Heat Riser Insulation using Sawdust

While building a Rocket Mass Heater we do not have perlite or vermiculite available to make the insulation of the heat riser. We plan to use sawdust mixed with clay contained in wire mesh. There is a lot of posts mentioning using a 50/50 mixture of clay and sawdust, but there are absolutely no follow-ups or mentions that this mixture worked or what happened. Does anybody have more info?

Also is there a way to test if the clay/sawdust mixture holds up or not? Thanks for the feedback.

Hi Ruben;  I have no direct experience with sawdust added to insulate the riser. I suspect that the sawdust would burn off leaving small voids and becoming insulating that way. I do not think it would last or be the most efficient, but its certainly worth trying.
I recently learned about a new way to make an insulated riser. Called the 5 minute riser , invented by a fellow with the nickname of Pinhead.  
It require's having access to ceramic fiber blanket, readily available in the US thru Ebay , it may not be so available where you are.
A 1" thick blanket of ceramic fiber stuck inside of regular wood stove pipe and your done !  That's it !!!  Able to leap tall buildings in a single bound and can handle any temperatures a batch box or a J tube can throw at it !  WOW! way cool in my opinion!
In your situation you may not be able do this at this time but you could search and see if ceramic fiber blanket is available for future use.
I know for me I will never build a fireclay/perlite  riser again!  

It didn't work very well for us, but I think with patient experimenting, one could find the right mix that would last. We made a cylinder of straw-clay and surrounded it with ashes. But the straw clay broke down within a month or two. Also it wasn't very well protected, because we kept the top of the barrel open to be covered with the pot for cooking, so things may have kept falling down inside.

thomas rubino wrote:
A 1" thick blanket of ceramic fiber stuck inside of regular wood stove pipe and your done !  That's it !!!  Able to leap tall buildings in a single bound and can handle any temperatures a batch box or a J tube can throw at it !  WOW! way cool in my opinion!

Wow, wow, wow! We found a dozen rolls of used ceramic fiber blanket about 1/2 inch thick, discarded in the desert, probably by the Indian military. We tried out a garbage incinerator using whole rolls as pieces of chimney. But we didn't try putting a single or double layer inside a pipe: that's a great idea! But how do you attach it to the inner surface of the pipe so that it doesn't fall down and block the pipe? Most of the rolls we found have chicken mesh on both sides, which I assume will burn out quickly in rockety conditions.

You don't say where you are in the world. I suspect that you will be able to find vermiculite without too much trouble. It is often stocked by garden centers as an ingredient in potting mixtures.

There is a stiffener product you can use , but pinhead claimed you didn't need it . He posts over at Donkeys pro boards . Here is the item # at ebay for blanket and that should lead you to the stiffener product.
Yes the chicken wire would burn off . And 1/2" doubled up might want to fall in . But I'm sure you can find a way.  

Hi all,

Thanks for the feedback but I am not sure what to make from it. Would anybody have a more clear answer specifically for the clay/sawdust question (Can anybody maybe also explain the relationship with ashes)? I am located in a small village in Romania. It will cost way too much to transport perilite/Vermicilite or anything from a garden store here so I am afraid this is not an option (closest relatively big city at 50 km and no car available).

Thanks all for the feedback

Ruben

Sorry Reuben, I can't help with your direct question. My best suggestion is that you do an experiment. Build a mini rocket burner of some sort, outdoors. See how it responds after a dozen or so fires. My suspicion is that - to get the degree of insulation you need - that it will be too fragile.

Can anybody here say for sure (and provide a citation) that ceramic wool is safe to handle without personal protective equipment? My current belief is that it is not.

Hi Ruben;  Sorry for drifting off of your question.    
To test your riser construction , build a quick rocket stove outside with no mass just a J tube, be sure to smear clay mud to seal your bricks .Mount your riser on this and see how well it performs.  The original concept of rocket stoves was to be able to produce them in under developed areas with local materials. Try one riser with sawdust , try another with wood ash , personally I think wood ash might last longer but I have not personally tried either. Try different mixtures . If neither of those works look for volcanic rock / pummis stone.  Try try again until you find what works best.   Your riser may have a limited life and need checking and or replacement each season but after having built one and gone thru a winter using significantly less firewood not smoking out your village with fumes and staying warm all night it will be worth it !  

