Instead of burning wood for heat, some Europeans now build a compost pile over plastic water lines that extract heat from the decomposing plant material. Temperatures can get as high as 149 degrees. With a circulating pump as the only moving part, the compost heater lasts an average of 12 to 16 mos. – and occasionally up to 24 mos. – providing heat and up to 80 percent of the hot water for a 1,500 sq. ft. home.
How To Build a Solar-Powered Water Heater — If the power goes out there is very little chance that you can produce enough hot water to fill your needs through wood fire alone. There are many methods of warming water with no power. This could be just the best available.
Build A Woodstove Water-Heating Attachment - Do It Yourself