Rocket mass heater

A rocket stove mass heater or rocket mass heater, is a space heating system developed from the rocket stove, a type of efficient wood-burning stove, and the masonry heater. Its fundamental characteristics are an insulated combustion chamber where fuel (generally wood) is burned with high efficiency at extremely high temperatures, and a large thermal mass in contact with the exhaust gases which absorbs most of the generated heat before the gases are released to the atmosphere.

In the earliest developed form, wood is gravity fed into a "J-shaped" combustion chamber, from where the hot gases enter a heavily insulated fire-brick or ceramic refractory vertical secondary combustion chamber, the exhaust from which then passes along horizontal metal ducting embedded within a massive cob thermal store. The thermal store is large enough to retain heat for many hours and may form part of the structure of the building. They have proved to be popular with natural buildings and within permaculture designs; they are normally self-built and are not yet recognized by all building codes which regulate the design and construction of heating systems within buildings.

More recent alternate developments have instituted a batch-fed horizontal firebox which feeds into the vertical heat riser or secondary combustion chamber. Also, the horizontal duct flow through the mass may be replaced by a massive "bell" of larger cross section where the hot gases slow and stratify and the cooled gases exit near the bottom as in some masonry heaters. These developments may be used in different combinations in specific instances.

An internal vertical insulated chimney, the combustion chamber, ensures an efficient high-temperature burn and creates enough draft to push exhaust gases through the rest of the system. Flue gases are cooled to a relatively low temperature within the thermal store, approximately 50 °C (122 °F), and steam within these gases condenses into liquid releasing the associated latent heat of condensation, which further increases the efficiency in the manner of a condensing (gas) boiler.

...
Wikipedia