greetings and salutations. New here. A few questions.

Hello.

I have been doing a lot of research on rocket mass heaters. I am quite certain I understand all of the principles to build rocket mass heaters as they have been built for the purpose of heating larger homes/spaces.

I have a few questions on materials and theory so that I may experiment with some variations on the theme. Hopefully they will spark some new dialogue and possibly forays in to new territory.

First question has probably been covered before but I have not found a definitive answer.

1. Is there a simple tried and true formula/ratio to create ideal refractory material for a chimney/heat riser? Ideal meaning high temperature, resist shrinking/cracking etc. Is this formula also the same if one wants to cast the entire core and heat riser from the same material? (different sections though) And if I pour a j-tube, is there a formula that will not require waiting 4 weeks before the unit can be used?

I would love to figure out a homogenous material I could cast the core from, I plan on casting it in a small galvanized feed type tub. For simplicity sake it would be great to come up with a simple j-tube form to hang in the tub and do a SINGLE simple pour in to the tub, rather than all of the multiple stages/materials that are used to currently build the cores.

I keep seeing people build these 2000 degree furnaces in a wood box. Metal seems like a much more suitable material to contain fire. Wat up wit' dat?

I have seen all manners of materials and ratios for heat riser. I understand firebrick wrapped with stovepipe and stuffed with insulation works best, but I am experimenting with MUCH SMALLER units, and the firebrick method requires too much space basically forcing you to use a huge drum as the final shell which creates a huge footprint.

I like the idea of perlite and clay mixture, that seems the most "rock like".

2. What kind of clay? where do I get it? I need to be able to purchase these materials, I can not dig any up.

3. if I decide to use a sacrificial metal liner for the riser, that burns off, leaving the refractory material, what happens to that material? do any toxic compounds become embedded in the refractory material to be slowly released in exhaust over time? The reason I ask is because one of the applications will be exhausted very close to a chicken coop. I do not want zinc or other metals being exhausted in to their environment/runs.

I also wondered about perlite in the heat riser. Does the surface of the riser release particles of volcanic glass in the exhaust? Are exposed bits of perlite in the heat riser and exhaust run a health issue for any humans who may be very close to the exhaust flue? This question also applies to COOKING rocket stoves people pour in 5 gallon buckets. Are they breathing in small particles of volcanic glass rocketing up in to their food and lungs?

4. Is there an ideal ratio for the inner dimensions of the core/heat riser/exhaust? Are there any rules dimension-wise which are especially critical while there is room for experimentation elsewhere? In particular, is the diameter of the airflow path the SAME ALL THE WAY THOUGH the system? Or do you need the exhaust portion of the path to have larger diameter than the core/riser to create lower pressure on the back end, to let the gasses slow down and get burned rather than being rocketed out the chimney cap before getting burned.

All of my designs will incorporate very tall heat riser to amplify the front end updraft as much as possible.

For smaller applications I may or may not be that concerned with a long exhaust run/large thermal bench. I will be trying to use a few horizontal runs made from metal to release the heat inside the dwelling rather than a huge slab of permanent cob using up all the floor space. I completely understand that less thermal mass means more fluctuation and fire tending. This is fine for my applications. Plus I have an idea to keep the fire burning unattended with zero thermal mass in the system which I will present as it's own topic.

Thanks for any help, and thanks to all who have pioneered these devices. The efficiency and ability to use scrap twigs and branches that are useless in conventional stoves is truly an evolution in heating technology.

Huge chunky cord wood trees could be put to much better use than belching heat and smoke out in to the cold night air!

: )

Welcome to Permies, Jacob! I'll take a crack at the question that is in my area of expertise.

jacob green wrote:
3. if I decide to use a sacrificial metal liner for the riser, that burns off, leaving the refractory material, what happens to that material? do any toxic compounds become embedded in the refractory material to be slowly released in exhaust over time? The reason I ask is because one of the applications will be exhausted very close to a chicken coop. I do not want zinc or other metals being exhausted in to their environment/runs.

Zinc is a fairly volatile metal and if you put galvanized steel into a fire, it flashes off in short order. If you stay upwind, this is not really a problem, because zinc is not a problem for biological systems as long as they don't get a huge dose of it at one time. Things that remain on the refractory material tend to be refractory themselves, so after the first firing, you are not going to have stuff "slowly released in exhaust over time".

You should be able to purchase fireclay from lowes or home depot im pretty certain,

i called a sand and gravel yard and they had a better price and i like to support local when i can.


i am in alaska and i was quoted at 27 dollars for a 50 pound bag of fire clay.

i have also seen it said many times that using some furnace cement while pouring your core helps to harden it.

i have the plans for my core up in the other thread.

John Elliott wrote: "4. Is there an ideal ratio for the inner dimensions of the core/heat riser/exhaust? Are there any rules dimension-wise which are especially critical while there is room for experimentation elsewhere? In particular, is the diameter of the airflow path the SAME ALL THE WAY THOUGH the system? Or do you need the exhaust portion of the path to have larger diameter than the core/riser to create lower pressure on the back end, to let the gasses slow down and get burned rather than being rocketed out the chimney cap before getting burned"

your burn tunnel needs to be the tightest of all areas.

all areas past the burn tunnel need to be at least as large as the overall area of the burn tunnel if not bigger.

make sure that where you connect your exhaust to the burning unit that you do not cramp the space at that joinder.

if any areas are smaller in surface space than the burn tunnel the unit will not draft correctly from what i understand.

as far as having efficient burn that all comes down to the height of your heat riser.

my comments are what i have learned in my research, i am in the middle of constructing my first RMH so i dont have every correct answer, just trying to be as helpful as i can.

