, stick with fireclay for all your RMH work and you won't have any issues.

Bart Leynen wrote:I also wonder how one can stabilize the perlite layer that comes under the fire tunnel and heat riser. That wasn't shown in the video. It has to be 100% solid and level for the fire tunnel to be built on top of it.


With your fireclay, you want to add enough water to it so its the consistency of a milkshake. This is what is called clay slip. Add this to your perlite and mix it in well....but just enough so that it will clump together and hold its shape but will still easily pop apart when lightly squeezed. Too wet and you loose a lot of the insulation value of the perlite, as clay is not an insulator, only a binding agent.

Bart Leynen wrote: 2) Durablankets, which seem really convenient and were also used in the video on the DVD, are hard to come by but I have found something else which I can order online here in Italy. It is a ceramic fiber with aluminum and silicium and can tolerate temperatures as high as 2000 °F so that already ticks another box. My question regards the thickness of the material. DO I need 1 inch thick, or will 0,5 inch be enough?


Not sure which ceramic fiber blanket your looking at but the safest one to use that has next to no health concerns (no more than standard fiberglass insulation) is called Morgan superwool. Working-Morgan-Superwool-ceramic-blanket
1" thick is a good size to work with. Half inch is a bit too thin for insulation and structure.

Bart Leynen wrote:3) One more question regards the liquid they dip the firebricks in before they put them on top of each other to make the heat riser? Is that just water mixed with fire clay? That's my mission for tomorrow: find a place where they sell fireclay. So far it seems like they don't have fire clay powder, but there's a local store where they sold me the firebricks, maybe I will have more luck there. They sold me some refractory cement last week but I have read it is far from ideal.


Yes, clay slip is what they dip the bricks in. Just on the edges and just enough to help temporarily stick them together, help seal the air gaps and level out any irregularities in the bricks so they don't wobble. Refractory cement is a one way deal. Clay is so much more forgiving and much nicer to work with. No gloves needed and if you want to modify your stove in the future, you just wet it down and it easily comes apart.




 
Bart Leynen
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Gerry Parent wrote:

Bart Leynen wrote:
1) So, I found some perlite, I bought a bag of 100 litres but I haven't been able to find the ultra fine powder I have seen the Wisners use in the project from the first DVD.


Actually, what you show in your picture of the perlite looks just fine. You don't want the ultra fine powder (if there even is such a thing).

Bart Leynen wrote: They call it 'argilla espansiva', which means "expansive clay". Can I use it in the cobb and in the clay mortar for the firebricks in the heat riser?


If you can't find any natural sourced clay in the hills, stick with fireclay for all your RMH work and you won't have any issues.

Bart Leynen wrote:I also wonder how one can stabilize the perlite layer that comes under the fire tunnel and heat riser. That wasn't shown in the video. It has to be 100% solid and level for the fire tunnel to be built on top of it.


With your fireclay, you want to add enough water to it so its the consistency of a milkshake. This is what is called clay slip. Add this to your perlite and mix it in well....but just enough so that it will clump together and hold its shape but will still easily pop apart when lightly squeezed. Too wet and you loose a lot of the insulation value of the perlite, as clay is not an insulator, only a binding agent.

Bart Leynen wrote: 2) Durablankets, which seem really convenient and were also used in the video on the DVD, are hard to come by but I have found something else which I can order online here in Italy. It is a ceramic fiber with aluminum and silicium and can tolerate temperatures as high as 2000 °F so that already ticks another box. My question regards the thickness of the material. DO I need 1 inch thick, or will 0,5 inch be enough?


Not sure which ceramic fiber blanket your looking at but the safest one to use that has next to no health concerns (no more than standard fiberglass insulation) is called Morgan superwool. Working-Morgan-Superwool-ceramic-blanket
1" thick is a good size to work with. Half inch is a bit too thin for insulation and structure.

