Rocket mass heater
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A rocket mass heater (RMH), also known as rocket stove mass heater, is a form of slow-release radiant heating system, designed to primarily heat people and secondarily to warm areas in
line of sight The line of sight, also known as visual axis or sightline (also sight line), is an imaginary line between a viewer/observer/spectator's eye(s) and a subject of interest, or their relative direction. The subject may be any definable object taken ...
around it. Variations of RMH can also be extended for the functions of
cooking Cooking, cookery, or culinary arts is the art, science and craft of using heat to Outline of food preparation, prepare food for consumption. Cooking techniques and ingredients vary widely, from grilling food over an open fire to using electric ...
, heating water, and producing warm air for distribution. Rocket mass heaters are developed from
rocket stove A rocket stove is an efficient and hot burning stove using small-diameter wood fuel. Fuel is burned in a simple combustion chamber containing an insulated vertical chimney, which ensures almost complete combustion prior to the flames reaching t ...
s, a type of
wood-burning 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 i ...
, and
masonry heater A masonry heater (also called a masonry stove) is a device for warming an interior space through radiant heating, by capturing the heat from periodic burning of fuel (usually wood), and then radiating the heat at a fairly constant temperature ...
s. A primary 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 building design, thermal mass is a property of the mass of a building that enables it to store heat and provide inertia against temperature fluctuations. It is sometimes known as the thermal flywheel effect. The thermal mass of heavy structura ...
in contact with 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 ...
es, which absorbs most of the generated heat before the gases are released to the
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
. According to anecdotes a rocket mass heater might reduce fuel consumption by 80–90% compared to "conventional" stoves. In contrast to conventional wood-burning stoves and
fireplace A fireplace or hearth is a structure made of brick, stone or metal designed to contain a fire. Fireplaces are used for the relaxing ambiance they create and for heating a room. Modern fireplaces vary in heat efficiency, depending on the design. ...
s, in a rocket mass heater, combustion is close to complete. In a rocket mass heater, by-products of combustion, such as
smoke Smoke is a suspension of airborne particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass. It is commonly an unwanted by-product ...
,
soot Soot ( ) is a mass of impure carbon particles resulting from the incomplete combustion of hydrocarbons. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolysed ...
, and
creosote Creosote is a category of carbonaceous chemicals formed by the distillation of various tars and pyrolysis of plant-derived material, such as wood or fossil fuel. They are typically used as preservatives or antiseptics. Some creosote types were ...
compounds, are sucked into the insulated tunnel of the unit, where some claim they further combust, releasing even more heat energy to drive the rocket process, unlike a normal fire, where they are blown out the
chimney A chimney is an architectural ventilation structure made of masonry, clay or metal that isolates hot toxic exhaust gases or smoke produced by a boiler, stove, furnace, incinerator, or fireplace from human living areas. Chimneys are typic ...
.


History

The word ''rocket'' was adopted from the
rocket stove A rocket stove is an efficient and hot burning stove using small-diameter wood fuel. Fuel is burned in a simple combustion chamber containing an insulated vertical chimney, which ensures almost complete combustion prior to the flames reaching t ...
developed by
Aprovecho Aprovecho is the name of two non-profit organizations located in Cottage Grove, Oregon. Aprovecho Sustainability Education Center is a not-for-profit organization based in the vicinity of Cottage Grove, Oregon. Its focus is on sustainable living, ...
for cooking, described in 1982.Aprovecho, "Whole stoves" ''Mass'' refers to the mass of masonry where it stores heat. Masonry heaters have been used since pre-historic times and have evolved into various designs in Europe, Russia, and China. The key principle is the incorporation of a large thermal mass built of masonry which absorbs heat from exhausting combustion products directed in a sinuous path through channels embedded in the masonry. The first prototype was built in the 1980s. Although in some sense simply a variation on a masonry heater, most rocket mass heaters are distinct in producing immediate radiant heat (from the metal "burn" barrel), in being constructed of much cheaper materials (usually a cob mass, 55 gallon steel drum, and small brick firebox), and in requiring less robust a base to be constructed on, since a rocket mass heater's weight is distributed over a larger area.


Different types of rocket mass heaters

There are several types of rocket mass heaters depending on how they are designed. In all designs, an internal insulated
chimney A chimney is an architectural ventilation structure made of masonry, clay or metal that isolates hot toxic exhaust gases or smoke produced by a boiler, stove, furnace, incinerator, or fireplace from human living areas. Chimneys are typic ...
called a "heat riser", produces an efficient high-temperature burn creating sufficient draft to push
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 ...
es through the rest of the system.
Flue gas Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. Quite often, the flue gas refers to the combustion exhaust gas produced a ...
es are cooled to a relatively low temperature within the thermal store, approximately at 50 °C (122 °F). In some designs, steam within these gases
condense Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...
s into
liquid A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, a ...
, releasing the associated
latent heat Latent heat (also known as latent energy or heat of transformation) is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process — usually a first-order phase transition. Latent heat can be understo ...
of condensation which further increases the efficiency in the manner of a condensing (gas) boiler.


