by matt walker » Fri Jul 05, 2013 5:48 pm
So glad you are liking our little spot here Ray, we're glad to have you here. We move pretty slow compared to the bigger boards, but I created this space so we can all share in a friendly environment free of some of the baggage of larger sites and I can't tell you how pleased I am with how we are evolving. We do have a great crew here!
On to your questions....
1) I don't believe that a 10" outer will give you sufficient wall thickness here. This is a crucial area as far as insulation goes, the more the better. If you are having a hard time sourcing the small barrel you could take the 10" flue pipe and open it up and screw sections together to create a larger diameter outer shell. It will be awkward filling it though since it won't want to be round.
2) Oh man, that's a tough one. My gut says it will work fine, probably even better than the fireclay, but I haven't tried it. The fireclay has the benefit of being basically immune to really high temps, even if you get it hot enough to start to vitrify you are basically just firing the riser. I couldn't say that the heatstop will react the same way, it might though. Whatever you use, make sure you really get the mix as light as possible, as much perlite as it will hold and still stick together. That's the key to having a high insulation value which again is crucial in the riser.
3) Yep, the 31" should give you the right height inside a regular drum, and you set the drum in a cob seal so if anything the top gap will grow on install which is fine. The key here is that you want as much riser as you can fit in the build, and 31" is probably the minimum you would want to go to ensure a strong draft in the feed.
4) I set up mine outdoors and fired it off, then carried it in in the two sections, core and riser. That said, if you can avoid doing that you guarantee success. Once these things are drying out they get very fragile, although your materials will alleviate that some. Don't forget what I said about cracks though, you will have them, so you will want to be very careful moving a fired core. Using the little drum as the riser outer also adds a ton of support while you carry in the riser, without that, I'm not sure I'd want to try to lift it.
5) If your stove works as well as I expect that inner pipe will be gone in a matter of a few firings, amazingly enough. That's why the mix in the riser is important, it's got to fire hard enough to stay in place once the inner liner is gone. My home system is made of just fireclay and perlite, and I'm sure if I took off my barrel and tapped on the riser it would collapse, but it's well protected in there and without any major shocks I imagine it will last a lifetime. I've played with enough of these outdoors to see them in all states, and it's pretty amazing how stable they are once fired. So yeah, your pipe will burn out and you'll pull out the pieces when you clean the burn tunnel. It's a little unsettling but it's totally normal and expected.