Market growers used to use hot-boxes, or heated cold frames, to keep crops growing in the winter. The heat was provided by fresh horse manure buried well below the earth that the plants were in -- you don't want the roots going into the pack of fresh manure, or they'll be burned -- too much nitrogen. So it's bottom heat. I don't know if it would be even remotely feasible to bury a couple of feet of fresh horse manure under the floor of a chicken coop. If you didn't have your own horses, you'd have to truck in quite a bit of manure, and then there would be all the digging -- twice a year, as it has to be removed in the spring. You'd probably have the best gardens around, though, with all that composted manure every year!
HUGE compost piles are being used to produce hot water, and I think even to heat buildings (via pipes run through the piles), but again, it would be a LOT of material to come up with. Might not be too hard to do in areas with more humidity and more plant growth, but in the high desert of Nevada and the eastern side of Oregon where I live, it could be difficult.
Kathleen
Yeah, that's one of the things I was thinking about but couldn't find the exact information I was remembering. Also figured it would let off too much humidity. But since its buried so deep, maybe not.