This is a simple question with a pretty complex answer. You might want to check this thread: Water Heater Coil
I'm don't want to rely on electric/pump systems to keep my living room furnishings from being blown into the yard. Many who do seem to add battery back up power systems for the pumps. Kinda kludgey, IMO. http://www.woodheat.org/dhw/dhw.htm
This link is broken, either the page no longer exists or there is some other issue like a typo.
The brits never got over open thermo siphon systems, which are probably the best solution for simple 'solid fuel" DHW. It's popular there in many cottages. Here's a fair example of what they do. http://www.solidfuel.co.uk/pdfs/link_up.pdf
Yanche recommended Dan Halohan's books on the subject and here's an example of why: http://www.oldhouseweb.com/stories/Detailed/830.shtml