How efficient are rocket furnace mass heaters

Rocket mass heating - Rocket mass heater

A pebble style rocket mass heater installed in a home in Montana

A Rocket mass heater ( RMH ), also as Rocket furnace mass heater is known, is a form of slow release radiant heating system designed primarily to warm people and secondary to warm areas in sight around them. RMH variations can also be expanded for the functions of boiling, heating water and generating warm air for distribution.

Rocket mass heaters are developed from rocket stoves, a type of wood stove, and masonry heaters. A major design of a rocket mass heater consists of an insulated combustion chamber, where fuel is burned with high efficiency at high temperature, and a large thermal mass in contact with the exhaust gases, which absorbs most of the heat generated before the gases are released into the atmosphere . According to anecdotes, a rocket mass heater could reduce fuel consumption by 80-90% compared to "conventional" stoves.

In contrast to conventional wood stoves and chimneys, the combustion of a rocket mass heater is almost complete. In a rocket mass heater, by-products of combustion, such as smoke, soot, creosote, compounds are sucked into the unit's insulated tunnel, where some claim they continue to combust, releasing even more thermal energy to drive the rocket process, as opposed to to a normal fire where they are blown out of the chimney.


The word rocket was developed by the adopted rocket stove, aprovecho for cooking, described in 1982. Dimensions refers to the mass of masonry where it stores heat. Masonry heaters have been used since prehistoric times and have evolved into various constructions in Europe, Russia and China. The key principle is the incorporation of a large thermal mass of masonry that absorbs heat from consuming combustion products that are conducted in a tortuous path through ducts embedded in the masonry.

Construction base of a rocket furnace (L-tube) DIY - time lapse.

The first prototype was built in the 1980s. While it's just a variation on a masonry heater in some ways, most rocket mass heaters differ in their ability to generate instant radiant heat (from the metal fire) as they are made of much cheaper materials (usually a cob mass, 55 gallon steel drum) and small brick firebox) and requires a less sturdy base to build on since the weight of a rocket mass heater is distributed over a larger area.

Different types of rocket mass heaters

A cob style rocket mass heater with a cob heat exchange mass
A pebble style rocket mass heater with a wooden frame that gives it a different aesthetic

There are different types of rocket mass heaters depending on how they are constructed. In all constructions, an internal insulated chimney called "heat head" creates efficient, high temperature combustion that creates sufficient draft of air to force exhaust gases through the rest of the system. Flue gases are cooled to a relatively low temperature within the heat accumulator, around 50 ° C (122 ° F). In some constructions, steam condenses into liquid in these gases, releasing the associated latent heat of condensation, which further increases the efficiency in the manner of a condensing boiler.

J-tube style

In the earliest and most popular form, wood is fed by gravity into a "J-shaped" combustion tube, from where the hot gases enter a well-insulated, refractory, vertical secondary combustion chamber made of firebrick or ceramic, the exhaust gas then running horizontally . The heat accumulator is large enough to store the heat for many hours and can be part of the building structure. They can be used with natural buildings and within permaculture schemes. where they are usually self-built and are recognized by many (but not all) building codes that govern the design and construction of heating systems in buildings.

Batch box style

Recent alternative developments have introduced a batch-fed horizontal fire box called a "batch box" that is fed into the vertical riser or secondary combustion chamber. Compared to a traditional J-tube style RMH, a batch box style RMH can be loaded with more wood at one time so it can run on its own for a longer period of time. The horizontal channel flow through the mass can be replaced by a heavy "bell" with a larger cross-section, in which the hot gases slow down and stratify, with the cooled gases emerging near the floor, as with some masonry heaters. To ensure proper airflow, the vertical port on the back of the box should never be blocked. Compared to J-Tube style RMH, it may take longer to create a Batch Box style RMH.

Cob style

The heat rise of a cob-style rocket rocket mass heater is made from cob, a mixture of clay and sand reinforced with straw. Due to the relatively low thermal conductivity, a mass consisting of pistons can absorb heat and store it over a longer period of time. This allows cob-style rocket mass heaters to heat a room for several days after the fire has ended, reducing the need for wood.

Pebble type

A pebble style rocket mass heater invented by Paul Wheaton in 2010 uses pebbles and stones to boost its thermal mass. Compared to cob style rocket mass heaters, pebble heaters weigh less so they can be easily moved and modified. The lightweight design also makes them more suitable for installation in places with wooden floors. A pebble core also offers a different aesthetic look than cob.


