What we must keep in mind here is the temperature of the water that will be used by people. It is not as simple as heating water with fossil fuel such as wood and letting this heated water somehow flow through the water heater for potable use.
The one set up that I am familiar with had circulating pumps that pushed non-potable water through coils in the water heater not the potable water in the water heater through the remote heating device.
If one was looking at any type of external heat to support their potable water supply I would suggest solar water heater. This is the only Green setup I am aware of. Using fossil fuel for hot water is not Green Technology but the direct opposite. Green Technology is to get away from fossil fuel such as burning wood. The carbon foot print left by burning wood is far from the same carbon foot print used to produce the electrical energy for the water heater.
Here is a link to one type of solar water heater
http://www.rheem.com/products/solar_water_heating/
There are several thousand different units that one could choose from and without a doubt the expense will be less than using a wood stove setup after the tax deductions one can benefit from.
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