The outdoor wood boiler is a variant of the classic

wood stove A wood-burning stove (or wood burner or log burner in the UK) is a heating or cooking appliance capable of burning wood fuel and wood-derived biomass fuel, such as sawdust bricks. Generally the appliance consists of a solid metal (usually cast ...

adapted for set-up outdoors while still transferring the heat to interior buildings.

Technology

The outdoor wood boiler is a variant on the indoor wood, oil or gas boiler. An outdoor wood boiler or outdoor wood stove is a unit about 4-6 feet wide and around 10 feet long. It is made up of four main parts- the firebox, which can be either round or square, the water jacket, the heat exchanger, and the weather proof housing. The fire box ranges from 2 to 5 feet long and can be as tall as 4 feet. The firebox and heat exchanger are surrounded by water or a glycol-water solution, which absorb heat from the burning wood. The heated water is generally circulated through insulated underground lines to a heating load, where the heat can be transferred from the water to various heat emitters.

Standard outdoor wood furnaces

Standard outdoor wood boilers heat the firebox and the smoke goes out the exhaust straight. If not run hot enough, the

exhaust gas Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline (petrol), diesel fuel, fuel oil, biodiesel blends, or coal. According to the type of engine, it is discharged into the atmosphere through an ...

will be thick and black, and the resulting

ash Ash or ashes are the solid remnants of fires. Specifically, ''ash'' refers to all non-aqueous, non-gaseous residues that remain after something burns. In analytical chemistry, to analyse the mineral and metal content of chemical samples, ash ...

will not be fully rendered. While functional, these models take more work and are far less efficient than some newer more efficient models.

Catalytic outdoor wood boilers

Some newer outdoor wood boilers have a

catalytic converter A catalytic converter is an exhaust emission control device that converts toxic gases and pollutants in exhaust gas from an internal combustion engine into less-toxic pollutants by catalyzing a redox reaction. Catalytic converters are usual ...

installed, to allow for a more efficient burn and treatment of the exhaust before it leaves the unit. There is a considerable difference in cost between these and standard units. These are far less popular than gasification outdoor wood boilers.

Secondary combustion (gasification) outdoor wood boilers

Gasifier wood boilers use secondary combustion air to burn additional wood gases. The most sophisticated systems use computer controls to direct air at different stages of the burn process at variable volume. High efficiency outdoor wood boilers use down draft gasification must meet strict EPA Phase 2 Certification. Outdoor wood gasification boilers can burn between 30% - 50% less fuel than its traditional counterpart. Outdoor wood boilers are a topic of environmental controversy. An improperly used or built outdoor wood boiler can produce wood smoke with excessive unburned

particulate matter Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM) or suspended particulate matter (SPM) – are microscopic particles of solid or liquid matter suspended in the air. The te ...

, but when properly burned, studies show that burning wood is still one of the most environmentally friendly heating options. In 2007, EPA began a voluntary program to encourage manufacturers to reduce particulate levels. In 2015, the U.S. EPA updated its clean air standards for residential wood heaters, this is a mandatory standard for sales in the United States. The revised regulations apply only to new products and will not affect existing wood heaters. At this point, the only boilers that pass these emission controls are gasification models.

References

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