Bob's Tip of the Day: Furnaces heat air. If your heating appliance heats water, then it’s a boiler. If your appliance sources heat from the air, the ground, or a water reserve, then it’s one of several types of heat pumps. If there are ducts through which warm air blows, then you have forced-air distribution. If you have baseboard radiators, your distribution system is hydronic (hot water). If the heat comes from your floors (or walls or ceiling), you use radiant heat distribution.
Geothermal heat pumps, or ground source heat pumps (GSHP) have been around for more than half a century. The technology exploits the constant temperatures we find right below the Earth’s surface, either for heating or cooling purposes, and has significantly higher efficiency than electrical heating, furnaces or even air source he
Compost Heat Your Home (see source) Also - http://www.instructables.com/id/Compost-Heating-System/ And - http://www.offgridworld.com/the-power-of-compost-can-you-power-heat-your-off-grid-home-with-compost/