The two major advantages of a wood-burning boiler are its low fuel cost and its ability to heat a large indoor area. Other advantages include the small space required by the heating system inside the home, the lack of woodsmoke smell inside, and the low environmental impact when compared to other methods of home heating.
Outdoor wood-burning boilers are heating systems that work by heating water in an outdoor boiler with a contained wood fire. The heat is carried in the liquid by pipes into the home. Those hot water pipes provide heat to the home with either a conventional baseboard hydronic system or a forced air furnace. The wood boiler is large enough that it only needs to be filled with wood once or twice a day.
Wood is a relatively inexpensive fuel source, and some homeowners even cut wood on their own properties to use in their wood boilers. Sufficiently large wood-burning boilers can heat multiple buildings. Using wood as a fuel source saves fossil fuel resources, considered one form of environmentally friendly home heating.
Wood-burning boilers aren't pollution-free, though. The low temperatures of the wood fires inside the boilers cause significantly more carbon to be released into the atmosphere than hotter wood fires.