I live in a rental home with no existing fireplace/chimney. I can't make any structural changes to the house. My idea is to build a Rocket Stove just outside my house and channel the heat pipe under the house and setup thermal mass there for radiant heat. Here are a few question I have in trying to design a system:
1. How can I build a rocket stove without the heat exchanger (barrel)? I won't need instant heat and don't want to loose heat outside with an exchanger. I wan't to channel all of the heat into the pipes that will go under the house.
2. If no. 1 is possible, would I still use a J tube design or is it possible to use an L tube with the L laying down (vertical woodfeed into a horizontal tube)?
3. Can dirt be used as thermal mass. If so, would I need to lay an insulating layer, then lay the pipe, then the dirt?
4. Lastly, what are the length limits of a 8" or 6" diameter system and do I count the rise of a chimney at the end in the the length?
Here's a couple words that may help you in your quest: hypocaust and ondol. These are old technologies invented by people who wanted heat coming up from below.
I believe that the thermal mass of the floor is what is important in these systems, so it is kind of redundant to have a thermal mass down in the crawl space, which then radiates up and warms the mass of the floor. The more efficient thing would be to attach your heat pipes directly to the floor.
A-ha, you didn't say you had a wood floor! That makes a big difference, since the two examples I gave you rely on the thermal mass of a stone or tile floor. Wood is more of a thermal insulator, which in the case of a floor will slow the rate at which the heat source below it loses heat to the living space above. This could be a good feature, as a high thermal mass, like you are thinking of, will keep the room above warm for a long time.
But in your search for a good thermal mass, don't limit yourself to dirt. In this table of specific heats, you can see that water has 5 times the heat holding capacity of granite. Maybe the best thing to do, since this is not your own place, is to use your rocket stove to heat a big tank of water in the crawl space. Then when you move, you take your tank with you.