Wood Boiler Definitions

picture showing a typical outdoor wood boiler

Typical Outdoor Wood Boiler

picture showing a heat storage water tank

Heat Storage Water Tank

Outdoor Wood Boilers (OWBs)

OWBs are wood-fired water heaters that are located outdoors or are separated from the space being heated. OWBs are typically constructed of a large metal firebox (15 to 30 cubic feet) surrounded by a water jacket and housed within a small shed-like structure with a smokestack. The fire in the large fire box heats water that is circulated into the home through underground pipes. The energy may be used to heat houses, shops, domestic hot water, greenhouses, swimming pools and spas. The fire is regulated by a draft fan that controls the burn rate of the wood depending on the heat demand. This causes the fire to cycle between full burn and smoldering modes. Conventional OWB units operate at relatively low fire temperatures and provide inefficient combustion causing a greater portion of the wood's heat value to go up the chimney as smoke.

Dual Stage Wood Gasification Boilers

Dual Stage Wood Gasification Boilers differ from OWBs in both construction and operation. Gasification boilers heat the wood in a primary firebox (typically 4 to 6 cubic feet for residential boilers) at relatively low temperature and low oxygen conditions to drive off the organic gasses from the wood. These gasses then pass into a secondary combustion chamber where super heated oxygen rich air is introduced. The high temperatures, turbulence and longer combustion time allow for almost complete burning of the available fuel, and substantially reduced emissions. Heat transfer occurs when exhaust travels through tubes that travel through the boiler's water reservoir. Gasification boilers are designed to provide for near complete combustion of wood and heating efficiencies frequently greater than 80%. This is roughly twice that of conventional OWB units. Gasification units perform best when operated with a heat storage tank because the boiler burns the whole wood load continuously at high temperature.

Heat Storage Water Tanks

Heat Storage Water Tanks are separate from the boiler and are used to store heat from an external source such as a fuel boiler. The tanks are filled with water and contain several heat exchange coils that are connected to the boiler and heating system. Use of a heat storage tank allows a wood boiler to operate at an uninterrupted full burn rate, allowing for higher efficiency and lower smoke emissions.