J-tube style

In the earliest and most popular form, wood is gravity-fed into a "J-shaped" combustion tube, from where the hot gases enter a well-insulated
firebrick A fire brick, firebrick, or refractory is a block of ceramic material used in lining furnaces, kilns, fireboxes, and fireplaces. A refractory brick is built primarily to withstand high temperature, but will also usually have a low thermal cond ...
or
ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
refractory vertical secondary combustion chamber, the exhaust from which then passes along horizontal metal
ducting In telecommunications, an atmospheric duct is a horizontal layer in the lower atmosphere in which the vertical refractive index gradients are such that radio signals (and light rays) are guided or ducted, tend to follow the curvature of the Eart ...
embedded within a massive
thermal store Thermal energy storage (TES) is achieved with widely different technologies. Depending on the specific technology, it allows excess thermal energy to be stored and used hours, days, months later, at scales ranging from the individual process, ...
. The thermal store is large enough to retain heat for many hours and may form part of the structure of the building. They can be used with
natural building A natural building involves a range of building systems and materials that place major emphasis on sustainability. Ways of achieving sustainability through natural building focus on durability and the use of minimally processed, plentiful or renew ...
s and within
permaculture Permaculture is an approach to land management and settlement design that adopts arrangements observed in flourishing natural ecosystems. It includes a set of design principles derived using whole-systems thinking. It applies these principle ...
designs; where they are normally self-built and are recognized by many (but not all)
building code A building code (also building control or building regulations) is a set of rules that specify the standards for constructed objects such as buildings and non-building structures. Buildings must conform to the code to obtain planning permission ...
s which regulate the design and construction of
heating system A heating system is a mechanism for maintaining temperatures at an acceptable level; by using thermal energy within a home, office, or other dwelling. Often part of an HVAC (heating, ventilation, air conditioning) system. A heating system may be a ...
s within buildings.


Batch-box style

More recent alternate developments have instituted a batch-fed horizontal firebox called "batch box", which feeds into the vertical heat riser or secondary combustion chamber. Compared to a conventional J-tube style RMH, a batch-box style RMH can be loaded with more wood at once so that it can run by itself for a longer period of time. The horizontal duct flow through the mass may be replaced by a heavy "bell" of larger cross section where the hot gases slow and stratify, where the cooled gases exit near the bottom as in some masonry heaters. To allow proper airflow, the vertical port at the back of the box should never be blocked. Compared to j-tube style RMH, it can take more time to build a batch-box style RMH.


Cob style

The heat riser of a cob style rocket mass heater is made of cob, which is a clay and sand mixture reinforced with straw. Because of the relatively low
thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
, a mass consisting of cob can absorb heat and retain it for a longer period. This allows cob-style rocket mass heaters to warm a space for several days after the fire has stopped, thereby reducing amount of wood needed.


Pebble style

Invented by
Paul Wheaton Paul Wheaton is an American permaculture author,Stollar, R.L“Paul Wheaton and Permaculture”Eugene Daily News. Retrieved January 4th 2013.Fredrickson, Erika"Grow Wild" Missoula Independent. Retrieved May 26th, 2020. Master gardener program, ma ...
in 2010, a pebble-style rocket mass heater uses
pebble A pebble is a clast of rock with a particle size of based on the Udden-Wentworth scale of sedimentology. Pebbles are generally considered larger than granules ( in diameter) and smaller than cobbles ( in diameter). A rock made predominant ...
s and
rocks In geology, rock (or stone) is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its chemical composition, and the way in which it is formed. Rocks form the Earth's ...
to facilitate its thermal mass. Compared to cob style rocket mass heaters, pebble style heaters weigh less, allowing them to be moved and modified easily. The lightweight design also makes them more suitable for installing in places that have
wood floor Wood flooring is any product manufactured from timber that is designed for use as flooring, either structural or aesthetic. Wood is a common choice as a flooring material and can come in various styles, colors, cuts, and species. Bamboo flooring ...
. A pebble core also provides a different aesthetic look compared to cob.


Components

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, named a ''wood feed'', a ''heat riser'' and ''heat exchange mass''. Depending on the design, a rocket mass heater can have different architecture within these components, and may also contain several other components designed for different purposes.


Wood feed

The wood feed 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 receives wood in a vertical orientation. A wood feed is designed in a way that when wood sticks are inserted into the feed, only the bottom end of the sticks burn. Due to the direction of the
airflow Airflow, or air flow, is the movement of air. The primary cause of airflow is the existence of air. Air behaves in a fluid manner, meaning particles naturally flow from areas of higher pressure to those where the pressure is lower. Atmospheric a ...
, the fire burns sideways, toward the combustion chamber and the heat riser. The horizontal direction of the fire prevents smoke from escaping the wood feed to the environment. As the bottom of the wood sticks burn, they get slowly pushed in to the feed. The wood feed in a batch-box style rocket mass heater allows more wood to be loaded at the same time.


