Now this is getting very interesting. Does anyone have any references for using the earth as a heater? I am not going to state that whatever I say has any particular authority, 'cause I ain't got any. However, having taken physics and spent my life dinking w/ "science stuff" (like many of us here) I am still not convinced about this earth-heater thing, and here is why. For one, if this method really works, why not use NO under-slab insulation and have even more heat coming back at night? Same concept, to me, just a larger version. The heat loss calc equation is: heat loss = reciprocal of R-value times the surface area times the temp differential, between and of two adjacent objects. Q=UA(delta T). Period. Heat transfer is independent of earth's meager gravitational force, ie heat does not go "directionally", but rather "to cold". (Matter in a higher state of excitation ("warmer", higher entropy) gives off energy in the form of heat to matter in a lower state ("cooler", lower entropy)). Period. Up/down/sideways matters not. What DOES go up are warmer liquids/gasses, because they are less dense than their cooler partner and therefore effected less by gravity. A concrete slab can only get heat from the earth if that earth is warmer than it. If a slab cools to 44 degrees and the ground is 45, the ground will warm the slab. Until that time, the warmer slab is warming the earth. Certainly, earth warmed by someone's wood boiler will eventually "give back" some heat if/when the slab gets cooler than it, but I don't think I want to send all those btu's down there to warm it, primarily because I can not control how far down AND sideways the heat will go. Heat goes to cold. Period. I've been told that a foot of dry earth has an R of 4-ish, about what 1" of Extruded Poly has. Do I want to heat a huge mass of earth below my slab so that I get SOME of it back? No. I don't. Analogy: The more money you pay for certain tax write-offs, the more you get back, but it is not a good investment over-all, (just looking at the money part). That said, would I heat a bunch of earth in a box ON TOP of my insulated slab? Sure, just like I'd heat water through which to run a coil to heat DHW, etc. The difference is, I can pretty well control that heat transfer, and keep most of it in my house. I wouldn't look at that system as saving me anything, but rather shifting the time when I get that heat into the house. Heating the earth is a limitless venture; you could run a heat coil down there and heat it all day, but I bet you'd get a very small percentage of it back at night. The Chinese are my buddies, but I am not going to send heat their way unnecessarily. Does that make sense? That last point is the critical one to me: how much do you spend to get how much back? AND, BTW.... we could extend that hypothesis and say "Well, I opened my door to heat the air around my house, so that I can get it back tonight." Assuming no wind and no Brownian motion in the air molecules to move the heat from right around the house, that would be the same concept as heating the earth, would it not? Finally,if you are sitting on a 'thermal", like in Yellowstone Park, by all means don't insulate one iota; let the earth do it all. So: Somebody respond and tell me where you think my reasoning is in error. I think it is ok, but then I tend to be prejudiced.... thnx. j