"J-style" combustion unit of a rocket mass heater profile view (top) and a top-to-bottom view (bottom). 1 : Wood lining . 2 : Burning tunnel . 3 : heater . 5 : Firebox (consisting of fuel supply, combustion tunnel and heating riser). 4 and 6 : Downdraft bell / barrel . 7 : Distributor.

Different types of rocket mass heaters are made with different components and different materials for specific purposes. All types of rocket mass heaters have three main components: Wood feed , Heat rise and Heat exchange mass . Depending on the design, a rocket mass heater can have a different architecture within these components and also contain several other components that are designed for different purposes.

Wood lining

The wood lining is the opening through which wood and air enter the system. In a J-tube design, the wood feed is a small and narrow opening that accepts wood in a vertical orientation. A wood lining is designed in such a way that when wooden sticks are inserted into the lining, only the lower end of the sticks burns. Due to the direction of the air flow, the fire burns sideways towards the combustion chamber and the rise in heat. The horizontal direction of the fire prevents smoke from the wood lining from escaping into the environment. When the bottom of the wooden sticks burns, they are slowly pushed into the feed. By adding wood to rocket mass heaters in batch box style, more wood can be loaded at the same time.

Heat the riser

The heat riser is an insulated vertical duct that draws flames up and supplies power to the rest of the firebox. The heat surge should be insulated to keep the flames for complete combustion at high temperatures. The low thermal conductivity of the insulating material enables the temperature rise within the heat rise to high temperatures. When the hot gases reach the surrounding cylinder, they give off a significantly higher amount of heat, which cools the hot air to a relatively lower temperature. This flow creates a strong convection current within the heat surge, creating a forward flow of air that helps wood in the wood lining burn sideways.


The barrel that encloses the rise in heat is typically 0.2 m in size 3, which serves as a radiant heat source for the room and the top of which can also serve as a cooking surface. To enable efficient convection currents, the cylinder should be made of steel or materials with higher thermal conductivity.

Heat exchange mass

The thermal mass that surrounds the exhaust duct and absorbs heat from the hot combustion gases is the interface that gives off heat to the environment. It can be made of corn on the cob, stones, or pebbles. The thermal mass is often shaped into a bench that can become an architectural feature of the house.


J-tube rocket mass heater in operation. 1. Wood inserted vertically burns sideways. 2. High temperatures generate strong convection currents in the heat increase. 3. That Exhaust gas escapes from the side. 4. Heat exchange mass absorbs, stores and radiates heat. 5. Final exhaust gas from carbon dioxide and steam)

A rocket mass heater is powered by the heat of combustion from wood, which is fed into the wooden feet in a vertical orientation. To start the combustion process, the lower end of the bundle of wood should be set on fire. The fire can only burn sideways, in the direction of the heat build-up, as the insulated pipe connecting the wooden feet to the heat build-up is the only available path for airflow.

When fire reaches the heat surge, the J-tube design directs the flames up into the highly insulated combustion chamber, which allows thermal energy to accumulate and create high temperature gases. The higher temperature also allows wood fuel to burn completely. When the hot air comes into contact with the surrounding cylinder, the cylinder's high thermal conductivity removes a significant amount of heat from the gas. Since the barrel has a high heat loss, it quickly heats the room.

The remaining exhaust gases, which mainly consist of carbon dioxide and steam, exit laterally through a serpentine channel through the heat exchange mass and give off a large part of their heat in the process. The heat absorbed by the heat exchange mass stores this energy and slowly radiates it to the environment.


  • Some people claim that it uses 10% of the wood compared to a traditional wood stove. Since the heat exchange mass absorbs most of the combustion heat and releases it to the environment, it also offers a higher thermal efficiency. As a result, the exhaust gas temperature of a functioning rocket mass heater can average between 60 and 90 ° C, which is 400 to 600 ° C (140-194 ° F) lower when compared to a conventional wood-burning stove. 752-1112 ° F).
  • Proponents claim that the exhaust is mostly made up of carbon dioxide and steam, as rocket stoves are generally more efficient at burning fuel.
  • The low thermal permeability of the heat exchange mass allows a slow release of heat after the fire has been extinguished. Some people claim that their rocket mass heaters are capable of adequately heating their home for several days after all of the wood is exhausted and the fire is extinguished.


  • Building a rocket mass heater can be a massive construction that can take up much of the space in a small house.

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