I have a hydronic system with a oil fired boiler as a base. Tied in is a pellet stove, wood stove and the main engines. Each stove has a water coil and plumbed to heat the boiler water. The oil burner doesn't come on unless the water temp gets cold. I heated a couple years with wood. A real PIA. The boat was either too hot or too cold. Wood is only cheap heat if you cut and split it yourself. Then you need storage. Wood pellets are the cheapest and most temperature consistent. Besides providing salon heat, the pellet stove coil with it's own circulation pump, keeps the boiler hot. The boiler circulation pump comes on when heat is called for. Cruising, either main engine provides the heat to the boiler.
If you haven't used coal, it's a dirty fuel and lots of soot leaves the stove pipe. Cleaning out the ash can leave a mess inside the boat, too. Quality wood pellets have little ash and produce much more heat than cheap pellets. They more than make up the difference in price. I have yet to clean my ash this winter. In really cold weather, I maybe clean the ash once a month. There are pellet stoves that don't require a blower, but probably too big for small boats. Also there are combination pellet stoves that also have a boiler for hydronics. About 2'x2'x4'. Mine is a smaller stove with a coil from pipe I made.
