Heat riser, duct/flue questions

I'm having some trouble with just what I should be using for the interior lining on the insulated heat riser. Seems to me that the typical metal duct material is much too thin for the job and will burn out in a hurry - but maybe with a proper cob and perlite insulation layer it doesn't matter that the interior liner burns out? Or is there something else I should be looking for to form the heat riser? I know of ceramic flue elements (uncertain what the proper name for them is) and I would think those would hold up, but I have not found them in my (casual, so far) looking about.

So, one question - what to use for the lining of the heat riser?

Second question, regarding the duct work through the heat mass and for the chimney. Is it just conventional light gauge galvanized sheet metal duct?

Getting ready to build a RMH for a seasonal greenhouse that should also supplement heating the whole house it's attached to.

Thanks for all you do.

Peter Ellis : Dealing with your last sentence 1st, if you are committed to a Rocket Mass Heater R.M.H. in your future then let me strongly advise you to go to
cobcottage.com to get Your PDF Copy $15.oo, of Evans' and Jackson's Great Book 'Rocket Mass Heaters ', there is STILL no other book in any
Language with as much Rocket Stove / Rocket Mass Heater Family information !!! ( and I dont make a dime!)

The use of Perlite and Clay Slip to make an insulated Heat Riser / Internal chimney is a recent improvement over most types of refractory 'fire' bricks, and certainly
how I propose to build my next R.M.H.

All of the stove pipe buried within the R.M.H. / Thermal bench should be considered to be a sacrificial form around which your cob, your Perlite And Clay Slip are
placed and a few bricks are stacked . As such we do not care whether they last weeks or years, here we can go with the cheapest grade of thin wall piping we can find,
the lighter the better, galvanized pipe having no inherent advantage !

For what we are trying to accomplish Ceramic flue pipe has limited advantages. If we try to start our R.M.H. up after we have allowed the whole house and the Rocket
to get cold, there will be sections of our ceramic material trying to come up to Temperatures in excess of 1500 F, while other parts of the ceramic pipe are still at room
temperatures or colder, This can cause catastrophic failure of our ceramic tile !

Many people add-on a Green house on the southern axis of their property, treating the combined wall as a Thermal mass heat storage/.Tromb Wall opportunity and are
very happy with just a simple sliding door between the two attached buildings, You will probably find that there are major periods of operations where your greenhouse's
humidity is higher than you might like it in the house, this depends a lot on location, climate and how you use water in your green house which again goes back to
regional preferences!

Because of the R.M.H.s appetite for small very dry wood for maximum efficiencies you will want to plan on Storing your wood for your R.M.H.s Dragon Outside of your
Humid Greenhouse, which can easily add 10% moisture content to 'tomorrows wood today' ! I know I have provoked more questions than I have answered, this is just
one of a series of steps moving you towards your rocket stove build !

For the Good Of the Crafts ! Be Safe, keep warm ! As always your comments and questions are Solicited and are Welcome ! Big Al !

Couple of points:
- Heat riser: yes, any steel (whether mild, galvanized, or stainless) exposed to flames in this area will likely warp and burn out.
Perlite/clay slip makes a moderately stable insulative material that can hold its shape as the metal burns away; it's friable but cheap.
Firebrick, refractories rated for 2400+ F (3000F would be better), or even ordinary clay building brick (2000 F) with a perlite or mineral wool insulation will probably last longer.
Insulating kiln brick also works as a stand-in for both structure and insulation, but I'd recommend slopping on a second layer of plaster or fiber to prevent any possibility of thermal stress cracks creating air leaks in the joints between the bricks.

- Greenhouse: Damp conditions mean cob won't 'set', so the pipe you choose may need to be self-sufficient. That said, galvanized will likely work for years as long as it's protected from crushing or pooling of accumulated moisture.
Other options include smooth terra-cotta drain or chimney liners, well casing, smooth-lined concrete drain pipe, painted stovepipe, etc. Avoid any corrugated or rough materials; if you have to build a square channel, make it somewhat shorter than you'd otherwise do with smooth, round channels.

