Thanks for all the thoughts and insight on this. I've been away from my computer for a while and tinkering. I also got a copy of the RMH book from my library so I've been scouring it over. So many of my questions are answered in that book, so I'm trying to relieve this forum from my basic questions that can be answered elsewhere...
I found this video on thermal siphons:
http://www.youtube.com/watch?v=5IRLVCJ1olA and I'm still working on how I'll incorporate this. I think you're correct, though, guy. I'll have about 3' of head and I think it will work. My tank is a copy of Gary's from builditsolar.com. It's a 2x4 framework sheeted with marine ply, lined with foil faced foam sheeting, then a EPDM pond liner. I didn't want any penetrations in the liner so I'm limited to draining it by siphon action with a hose, which would take a long time, but I shouldn't really ever have to. It's been operating just fine for years now. The cold outs go up over the side and then back down. The pump is located about 2.5 ft. off the floor and there remains enough head in the piping to keep the pump primed. This may limit the siphon action, though. Really, I don't mind running the pump anyway. It doesn't use much energy and it's running on solar anyway. The key to solar is keeping your energy usage down. Just like money, it's easy to keep it if you don't use it!
Right now I'm thinking of running a grid of copper in my mass above the ducting, kind of like the pex in the floors. I'm not sure how pex or other plastic will hold up to the temps of the mass, so I think I might stick with copper. I think I can dig some up. My current system is just copper wrapped around my stove exhaust and clad with mortar. It doesn't work great but it works. I figure it has only about .3 gal of water sitting in the heat exchange portion of it so there is plenty of room for improvement. If I could run a long grid of copper that is larger in dia. I could get much more exchange action going on. We'll see. I'm just going to do it one step at a time. Ideally I wouldn't have the cold out line running up over the side wall of the tank but I'll just get this RMH dialed in for now and worry about the tank later. While it's easy to improve and probably not too costly, I hate wasting anything or getting rid of something that works.
I spent some time last week making the casting mold for a 6" stove. Nothing innovative, just a copy of Matt's design scaled down to a 6" version. It's ready to pour, but the closest fireclay to me is 2 hours away. Sometime in the next couple of weeks I might make the trip, but I'm researching other materials that I could use which might be available nearby. I've been reading about some things that people use to make crucibles, which should be very durable, but everything comes at a price.
I'm taking pictures all along. One of these days I'll get some time to set up one of those photo site accounts so I can include them on here. I hate opening all these different accounts, but I've benefited so much from all the info in this forum I'll bite the bullet to include some for everybody else. I'll keep you posted!
"Knowledge is power. Arm yourself."