Wood Boilers 
Wood boilers can use wood chips, pellets or logs
Pellet boilers are the most commonly used. They tend to take up less space than the other types. Hopper or automatically fed they need to be located close to the fuel store and can therefore be placed in an outhouse or utility room.
Wood chip boilers require large fuel stores and can require a large buffer as they are slow to react to changes in loading. Less suitable for domestic situations.
A log boiler itself is quite compact but requires a larger fuel store. This is more suitable for non-