Troy Fairclough wrote:

your burn tunnel needs to be the tightest of all areas.

all areas past the burn tunnel need to be at least as large as the overall area of the burn tunnel if not bigger.

make sure that where you connect your exhaust to the burning unit that you do not cramp the space at that joinder.

if any areas are smaller in surface space than the burn tunnel the unit will not draft correctly from what i understand.

as far as having efficient burn that all comes down to the height of your heat riser.

Excellent thank you. These type of concise "rules"/comments are exactly what I am looking for. I understood them and their significance immediately. With enough of these types of rules, I simply make sure not to break them and I can design within those parameters.

One question on the "burn tunnel". Do you mean inlet where the sticks go? Or a litle further in where the combustion occurs? and what do you mean tightest of all areas? Should this diameter be smaller than the heat riser?

For simplicity's sake I was planning on having the entire inlet, combustion area, and heat riser be the same diameter. UNLESS people here tell me the burning will be more efficient with different diameters in each of the sections.

Hi Jacob ;I found Fireclay at a building supply . It seemed that most stores did not even know what it was ,then when i found the correct store they had a pallet of it ! I paid 16.00 a 50 # bag I did not find any at home depot or lowes . You also will want 4 cubic foot bags of perlite, I found this at a landscaping store for 17.00 a bag. Mix 50 lbs fireclay with 2 cubic feet of perlite add as much dry refractory as you can afford. This mixture will work to cast a core and to cast a chimney. there is a post here at permies from matt walker that shows his cast core , how to build it and how to cast a heat riser. I followed his ideas and had good success with it.

thomas rubino wrote:add as much dry refractory as you can afford.

Thank you very much for the specific examples and the prices too are helpful so I know what to look for and where to look.

I did not understand the refractory comment.

Is 'dry refractory" a material separate from the the fireclay and perlite? Or is the dry refractory the fireclay itself?

Also as I understand it, if the core gets to hot it will compete with the chimney effect, so is this where the "dry refractory" becomes important? To add insulative qualities to the core rather than heat-sink qualities?

Hi Jacob; First I should ask if you have a copy of ianto evans book (rocket mass heaters) ? If not you need one. Now, i have enclosed a picture of a can of refractory cement, it is added to the fire clay & perlite to make the mixture harder and more abrasive resistant ,also i neglected to add that you need some shredded fiberglass to add to your mixture to help bind it all together. I bought this refractory off of ebay a 2 gal can was apx. 60.00 , not cheap! The perlite mixed in with your fire clay does not hold heat, keeping the core hot only in the burn tunnel and heat riser allowing all your heat to fly up your heat riser slam into the barrel lid complete a secondary burn and get pushed into your horizontal transition area . The top of my barrel in an 8" system is running 1100 degrees side of the barrel is 940 degrees. I have no ash buildup yet after 4.5 months of hard running. Now where your wood goes in is the feed tube, the horizontal tunnel is the burn tunnel and the chimney is the heat riser. Another option that i am considering for my next cast core is to use perlite and 2600 degree refractory cement available from EJ Bartells, product KSV4 i am told this runs anywhere from 35.00-60.00 a 55 # bag and for an 8" system i would expect to use at least 3 bags. When you build your heat riser use a piece of sonitube as a sacrificial form it is cardboard and will burn off with your first fire, no worries about galvanized and no pieces of metal pipe plugging your chimney later.

Hi Jacob I'm new here as well. Hard to imagine, isn't it? Everything is SOOOOO vague. I'm sure to the old hands all this stuff is second nature. It seems odd to me that these guys have this stuff "just lying around". I mean, I'm an accountant? I don't usually have these things? It's funny how they chat casually as if money doesn't matter and of course these materials don't cost anything at all, right?

Anywho.... Matt's tips are great! And so many others too I cannot remember them all. Perhaps Donkey?

I am just about to build my first Rocket Stove; still gathering the last few materials. I have several barrels - 20 gallon and 30 gallon. I previously purchased 2 bags of 2 cu ft vermiculite, though have found on these boards that perlite is preferred, hence I exchanged the vermiculite for two 2 cu ft bags of ... perlite.... (Home Depot, $16)

This afternoon I stopped in at a ceramic supply (potters' supply, local, Lodi NJ) and picked up 50# bags each of refractory mortar and Hawthorne fire clay. (Hawthorne seems to be a generic type of clay, rather than a brand). The refractory cost $45. The Hawthorne fire clay was $16.

Good luck!

I have been doing some investigating into refractory clay/cement with perlite materials.
Here is one of the links I came across. I figure if you can melt metal in a crucible, this is what is needed. It does not seem hard to make and looks durable.
I intend to make a rocket stove with one eventually.

http://boyerfour.com/notes/refractory.htm#Recipes

MAKING LIGHT WEIGHT REFRACTORY CERAMIC FROM PERLITE AND CLAY

Edited by moderator to shorten link