"Hydronic wood furnace" is the term newly adopted by the American Society for Testing and Materials (ASTM) to more accurately describe what has typically been called a "wood boiler." Use of the word "boiler" suggests that  these furnaces operate under pressure and produce steam for heating. In fact, most wood boilers operate under zero pressure and produce only very hot water.

HYDRONIC -- Of or relating to a heating or cooling system that transfers heat by circulating a fluid (in this case water) through a closed system of pipes.

WOOD-FIRED BOILER – the term “boiler” typically refers to a device that converts water to steam for the purpose of heating or power generation. In the home heating industry, the term boiler refers to a device that produces hot water, not steam. So, a wood-fired boiler is a furnace that burns wood to produce hot water for home heating. To eliminate this conflict in terminology, the Association for Testing and Materials now refers to “wood-fired boilers” as “wood-fired hydronic furnaces.” 

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