Outdoor wood-fired boiler

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The outdoor wood boiler is a variant of the classic wood stove adapted for set-up outdoors while still transferring the heat to interior buildings.


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 newer, somewhat controversial method of wood-based hydronic heating.[1] It has come into popular use only in the last decade, and the technology is still being developed and standards set.[2]

An outdoor wood boiler works by heating a water jacket that surrounds the firebox. That water then is used to transfer the heat to the existing heating system by a heat exchanger.[3][4] There are many different manufacturers both in the United States and Canada, with different regulations by region or state. One such U.S. manufacturer is Central Boiler.

Standard outdoor wood boilers[edit]

Standard outdoor wood boilers heat the firebox and the smoke goes out the exhaust straight. If not run hot enough, the exhaust gas will be thick and black, and the resulting ash will not be fully rendered.[2] While functional, these models take more work and are far less efficient[1] than some newer more efficient models.

Catalytic outdoor wood boilers[edit]

Some newer outdoor wood boilers have a catalytic converter 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.[2]

Secondary combustion wood boilers[edit]

There is a new breed of wood boilers that use naturally occurring secondary combustion 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.


Outdoor wood boilers are a topic of environmental controversy.[4] An improperly used or built outdoor wood boiler can produce great amounts of wood smoke and waste material. There are efforts among governments, citizens, and manufacturers to try to ensure that the technology is produced and regulated right while it is still in relative infancy.

One of the largest problems now is that there are no one set of standards regarding the manufacture of outdoor wood boilers or their allowed emissions. In the United States, the EPA has established a voluntary program to qualify outdoor wood boilers that meet certain emission standards.[1] However, as of February 2012, in the United States only 8 states have adopted this voluntary standard and only allow the installation of these cleaner, EPA qualified outdoor boilers.[1][5] Two states — Washington and Oregon — effectively ban them altogether. The EPA is in the process of making new regulations and will establish a national standard that all outdoor wood boilers have to be certified to meet emission standards.[1]

The only truly effective test that exists now is to measure the plume data of the chimney exhaust.[6] Currently, very few outdoor wood boilers have been submitted to this test, making it harder for the consumer to make informed environmental choices.

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