Michael Ahlefeldt

Glad to hear you got it working after all -seemed such a waste not to be able to use it.
Again being a newbie here and not having gone through all the extensive discussions yet, I may be kicking in open doors, but I downloaded Ernie and Erica's plans and I don't see that issue of the joint between the metal barrel and cob/plasterwork being addressed plainly- there is a vermiculite expansion but it is further down if I got it right.
That barrel is going to expand and contract constantly and you're fighting a losing battle to try to fix the gap between the materials with plaster or other “hard” materials. Its probably not even be a great idea to fill the gap because it needs that space to expand when hot.
It wouldn't be a big job to remove a couple of inches of the- Ill call it stone work- and then wrap a strip of say foam rubber (or plastic) ¼ or ⅜” thick around the part of the barrel below the stonework and then fill the stonework back against the foam. After it has set you pull the foam out leaving a open joint between the materials that you fill with a high temperature fiber caulk- like this for example-

http://www.unifrax.com/web/UnifraxHome3.nsf/HTMLNews/A383A07E194B305C852569420055BCB2?OpenDocument

Its a lot of a over kill but its a great material- it resembles papier mache when its dry- but it stands up to very high temperatures. It also has the ability to be applied on a hot surface- like a repair on a running smelter etc
You fill that space with that caulk and it will seal it and take up the movements of the materials and if you slick it before its dry it will have the same appearance as plaster- dull white surface.
If you do lift the barrel you can deal with cracks inside with the same stuff.
Whats inside the stainless rising tube? Any pictures?
I do realize in reading here that there are various different schools of thought, concerning using “natural” materials vs man made (in lack of better words) and I respect the reasoning behind it, but Ive seen some less than great solutions to normal issues in dealing with high temperature applications. On the other hand Im learning a lot of things I haven't run into before- great fun!
James- What does one do you get a bad reputation here? Not read all the threads before posting???

Hi Everyone
I just joined this forum having found these rocket heaters a while back- having a general interest in wood heating and living in a cold climate- Sweden. There are similarities to these cob heaters and our old tile covered fireplaces used here since the 18th century -small fire and a large mass to store the energy. I have two of these old heaters in my house and a cast iron stove and a water mantled wood stove in the kitchen
Ive also run a glass studio for a long time so I know a bit about combustion and high temperature materials and systems.
Im not entirely sure of the terminology and all design features of these cob heaters but reading about Yvette's problems begs for some comments and questions-
All fireplaces have a vertical chimney- thats to create a draft and get the combustion gasses out- the higher it is, the better draft you get because hot air is lighter than cold air and thus rises- going above the roof apex has nothing to do about catching a draft- its all about achieving height and draft and also a fire safety thing- fireplaces can have sparks coming out of the chimney and its not a good idea to have coals or sparks blowing onto your roof.
Im just generalizing here but off the top of my head you need a minimum of about 15 feet of chimney to create the draft for a normal wood stove
The draft is governed by a adjustable damper on the stove pipe or the chimney itself so you don't create a blast furnace in your wood stove.
For best results if using pipe as a chimney it should be insulated as not to be cooled and defeat the whole idea of more height giving more draft.
So what is the idea of just having a horizontal hole out of the wall at more or less ground level? It will not create a draft.
On a cold day and/or if a chimney has not been used in a while, even the best of chimneys might need help by lighting a newspaper in a place where the heat can rise unrestricted out the chimney and get the draft started
So by design the chamber in the cob heater restricts the draft by having the gasses go first up then down into the long chamber- hot gas does not want to travel down unless something is pulling on them from the other end.
The idea of the cob heater is also to save as much as possible of the generated heat- that means that the you would want to keep the flue gas flow as low as possible and still maintain a draft- even bigger reason to go high with the chimney. (and then damper it to perfect draft)
It is essential to restrict the exit rather than your air supply to regulate the draft- you can oxygen starve the combustion and thereby not get the complete burn you want to achieve.
EDIT- got the J part worked out
I cant tell by your description and movie where your crack is but I doubt very much it is “catastrophic”- just fill it with a furnace cement
A wild guess would be that the material was heated too quickly and/or too hot before it was dry
I just saw the new pictures and I could add that its probably a matter of your stonework drying and shrinking and the metal barrel does not- there is also different movement in the different materials- the metal expands in the heat more than the stonework and something will give.
I wouldn't even bother with all the fine points of the air supply until there is a proper draft in the cob heater- a vent of 10 sq. inches will do in the room.