Interesting idea. A boiler that doubles as a fireplace insert. http://www.aquaflam.cz/en/ My chimney backs to the attached garage, and has a water heater behind it. My electric bill is mostly hot water and the refrigerator. They're in Czechoslovakia, wonder if anyone has heard of something being made like this in the United States
Prices are on the website. I have found other companies searching "heat water with fireplace insert" They all are pretty pricey and made in Europe. I have a electric water heater directly behind my fireplace, in the garage. I would probably have to tear down the back side of the chimney to access the flue. Thinking maybe I could wrap the flue with copper pipe. I have see videos on you tube where the stack of a wood stove is wrapped in copper pipe to heat water. Might worth it, as the water heater easily uses $400-$500 worth of electricity every year.