Matt Coston;

From a MSDS sheet on ceramic fiber blanket.  
#2 Moderate health hazard, inhalation ,eyes and skin.  
PPE suggested is Glasses/Gloves and a Respirator.
The kind of things that you would want to wear when working around any inhalant.

Hi all,

Many thanks for the useful information and sorry for the late reply.

@Thomas Rubino:I asked a bit around and should be able to have some wood ash tomorrow (I found on some other forums that this should indeed work better than sawdust)
However, how long should it actually burn before I would know if it is a good insulation or not?

I will post a picture of the first set up today or tomorrow and it would be great to have your feedback on this

Thanks

Ruben

Ruben; If it isn't insulating well enough ,the entire riser will get too hot and crumble.  The idea of insulating the riser is to keep the heat going up not out.  Are you able to use a barrel with a removable top?  You may need to check your riser condition often, to confirm it is holding shape.  If it starts to fail ,be prepared to rebuild and have all the materials on hand to build a new one , even (especially) in the middle of winter.  
As I mentioned earlier , after you have a RMH thru the winter ...you will see how well it works... see how much less wood you will burn... Then Show everybody in your village ! Soon somebody who has the means will get/bring perlite back to town or way way better yet, ceramic wool blanket and you can upgrade your riser.  
Remember this;  The better insulated a riser is , the more heat is rising to the top of the barrel & circulating for a complete secondary burn and then more heat is going into your mass!  

Ruben;

Did you study the picture that Rebecca Norman posted?  If you can, build it like hers .  Mix your clay and ash and form your riser. Then surround that with sheet metal and pour straight wood ash in between. Wood ash is an excellent insulator.
You wont need to do that for testing your riser BUT when you build ... you really need to keep the heat rising or the draft on your rmh may stall and it WILL smoke backwards.

Hi Thomas,

According to what I understand from Rebecca, the design didn't work out for her so what should I do it exactly the same way? In all cases, I was thinking to build the heat riser out of bricks as I have a lot of bricks available in the village (although most bricks or not shaped very straight). I tried to make a first set up of the feed tunnel and the heat riser (first layers) today as you can see on the attached pictures (attachment 1-4) and I also gathered some wire and a 55 inch barrel  (attachment 5-6). So I have a few questions on this if possible:

1) Does this set-up entirely out of bricks make sense? I was thinking to put everything together with mortar in the coming days as the bricks are so unevenly shaped that I cannot build the heat riser without the rest of the structure collapsing. I would therefore also only test it the first time (put the fire) when the basic structure is with mortar

2) Do I understand it well that you suggest to make a mix of 50% wood ash/50% clay, put the wire around it and than poor more wood ash in? In the attachment a picture of the wire I found.

3) Do you think the barrel in attachment is fine as it is or should more paint be taken out? It doesn't allow to take the top off but maybe you know if we could cut the top with a tool and attach it with something?

Thanks for the feedback and I really start building tomorrow

Ruben

Hi Ruben;

I'm Sorry; I was thinking that you wanted to make your riser with a clay/ ash mixture.  If you are using clay bricks then things become much easier.  
Even with a brick riser you would still want to insulate it. After your brick riser is installed but before placing your 55gal barrel over top , wrap a piece of sheet metal loosely around it ,tie shut with wire. Fill the gap in between with wood ash. This will help insulate your riser.
Question #1) Yes building with old clay bricks will work fine for a rmh. Your mortar should be clay based not concrete.  Put local clay in a bucket of water and let it separate, then use the slimy clay (slip) to stick your bricks together and to wipe over the outside to seal air leaks.
#2)  As explained above if building a brick riser, you would want to insulate it with straight wood ash poured outside of your riser and contained by sheet metal.
#3) Barrel appears fine, to remove paint I simply build a fire inside of it like a burn barrel. That will help remove the paint.  The issue with paint is, when you build indoors the fumes are very bad as it burns off.
Do not cut the barrel top. It would vent inside your house.   With a brick riser you will not have collapsing problems like a clay riser would. Some time maybe you will find a barrel with a removable top.
#4)  You don't have a #4 but I do.    When you build ...  MEASURE carefully!  inside dimensions must be the same all the way to the top of the riser !!  19.05 Cm square for an 8" system.  Do not try to go smaller than a 6" system (13.97 CM)

