The two types of wood boiler are the outdoor wood boiler and the dual-stage wood gasification boiler. Gasification boilers are the more expensive type, but are also more efficient than outdoor wood boilers.
Outdoor wood boilers are reliable and inexpensive, but their heavy smoke output limits their use in urban and suburban environments. The smoke also precludes indoor installation and contributes to air pollution. Gasification boilers have two burn chambers and burn at higher temperatures than outdoor wood boilers. Consequently, they produce little smoke and are suitable for indoor and outdoor placement. This style of boiler is often used as a water heater.
The heating power of a wood boiler depends on many factors, including its size and the type of wood used for fuel. Thoroughly dry wood is the most efficient fuel for a wood boiler. Moist wood is difficult to ignite and generates steam, which rusts the boiler.
Burning moist wood also creates creosote deposits. Creosote is a major threat to wood boiler safety and efficiency. Sticky creosote deposits plug chimneys and increase the risk of chimney fires. Short chimneys and hot temperatures help prevent creosote formation. It is also important not to overload the firebox, and to rake the ashes before inserting additional wood.