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Hello Permsteading Community! I have been so inspired by this community that I am in the middle of a RMH build. It is an 8" system with a cast core and a 12' cob bench/bed L, here in Santa Fe, NM. The general exhaust layout has the barrel in the middle of the house with an out and back through the bench, then a vertical rise out the roof. We live in a windy location which has pushed our design to use the existing 8" flue out the roof. The ceilings are slanted and high (16') so I thought if we put the vertical exhaust pipe as close to the barrel as possible, it would help heat the air in it, thus starting the flow up and out. Does this make sense? Should the vertical stove pipe touch the barrel to make this work? I want to make sure I am not overloading the system with the overall run. Thanks for any help!

Hello, Lily, I am happy to hear that the wonderful community here has inspired you. I think you will be just fine in your plans. I don't think placing the exhaust close to the barrel will have much effect on your exhaust temps and the layout of your system should be just fine. I live in a very windy heating climate with a similar system size and have experienced no problems. I say, go for it!

Lili, what you're describing about barrel to vertical exhaust placement makes perfect sense. It has actually become somewhat common practice, where practical, among those who have built hundreds of cob RMH. As you've gathered, with the vertical exhaust pipe placed close to the barrel, the consensus is that a bit of barrel waste heat warms the vertical flue pipe and helps with driving the draft. There are other advantages as well, such as "shading" a direction where less barrel radiant heat is desired.

There are some good photos of a recent E&E Wisner cob rocket workshop built heater here, and you'll see how close they place the vertical flue to the barrel:

http://www.permies.com/forums/posts/list/1000/26232

From the description of your RMH layout with rather modest out and back horizontal flue run, and the existing indoor 8" flue, it should draft really well nonetheless.

Thanks for the quick reply and support. Much appreciated.

Hi Lili, welcome. I do like to run the chimney next to the barrel to help power it a bit, if it's possible to do so. I don't think it's crucial, but I believe it helps. Thanks for posting the photos! Great to see your build, keep 'em coming.

Here is a link to the pics and video of our build. https://plus.google.com/photos/11615040 ... 6985578881

I wasn't happy with the first core so we cast the second one in place. So glad we didn't have to move the first one inside! It will be put to use on the outdoor sunken core, cook stove idea. So simple and so functional. Thanks Matt for making the cast core such a simple process. Yes there has been some trial and error, but exciting nonetheless. It became our motto "WWMWD" What would Matt Walker do? or "Just cob it together". The Polar Vortex has inspired us to get in gear and finish the last bit. So cool to see all the people inspired by this technology/art/heater. Love it when form follows function. Keep dreaming the big dream. Excited to have found a local person in Santa Fe to give me advice. The web is growing. Lili

Oh, that beautiful southwest lighting! You're off to a great start, Lili. As for your question at the end of the video, I assume you realize the hardi-backer is meant to burn away, and yes, you can count on adding a plaster coat to the core from time to time. My latest stove has actually held up pretty well around the feed but I patched the top of the burn tunnel shortly after the first firing, and had to put a large patch on one of the sides in the burn tunnel where the surface was disintegrating. It may seem like a chore right now to have to do the constant maintenance, but once you finish your stove you'll be like, "but I want to keep building?!" And then every time you need to patch or modify you'll be happy you get to play with it again.

Lili, thanks so much for the kudos to my sloppiness! I do appreciate it, truly. If you read around the net you'll find folks agonizing over the details and I do love to encourage people to dive in and make something, perfect or not. You won't need fire cement all throughout the interior, and really don't need to worry about the cracks. They will be there, regardless of material. The trick is to get better at mixing cob as you work with your materials, and then make a really good outer case around the whole core. The softer perlite/clay core will crack, and frankly anything will, but with a good surround around it you won't need to worry about cracks. Stuff falling off and wearing off the interior as Manny describes is what needs attention, and it will happen. I have never found it to be too troublesome, and as time goes on you will sort out your preferred patching mix, and I found it a beneficial process as I tuned my feed over time to my fuel and use needs. Great photos, so fun to follow your build.