2 Outside the Box RMH Ideas

First off, I've been lurking for about a year but this is my first post...

I have two thoughts that I haven't exactly seen addressed so I thought I would seek your opinions.  If they've been discussed before, please forgive me and leave me a link to the discussion.  I have Ernie and Erica's book but don't have the Wheaton DVD's and have only made a small temporary Rocket Heater for proof of concept.

1) Sub Floor Pebble Style RMH - Imagine a loft style log cabin here in Alaska, with a crawl space and wooden floor.  Would it be a good idea to build a RMH in the corner under the floor with the fuel feed flush with the floor and nothing but a barrel and chimney above the floor?  My idea would be to either run a circular mass path or square as long as possible to keep the draft calculations in line.  (I figure with a circle I could avoid reducing the length since I would have no 90 degree bends, just one large sweeping circle)  This might provide pretty awesome floor heating but would it get too hot?  Also, I was thinking the profile of the pebble mass to be V-shaped with the hope that the heat would spread out more laterally instead rising strait up making a hot circle.  Obviously, heat rises, but would I lose/waste a significant about of heat into the very cold earth below the cabin?  

2) Copper Coil for Hot Water Heater Within the Heat Riser - Like the title says, building the heat riser with a long coiled copper water line inside.  I would like to heat water within the copper and run it to an adjacent room/closet with a hot water heater.  The water would draw in the lower line, heat up and rise and return out the upper line creating a loop of cooler water drawing in, heating and then returning.  I've seen a number of people on YouTube who build a fire around a coil to heat water for a hot tub, it just takes a few hours to heat 100 gallons.  I thought by locating the coil within the heat riser, the coil would get much hotter and thus draw more and heat a higher volume of water faster.  Maybe a 30 minute burn could heat a 50 gallon water heater?  I know the internal temps of the heat riser can be staggering but as long as there is water within the coil the copper should hold up fine right?  I could certainly wrap the riser with the copper, or even wrap the barrel with the copper, but I wanted to super heat the water during a short daily burn.  Am I asking for trouble and possibly building a steam bomb?  Will I melt through the copper?  I figure I couldn't run this to an adjacent bath tub/hot tub because I would have to keep it full with water to avoid burning out the copper.

Do the Permies have any ideas or thoughts to contribute to these ideas?

Thanks,
Rick

The underfloor idea has been done in various ways; yours sounds like it could be promising.

Putting a copper water coil inside the heat riser of a RMH would be a disaster, as the possibly 2000 degree F atmosphere would make the water boil in a fraction of a second and explode the piping. People have wrapped copper coils around the outside of the barrel and successfully heated useful amounts of water, without serious danger of explosion. You still would need to take pressure relief precautions.