Bart Leynen wrote:3) One more question regards the liquid they dip the firebricks in before they put them on top of each other to make the heat riser? Is that just water mixed with fire clay? That's my mission for tomorrow: find a place where they sell fireclay. So far it seems like they don't have fire clay powder, but there's a local store where they sold me the firebricks, maybe I will have more luck there. They sold me some refractory cement last week but I have read it is far from ideal.


Yes, clay slip is what they dip the bricks in. Just on the edges and just enough to help temporarily stick them together, help seal the air gaps and level out any irregularities in the bricks so they don't wobble. Refractory cement is a one way deal. Clay is so much more forgiving and much nicer to work with. No gloves needed and if you want to modify your stove in the future, you just wet it down and it easily comes apart.






Thanks for all the advice. It really helped me to get more clear on the what and how of building my first MRH. It's been really a challenge to get all the materials over here in Italy. I drove two hours to Rome last week just because fireclay is so difficult to get one's hands on.  

Having said that, yesterday, I started with the fireclay slip and the perlite which seemed to work really nice. Only problem is that the mix won't harden and I can't seem to get a solid foundation upon which I can build the fire tunnel and heat riser.

Any tips on how to make this mix harden (more quickly)? It's pretty cold here, 40 to 50 Fahrenheit right now. Maybe I need to add some heat first to speed up the hardening process?
 
Gerry Parent
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Bert, the clay slip / perlite mix needs to be kept as dry as possible -  just wet enough to barely hold itself together when squeezed together with your hand into a ball.  If you were to then squeeze it with two fingers, it should pop apart.
Secondly, it doesn’t really harden but rather dry enough to handle gently.
This soft delicate nature makes it very insulative.  Surprisingly though it will hold up a core when the weight is distributed over it.
You could add an external source of heat to help it dry faster but adding minimal water to it at the very start is key.
 
Bart Leynen
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Gerry Parent wrote: Bert, the clay slip / perlite mix needs to be kept as dry as possible -  just wet enough to barely hold itself together when squeezed together with your hand into a ball.  If you were to then squeeze it with two fingers, it should pop apart.
Secondly, it doesn’t really harden but rather dry enough to handle gently.
This soft delicate nature makes it very insulative.  Surprisingly though it will hold up a core when the weight is distributed over it.
You could add an external source of heat to help it dry faster but adding minimal water to it at the very start is key.



Thanks, Gerry. I think the texture and the amount of moisture was OK. It is starting to get dryer now as I can already see a few cracks appearing on the surface. One question comes to my mind now. I can distribute the weight of the fire tunnel and the heat riser evenly from left to right but the heat riser will have a lot more firebricks than the fuel feed. Hence, the corresponding part of the perlite/clay mix underneath will have to support more weight. Won't that create any problems as the firebricks there will sink in more compared to the firebricks underneath the fire tunnel and the fuel feed?

I have attached an image just to show what it looks like now. I think it looks OK but as I can't find any info with pictures online on how it should look like I am posting the image here just to be sure
clay-perlite.jpeg
[Thumbnail for clay-perlite.jpeg]
 
Gerry Parent
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From your picture it looks like you added a lot more clay than necessary.
As with the water, you only want to add just enough clay to hold everything together. Excess clay reduces the insulative properties of the perlite and will also crack. Hopefully not too much in your case.
If you are concerned about the weight of the heat riser then perhaps you should go with a "5 minute riser” made out of ceramic fibre blanket wrapped inside a steel tube.
An insulated heat riser made of lightweight material is much better than a heavy and dense one.
 
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Hey Bart, if you look at my post on Building a rocket mass heater you will see on the first page a bunch of pictures of clay stabilized perlite.  You might check the thread out.  Its RMH builders guide build.
 
That feels good. Thanks. Here's a tiny ad:
3D Plans - Pebble Style Rocket Mass Heater
https://permies.com/wiki/193712/Plans-Pebble-Style-Rocket-Mass
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