You are better off installing a heat exchanger in the firebox itself. If you put a coil or other device on the stove pipe, you risk cooling the exaust gasses too much too fast and coating the pipe with creasote. I could go into detail on how to do this right if you want but I think you can find all the information on yahoo. Look up "wood stove hot water"

P.S. extra points are awarded for using just a thermosyphon