Jonas Fritzsche

Here are the pictures of the bypass damper. It is build like the ordinary throttle valves used in stove pipes but from a full round piece of metal and some CMS (calcium magnesium silicate) fibre sealing. The sealing is not visible on the pictures because I added them later and I'm not shure wheter they were neccesary because I think the passage through the bypass doesn't need to be closed almost airtight for the hot gases to get forced into the bench bell when closed. So I think maybe a well fitted piece of thick round metal screwed on a round bar of metal could work aswell. Other people in this forum may have more experience on that.

Hello Linden,

thanks ! Yes, the heater is working. It's not finished yet because a lot of the bench shaping and adding more mass still needs to be done, but the performance is already quite impressive to me. The bypass damper I finally build by myself because I have access to a good metal workshop and I developed a kind of construction that I considered easy enough to build on my own.

Attached you find some pictures of how the whole RMH looks at the moment. I will add some pictures from the bypass damper as soon as I have them on my laptop.

I finally build my perlite clay riser today. I decided to form an inner tube from cardboard stabilized by rings of plywood, so i can burn out this whole inner structure when dried. For the outside i used a sheet metal container (a ventilation duct) srewed in plywood as bottom for the perlite-clay. To keep the inner tube centered I added a form of plywood on the top that was removed when I got close to the top with the perlite-clay. Attached you find some pictures.

Thanks! Yes, it's Cone 10 and I also thought that a quite liquid consistency would be good for the inner coating. Maybe even as liquid that it would be possible to lay the riser on the side (after the liner is burned out or removed in another way), splash a line of the mixture and cast it on the surface by rolling it. The dried perlite clay will soak a lot of water I think so it shouldn't remain that liquid for long and will possibly build a more stable connection to the perlite clay as you suggested also. I will see and test it with my small scale version.

Is there a reason why the perlite riser shouldn't have a thin layer of fireclay mortar on the inside to prevent it better from crumbling in this area and for a smoother and more long lasting shape? The mortar I would use is rated for 2370°F. On the outside I will use a metal cylinder so crumbling on the outside won't be a problem. Someone got experience with that? From my understanding that layer would'nt absorb much heat if it has a thickness oft just 2mm or something like that?

Today I formed a little riser to test the building technique and to find the right mixture. It seemed to work but It's still enclosed in the cylinders I used to form it so I'll see the final result when removing the cylinders. I could test the idea of coating the riser with fireclay mortar with this small scale model.

And I found out that the ready mixed fireclay mortar doesn't work as clay slip for the perlite mixture since it contains too much sand and therefore isn't sticky enough. So I used a clay slip from fine stoneware pottery clay instead

The chimney will rise outside above roof level after leaving through the wall and end at a height of around 11,5 ft. See the attached second drawing of the chimney planning.

Thanks again!

I purchased 6" stove pipe and now got the question where to plan the connection of the pipe for the bypass to the radiant chamber. Glenn wrote "at the side of the Bell". But which height? I suppose that closer to the top oft the radiant chamber respectively the same height where the riser ends means better draft in cold starting conditions, right? But since I want the upper barrell to be removable a connection there would make it necessary to remove this connection for maintenance aswell. So I would prefer to place the bypass connection right at the end of the first barrell which will be in a fixed (cobbed in) position anyhow. So the hot Gases would need to sink down around 16" from the top of the riser to the middle of the pipe. See drawing attached. Do you think that will work?

I tried to find out where other builders placed this connection. If I understood it right Thomas rubino f.e. placed the connection on one of bis builds quite at the bottom oft the radiant chamber and said that it works good. Its described in this post: https://permies.com/t/159700/Retrofitting-Piped-Mass-Bypass-Install . I think the last pictures show the radiant chamber at the left of the pictures.

Thank you for you answer Coydon.

Water from the roof can't enter but I'm not sure wether the concrete foundation slab stays dry in our humid climate respectively I don't know how it was built and if it has a good drainage. At the moment there is a sheet of plastic between wooden floor and the polystyrene floor insulation on top of the foundation.

So I think I would like to have such a drainage layer just to be sure. I was just hesitating because I thought that it might be an unstable base for the heater because its loose material. On the other hand: if it is well compacted so that it can't move it should work? I once built a rubble trench as foundation of a strawbale-building, so I have some experience but the difference is that in case of the rmh it would just be a thin layer. I'm lacking of more detailled information or experience on such a layer and don't want to mess up the whole project with this first step... So any experience if rubble can be a stable base (under both: the bench and the firebox) and how thick this layer should be will be welcome

And are there any opinions on my first question?

Jonas Fritzsche wrote:

1. Should there be a layer of compacted rubble between the concrete foundation slab and the perlite-clay layer under the RMH? Ernie and Erica write in the builders guide that they almost always start with such a layer to protect the cob from damp. The cabin is located in a forest in humid climate. Or does the perlite-clay layer (that I'll put under the whole structure anyhow for not losing too much heat into the foundation) act as a drainage aswell? And if a layer of compacted rubble is recommended: would a layer of around 3/4" sufficient?

Now I actually found a bagged product from a german manufacturer. Its a fine clay mortar for oven building (grain size 0-1mm but mostly <0,3mm) that can be used for a layer thickness between 2-8mm (around 1/13" to 1/3") and is rated for 2200 °F. It is this one: https://www.primus-ofenshop.com/produkt/universalmoertel-plus-keramisch-sack-25-kg?_gl=1*jntsc5*_up*MQ..&gclid=EAIaIQobChMIntXY8aSJ_gIVhM1RCh3FHAaPEAQYCCABEgKSGvD_BwE

Infos about the manufactuerer from Wikipedia: Wolfshöher Tonwerke GmbH & Co. KG is a manufacturer of refractory fireclay for craft stove construction, ceramic chimney inner tubes and shaped bricks for heating inserts

So I think I will use it in its thicker consistency as mortar for the bricks but also mix a clay slip from it for the construction of the riser.