Position of expansion joints in Brick J tube?

There are plenty of references in threads to convince me that expansion joints are needed but I'm really unclear as to where they go, and what they should be made of!

The floor and roof of the burn chamber and the end brick of the first course of the riser is to be made of higher temp firebrick, everything else in slightly lower (cheaper) temp firebrick. Oh except for side of feed tube, it's in house brick as it's furthest from heat, also, the oven extension off the feed tube will be in same house brick.

So I have three types of material, how/where should expansion joints be?

Am I right in assuming that another area is the join between cob and barrel? Could I stuff ceramic cord in here?
I also have a small sheet of foil lined ceramic board about .25" thickness, would this serve between bricks.

Advice appreciated

Lesley

Hello,

I am a new member at permies. I have been following along for a while now, but I signed up today because I too am confused about expansion joints, and here I see someone beat me to it. I too have seen the necessity of expansion joints mentioned multiple
times, and I believe that there are many people that would appreciate a more thorough explanation.

I understand that materials expand when they are heated. What I don't understand is how to put expansion joints, where and with what materials. For example, I know that my barrel will expand, pushing against the cob and causing it to crack. One solution that someone posted somewhere (not sure if it was on permies or not) is using cardboard, that will then be burned out. The problem is that when the cardboard is burned out, there will simply be a gap left for exhaust gases to escape. What I am picturing is some sort of high-temp tape attached to the barrel (how?) and held firmly in the cob, with a gap between the cob and barrel. The tape would provide a seal to keep the gasses in, and the gap would allow the barrel to expand. Do I have this completely wrong? What material should I use? What other locations require expansion joints. How about the cob and rocks that make up the bench - will the exhaust pipes cause cracking there, or is the mass simply too large?

Hi Daniel, Welcome and fingers crossed we get a reply!

i'm sure it was Erica suggested that to slow fire whilst building, so that the bits that want to expand can do so before the rest sets, was part of the solution.

Lesley

Ps if you go into your profile, you can add your location info so we all know where you are! Where in the world are you?

Yes, I've read that in several places. Problem is, with the barrel you need a tight seal to prevent hot gases from coming into the room. Perhaps the expansion is more in the vertical direction? All the pictures of rocket mass heaters I've seen appear to have the cob tightly around the barrel.

I live in Israel, the much-maligned.

If the barrel is seated firmly in cob at the bottom, when it expands it will slide a bit outward and still be sealed. This area probably depends somewhat on your total system and whether you have a good chimney that will add draft. You want the cob to set when the barrel is hot so that it won't have stress that would crack it; you can put a woven fiberglass rope as made for woodstove doors around the base of the barrel if your heat expansion will warrant it.

Thanks Glenn, i do appreciate your help,

When you say 'around the base of the barrel', do you mean inside the structure under the cut 'rim' of the barrel, or, where it emerges from the cob?

Do further expansion joints need adding within the brickwork itself and if so, where do you recommend I put them?
Lesley