Yours,
-Erica

I have access to some Aluminum Flex Duct (not aluminum foil duct) and was wondering if I could use that in the thermal bench part. I was thinking I would pack it (all around) in Perlite/Clay slip and surround it by bricks and big river rock I have laying around. I was curious since Big Al's sacrificial form comment made me think that this may be an option.

This is for an outdoor bench. The actual connection to R.M.H. would be stove piping. I am only interested in using the Aluminum Flex for the bench part and possible leading up to the horizontal exhaust pipe.

Thanks a bunch for your input.

-Lars
(n00b to R.M.H.)

Lars Fabiunke : If you have 6'' inside diameter Flex duct you should be able to use it after about 5-6' of your thermal mass, I would like to see a picture, but due to the
increased turbulence caused by the bends and creases of the inside walls the total number of feet of horizontal piping that the Heat Riser can 'push' the hot exhaust gases
through will be reduced ! How much? I don't Know !

You do not want Perlite in your Thermal Bench, Perlite is insulative! Not what you want there. You want to use a good grade of aluminum tape on the first 5-6' to seal your
ductwork, paint everything with clay slip, and then pack the outside of the piping with cob, adding more dense rocks to increase both amount of heat stored in your thermal mass
and the speed that the heat travels thru to the surface of your bench !

It is possible to have a R.M.H. out doors but generally it will need to be under a Carport Type structure, and special attention must be made to sealing the top and protecting the
sides ! Most of the time a warm bench with a fire going inside will be able to 'handle' an occasional rain storm, but not several days of rain without a fire the whole time !
Hope this helps a little !


For the Good of the Craft ! As always your comments, questions are solicited and are welcome ! Think like Fire, Flow like a Gas, Don't be the Marshmallow ! Big Al

wow.. here I go off to get some breakfast and mull my day's schedule and there is a reply already . Thank you for the answers and thank you for the correction on the Perlite error!

First of all I am using Ernie & Erica's Plan for 2 Chamber oven. Then I thought man it would be nice if I could make it so that we could enjoy a heated bench for the cool evenings. So dreamed up a plan and built it in 3D.
Enclosed is my ..ah-hem plan. The dream view (external), what the base structure looks like and my envisioned "pipe dream"

I am using old bricks, old blocks and new, fence posts, you name it. The base is done, the bench, the pipes, the cob oven and the rocket stove is not (yet)

Once you are done laughing, be gentle with your critique

Lars Fabiunke : Ernie and Erica have a damper control that was originally set up to bypass the thermal bench and pre-warm the chimney. The idea was to then switch
back to sending the hot exhaust gases thru the Thermal mass !

In your case I Think that you could build your Rocket Mass Heater to preheat your cob oven, or even to do all the heating and then switch to your Thermal
Mass !

In any case I am not sure about your plans to use Cement blocks without extensive protection from the heat of ether the R.M.H., or the two chamber cob oven, Cement
doesn't do well with long term exposure to temps above 400 F ! And your slab under the oven will need a solid core to take that weight ? I Think ? Big Al

I will have to check that out, thank you for the tip. I had thought that perhaps I could heat up the oven with the rocket, but I got my brain in a pretzel trying to figure out the damper.

I am adding aluminum foil plus 4 inches of perlite/clay slip as the base, then placing fire brick on top of that. E&E recommend, when a combustible base, 3 inch perlite/clay slip, I padded up. all that rests on corrugated aluminum. That should be enough for the cob oven, no?
The slab under the oven is pretty much all urbanite bricks from an old BBQ, stacked, packed and mortared together. I have some tile I was going to use for the inside flooring.

I am also worried about the RMH under the structure. One I don't have that much space, since I tossed the idea for a RMH being the pre/main heater. Two, if I end up making a small RMH, it may not even heat the bench...