Thanks for the responses everyone!
To answer your questions Allen, I am located in Philadelphia. This is a row home basement with concrete floor poured over dirt and a high basement ceiling. I own the house next door as well, which is where we currently live. Although it is possible I might build on wood floors in the future, these old houses are build so much stronger than new houses, so the weight is not nearly as much of an issue. I always design my rooms to center any heavy weight near the side (structural) walls rather than near the middle of the rooms or the front or back sides of the rooms (unsupported, as the joists go from side to side, not front to back). Also, I have been pouring concrete radiant slabs over the wood floors and they have no trouble with this weight. At several points we actually had several tons of sand piled in the middle of the living room floor inside a dumpster, and the floor didn't seem to notice (that's not to mention another several tons of cement, furniture, and other supplies stacked around it! Oh yeah, and about 10 more tons of bagged rubble, around 800 bags total!)!
This brings up another question, are RMHs usually built along the outside wall of a room like conventional heating systems usually are? I could probably make an L shape from the front wall of the house to the side wall where the chimney is, but the thermal bench would only have a single run of stove pipe running through it then, which is why I was thinking of having the bench on the side wall only with a the pipe going out and back again for a smaller but warmer bench. Is the reasoning for putting a heater on the exterior wall of a house mainly for uniformity of heat distribution, or are there other factors as well? I often think you would lose more heat that way, because locating the hottest part of the house next to the coldest part would create the biggest heat differential, which would cause the greatest heat loss over time. I like and agree with the concept around here that you should focus on heating the person, not the house, so why not locate the heat bench AWAY from the exterior walls, and then just agree to locate your body on the heat bench rather than over by the cold window?
Regarding insurance, I am much more concerned with it actually being safe than keeping the insurance company happy. If they really have a problem with it, I can either find another insurance company or change the design to appease them. The state of the house right now (and any of the other 100+ year-old houses around here) is so bad that there would be a dozen other much bigger insurance concerns, and whatever I do is going to be a MAJOR improvement on how things are now. I've also seen the extremely shoddy work of most contractors, and although I don't consider myself an expert, I know I am doing MUCH higher quality work than most professionals! In fact, I hired two contractors to do my electrical and plumbing on my first house, and when the electrician came to inspect it, he made me totally tear out and redo everything they had done. I did it myself the second time, and he was happy with my work. As I'm living in my own house, I know the work I've done is good quality, and I'd much rather have it ACTUALLY work than to have someone who does "certified" work that is actually a mess of leaking pipes and shorting wires. I realize that there are legal risks with anything though, which is why I am moving forward with caution.
The existing chimney is brick, starting in the basement, going through the first, second, and third floors, then out through the roof. The roof is a slightly sloped flat roof with torch-down rubber on top of wood decking. There is an existing 6" round metal chimney sleeve in the chimney now. It's the same as the chimney on the house that we are living in, which we sweep out ourselves every few months. We are mostly burning old wood that we collect around our block for our wood stove, so there is very little build-up anyway, but always better to be safe than sorry!
One thing I haven't been able to figure out is why people here keep saying that concrete is forever or that it is hard to work with. Am I missing something? I mean if you don't want it in the future, why not just take it out again? I understand the comments on using cob to withstand the temperatures though, so I'll have to look into how to build cob.
Here is a picture that I found of what my fire bricks look like: http://www.hotelrestaurantsupply.com/TWN-225044.html?utm_source=froogle&utm_medium=shopping&utm_campaign=products&gclid=CIvWzvX2r7oCFYqi4AodajsAsw
Mine are probably bigger than the ones in the picture, as this furnace was maybe three feet across, but they look exactly the same! I actually burned out several diamond metal-cutting blades cutting into the fire brick before I realized it was there! I thought I had run into some super-strong metal or something! I was really excited because I'd been wanting to build a RMH for a long time, but needed to find an affordable source of fire bricks, and there it was! They are shaped a little funny though to fit a large round chamber, but I was thinking of maybe breaking them.
So I still wasn't quite clear on the fire-brick portion of the design. Would I just build the feed tube, burn tunnel, and heat riser out of fire-brick without using any mortar, then coat just the outside with cob? And which parts can be normal brick, just the outside wall of the bench?
I did look at that page again for the book download, but there doesn't seem to be any download link, only a link to buy a physical copy of the book. Is the e-version no longer available?