I normally heat water using home built solar panels. When it is below freezing it's a good idea to circulate anti-freeze in the solar panels and have a heat exchanger beside the water tank. I've never got around to that. So, in winters past I've instead run a very shoddy loop of copper coils in front of the air outlet on the biodiesel powered furnace. By pumping water to this loop and back to the water tank I've had very hot water.
Last year I installed a wood stove and have been pleased with wood heat. It seems to offer savings compared to biodiesel or other fuel oils. I've wondered if it was possible to heat water using wood and without consuming any electricity for the pumping. It's kind of a crappy idea to run a 15 watt pump all winter long if it can be avoided.
After some reading I built something very simple and it's worked first try. Here are Flickr Photos.