This idea’s a bit impractical, but still awesome.
http://www.permacultureactivist.net/PeterBane/Jean_Pain.html
Compost piles properly made, of course, heat up. Reaching 60°C (140°F), a heap of this volume would ferment for up to 18 months and provide (through a simple plastic coil embedded in the pile) heated water for domestic use throughout the run of the reaction. Pain reports that he heated his five—room house of 1000 square feet (100 m2) and provided hot water (at a rate of 4 liters! minute) for its occupants from a 50 ton pile for six months, but that a 12 ton pile maintained that output for a full 18 months
(Aerobic composting would need some kind of ventilation to avoid the generation of methane due to anerobic decomposition.)