Podcast 267 feedback- cob ductwork??

Paul,
In the podcast you kept hammering on the idea of cob ductwork. Don't know if you are just trying to do this with ZERO dollars but it sounds to me like what you want is actually clay sewer tile. This is a fired clay product was used in residential building for a long time. Kind of being replaced by ABS or PVC piping these days, but the point is that this stuff had accessory fittings like elbows,45's, wye's, etc. this stuff is probably still hanging around in plumbing wholesalers yards. I would imagine this would be perfect in the applications you are talking about. Certainly would lend itself to uniform structures.
Jim

Just did a quick search and there is a company in Ohio that still manufactures them. Their website does not say what the price is, but this could be a good option.

Here is a link to their website.

When I heard that, I wondered if you could use something flammable, like the cardboard tubes for making post anchors with concrete. You need something that's strong enough to not collapse when it's covered in cob. If you let it dry slowly I think it could work. . . Eventually the cob vitrifies, right?

One of the problem I can see with cob pipes is that the cob could crack. I think the advantage with the precast pipes is that they should not crack. Although, they might not work well with temperature changes and end up cracking too

Ceramic chimney liners? Designed to handle heating cycles.

Raw clay has a weak point around 500-600 degrees, then fires hard like terra cotta somewhere above 1000. (No guarantees whether I'm remembering degrees F or C; some pottery has to go up to 2200 to 2600 F to fully harden).'

The ducting in the bench does not get much above 350 F in most cases; only the first run of pipe coming straight out of the manifold might get slightly hotter. Any kind of smooth, sealable clay pipe should work fine, and has been used successfully on several occasions I've heard about. It's not even hot enough to strain itself.

One thing to watch for: make sure the internal dimensions are what you are aiming for; it's easy to get a pipe that looks right because it's 8" outside diameter, and not realize the wall thickness shrinks the available space inside by almost half.

Yours,
Erica W

I bought some of this, but to use as a glory hole drain on new ponds. It does crack just sitting out in the elements over time. there were several fractured pieces in the pile when I went through it. It also has a thick rubber ring around it which may have to be replaced with a cob mortar for a RMH.

http://dallas.craigslist.org/dal/mat/4234536989.html

I co-own a pottery studio, I have plenty of stoneware and a wheel! When I make my first RMH my plan is to throw the manifold on the wheel and fire it to Cone 6 which is apx 2260 f. Will stoneware stand up to the internal temps, I've seen Erica mention it a few times in youtubes but wanted to ask to be sure!? Thanks. Also, if I make the other parts (not the exhaust just the Rocket part) out of stoneware, any suggestions, things to avoid?

David Miller wrote:I co-own a pottery studio, I have plenty of stoneware and a wheel! When I make my first RMH my plan is to throw the manifold on the wheel and fire it to Cone 6 which is apx 2260 f. Will stoneware stand up to the internal temps, I've seen Erica mention it a few times in youtubes but wanted to ask to be sure!? Thanks. Also, if I make the other parts (not the exhaust just the Rocket part) out of stoneware, any suggestions, things to avoid?

Ceramic parts have been used for heater heat exchange for 100s of years, so it is certainly something that can work. I am not so sure about load bearing large pieces. but I have seen some of those too... let me see if I can find some URLs to go with it...

This lady makes all her own tiles for the high mass wood heaters she then builds. All her designs are a work of art and I would expect pricey as well. Really well worth looking through though. Here is a Ceramic heater build video by the same artist. She is not the only person making these, they are quite common in Europe.

The page I wanted I can't find, but there was a company in Europe that made ceramic heaters in round sections that were about 24inch diam. and about 12inch high that they would stack to about 6ft high. It was unique in using large pieces. I do seem to remember that the manufacture of the pieces was more critical that with other processes. Most tile stoves are made of smaller pieces probably so that any one piece has only one rate of expansion to deal with. I think that is the reason the round one was made in sections as well.

With the RMH and a cast base I would worry about the temperature differential from the feed/tunnel/riserbase to the outer manifold. I would be inclined to make the manifold/base and feed/tunnel/riserbase two pieces. I would use fireplace rope or rockwool between the two to allow both sealing and room for expansion. (sheep wool may work in this area too)

While we are at it... Here is an article about the "Russian Rocket", Which is NOT a RMH, but rather a design based on the same starting point.... Making an almost free High Mass wood heater that is very efficient. I believe it is actually a contraflow with a bell on top. The bell on top could just as easily be a bench across the floor to harvest more heat. Anyway, enough of that. The reason I thought it would be of interest in this thread because of the materials. This was built with uncured bricks using a cob mix with the straw replaced with more sand, I don't think they thought of wool Anyway, they used the same mix for mortar just thinned out so that the finished unit is all the same material. Here is the article which would be worth watching as they will be doing comparisons between this and a RMH.

Wanna watch a build? Here it is.

The gist is that is that cob would work fine in brick form... duct forms could be made for ducts or maybe easier to make half duct forms.

Could one use sonoform tubing for the exhaust in the cob, & just let it burn out? One could flare the end of a piece of metal pipe & embed that a foot or two into the cob bench where it emerges to form the chimney.

...Or use the sonoform as a mold for making cob tubing...

A friend has built several rocket stoves, including a tipi RMH riser, using cardboard molds filled with twigs to prevent collapse during forming. They burned out easily during the first firing.

You have to let the cob dry enough to be stable while it burns. Build it 3-6" thick, then fire it until almost dry before continuing to build.

Maybe you could start the twig on fire from the end of the bench (last cleanout or where the pipe comes out of the bench to become the exhaust) and let them burn their way back to the barrel?

R Scott wrote:... you could start the twig on fire from the end of the bench (last cleanout or where the pipe comes out of the bench to become the exhaust) and let them burn their way back to the barrel?

- Making biochar in the process. Brilliant.