Hi Thomas,

Thanks a lot for the very useful feedback.

So for the isolation after the clay brick heat riser, you would use only wood ash without putting any clay? I am not sure if it will stick together in the case of the wire I have, I think it will start falling through the holes without any clay. I can try to find sheet metal but not sure I will find it.

With respect to dimensions, the thing is that the bricks I have are all 26.5*13.5*8.5 centimeters. As it is not so easy to cut these clay bricks in straight shapes, It therefore decided to use 13.5*22.5 as cross section for the feed tube, horizontal burn tunnel and heat riser so that it equals 17.8 square as described in Ianto Evans book for 8 inch tube systems (This is also the reason why, on my pictures, the wholes are more rectangular than square). Does this make sense.

A last thing is that I will also have to build the exhaust system with the same type of bricks as I don t have big steel pipe available. Would it be ok if I also use also 13.5*22.5 cross section for this exhaust system (with bench of around 5 meters long) or is this not appropriate (I will use bricks from surroundings and cob as layers on top of it (around 7-8 on top of the bricks for the exhaust system)).

Thanks a lot again

Ruben

Ruben;
If you can't find sheet metal then you will have to skip trying to insulate your riser. Of course that wire won't hold wood ash. You can always make improvements over time.
Yes a rectangle will work if the inside dimensions are all equal and match Ianto's numbers.
Also , you can use your bricks for the tunnel thru the mass .They will work fine.
Two things , I would size the tunnel bigger than the dimensions of your core.  Not a lot but definitely larger.  Second you will want to create a smooth clay surface on the inside of the tunnel bricks with clay slip.  Rough edges will slow down the heat  , you want a nice smooth surface for the heat to travel down. Just the bottom and the sides don't worry too much about the top brick.

Hi Thomas,

i searched a bit around and there is a guy in the village who can cut  top and bottom from a barrel so that I can use the part in the middle as metal sheet (he will cut a part from the sides if needed to make it smaller). You think this should be fine to keep the wood ash as isolation (I also obtained wood ash meanwhile)?

Thanks

Ruben

Ruben;
Yes , that would work and it would, need trimming down to a proper size but hold off on the sheet metal and concentrate on building your test rocket.
It is important to try building your rocket outdoors to see if your design works well.
 Because only clay is used  to build with, disassembly and moving indoors to reassemble is much less stressful than building indoors and finding out that something isn't right.
Another thing that is important to remember is ) You wont know how well it is working until you put your barrel over top of it.     Its summer , build one  outdoors first.  

Hi Thomas,

We wanted to test the combustion chamber today (in the we made the feed tunnel and horizontal burn tunnel at 13 to 22.5 cm whereas the heat riser at 18 cm squared) but the bricks are so uneven that I am afraid we cannot build the heat riser all the way up without putting any mortar under it. Also, it seems quite difficult to lite it as in attached set-up (difficult to get the flames going)

Do you think it would be fine if we mortar (1:1 mix of sand and clay) the ground base, feed tunnel, horizontal burn tunnel and first part of the heat riser and start testing in from that  point?

Thanks a lot for your answer.

Ruben

Hi all,

I am building a rocket mass heater with feed tunnel, heat riser (normal barrel over it) and exit haust from bricks and we are trying to test the combustion chamber today. In this setup, we built the heat riser of bricks with a big barrel over but without isolation (we will add metal sheet with 5cm of wood ash isolation but should not matter for testing). We tried to light it on a few times but we don't manage to get it going. I have the impression the smoke is going in the right direction when we put the paper but than the flames stay rather outside high on the wood. I included 2 pictures of the setup and 2 pictures from when we light it on.

Does anybody have an idea why it is not working and if we should light it in another way? Maybe it has something to do with the fact that there is a lot of air between the old uneven bricks?

Thanks a lot for the feedback

Ruben

Yes, the large gaps will pretty much kill the rocket draft effect. You do need to make the core airtight for the test; clay and sand will work fine and can easily be removed for moving the core inside. The clay mortar can also be reused just by rewetting it. (Bits that have been exposed to high temperatures may not go back to mud when wetted; mix that in with fresh clay if you want to reuse it.)

Hi Ruben;  I see glenn answered you. Yes it must have cob over all the air leaks or it won't work.
By the way... sure looks like that rmh is indoors...

Hi guys,

Thanks a lot for the feedback. We built the combustion chamber the last 2 days and it is working fine.

Today we are starting to build the exhaust duct out of the same bricks. Would it be fine to make this 18 by 18 cm if the cross section of the combustion chamber is 17 by 17 (exhaust duct will be around 4m excluding the chimney).

Also, we are thinking to first put a thick layer of cob and make the duct directly on top of it. Would this work well (so no other layer of bricks under it)?

Thanks a lot,

Ruben

Hi Ruben;
Yes, 18 x 18 will be fine for your tunnel.  Remember to put a smooth layer of cob on the inside of the bricks to keep your heat moving towards your chimney.
As far as putting cob down  on the floor .  That is what I did on my rmh. Apx. 4" of cob loaded with straw for insulation , this will keep the heat rising up thru your mass instead of sinking down and attempting to heat the  earth.
I have to ask what your floor under the cob is made of ?  If it is dirt or concrete your good... if it is wood it must be spaced up several inches with bricks or it will char and eventually burn the floor.

Hi Thomas,

Thanks for the answer. Yes, we were thinking about putting around 8 cm of cob (3-4 inch). The cob is however made with manure instead of straw but i suppose that is fine?

The floor of the building is also made out of cob, do you think this will decrease the capacity of the system to heat?

Thanks

Ruben

Ruben;  That will work fine ! No problems with manure over straw.  No issues with a cob floor either!
Keep building and send pictures when your finished.

Ok, great. How many inches of cob and rocks would you add on the sides and on top as isolation? Maybe around 6-inch (considering also that we have thick bricks that form the exhaust pipe (around inch thick)) on all sides? Purpose is really heating the room, not cooking. I will put pictures! Ruben

Ruben;
On my mass I have 1' to each side and easy 2' over the top.  It is 15' long (4.75 meters) by 26" wide(66.04 cm )  by 30" tall (76.2 cm) . All stone and cob encased with clay bricks.
Cooking would be done on the barrel,  for heating you can not have too much mass.  The more the better.   6" is not enough . You can start with 6" to dry out the cob but you really want 2' of rocks and cob over your tunnel.
Speaking of drying cob are you aware of how much water has to come out of the cob before it heats ?  A lot !!!  Do not be surprised, it can take over a month to truly dry out your cob.  

Hi Thomas,
Thabks again. Another small question. Is it possible to make the clean-out holes and ash-hole entirely from cob or do you a way of making it from entirely recycled materials? Thanks again.

Ruben;
My clean out / ash pit hole is formed by hand with cob and partly with brick.
 I used a store bought sheet metal 6" stove pipe cap. It fits snug but I smear cob all around as an extra seal to ensure it isn't leaking dangerous gasses into my room.
You can use almost anything metal as a plug/ cap.  Form your bricks & cob to the shape of your plug ... set your plug in place and seal it with cob.   It may  crack as it drys , just smear more over the edges.

In this older photo you can see the metal cap in place.  Really anything with a handle can be used .