Heat riser

The heat riser is an insulated, vertical channel that draws
flame A flame (from Latin ''flamma'') is the visible, gaseous part of a fire. It is caused by a highly exothermic chemical reaction taking place in a thin zone. When flames are hot enough to have ionized gaseous components of sufficient density they ...
s upward and powers the rest of the firebox. The heat riser should be insulated to help hold the flames at high temperatures for a complete combustion. The low thermal conductivity of the insulation material allows temperature to build up within the heat riser to high temperatures. When the hot gases reach the enclosing barrel, it gives off a significantly higher amount of heat, which cools the hot air to a relatively lower temperature. This flow causes a strong convective current within the heat riser, which creates a forward airflow which helps wood to burn sideways in the wood feed.


Barrel

The barrel which encloses the heat riser is typically of 0.2 m3 which serves as a radiation heat source for the space, and whose top can also serve as a cooking surface. To allow efficient convective currents, the barrel should be made of steel or materials with higher thermal conductivity.


Heat exchange mass

The thermal mass which encloses the
exhaust Exhaust, exhaustive, or exhaustion may refer to: Law *Exhaustion of intellectual property rights, limits to intellectual property rights in patent and copyright law **Exhaustion doctrine, in patent law ** Exhaustion doctrine under U.S. law, in p ...
duct and absorbs heat from the hot combustion gases is the interface that radiates heat to the environment. It can be made of cob, rocks, sand, or pebbles. The thermal mass is often sculpted into a bench which can become an architectural feature of the home.


Operation

A rocket mass heater is powered by the
heat of combustion The heating value (or energy value or calorific value) of a substance, usually a fuel or food (see food energy), is the amount of heat released during the combustion of a specified amount of it. The ''calorific value'' is the total energy relea ...
of wood inserted into the wood feed in vertical orientation. To start the combustion process, the bottom end of the bundle of wood should be set on fire. The fire can only burn sideways toward the direction of the heat riser, since the insulated tube which connects the wood feed to the heat riser is the only available path for the flow of air. When fire reaches the heat riser, the J-tube design diverts flames upwards into the highly insulated combustion chamber, which allows heat energy to accumulate and create high temperature gases. The higher temperature also allows complete combustion of wood fuel. When the hot air comes in contact with the enclosing barrel, the high thermal conductivity of the barrel draws a significant amount of heat from the gas. Because the barrel has a high rate of heat loss, it quickly warms the room. The remaining exhaust gases, which mainly consists of carbon dioxide and steam, exit sideways through a serpentine duct through the heat exchange mass, giving up much of their heat in the process. The heat absorbed by the heat exchange mass stores that energy and slowly radiates it away to the surrounding environment.


Benefits

*Some people claim it consumes 10% of wood compared to a conventional wood stove. As the heat exchange mass absorbs and delivers most of the
heat of combustion The heating value (or energy value or calorific value) of a substance, usually a fuel or food (see food energy), is the amount of heat released during the combustion of a specified amount of it. The ''calorific value'' is the total energy relea ...
to the surrounding environment, it also provides a higher
thermal efficiency In thermodynamics, the thermal efficiency (\eta_) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc. For a he ...
. As a result, the exhaust temperature of a functioning rocket mass heater can average around 60–90 °C (140–194 °F), which is lower compared to that of a conventional wood stove, which averages around 400–600 °C (752–1112 °F). *Proponents claim the exhaust gas mostly consists of
carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
and
steam Steam is a substance containing water in the gas phase, and sometimes also an aerosol of liquid water droplets, or air. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization ...
, because a rocket stove in general is more efficient at burning fuel. *The low
thermal transmittance Thermal transmittance is the rate of transfer of heat through matter. The thermal transmittance of a material (such as insulation or concrete) or an assembly (such as a wall or window) is expressed as a U-value. The thermal insulance of a structu ...
of the heat exchange mass allows slow release of heat after the fire is out. Some people claim their rocket mass heaters are capable of sufficiently heating their home for several days after all the wood is exhausted and the fire is out.


Disadvantages

*Building a rocket mass heater can be a massive construction that could take up much of the room in a small house.


See also

*
Masonry heater A masonry heater (also called a masonry stove) is a device for warming an interior space through radiant heating, by capturing the heat from periodic burning of fuel (usually wood), and then radiating the heat at a fairly constant temperature ...
*
Appropriate technology Appropriate technology is a movement (and its manifestations) encompassing technological choice and application that is small-scale, affordable by locals, decentralized, labor-intensive, energy-efficient, environmentally sustainable, and locally ...
*
Ernie and Erica Wisner Ernie and Erica Wisner are a couple from Tonasket, Washington, United States, best known for their innovative rocket mass heater designs. They are often referred to as the worldwide leaders and trainers in rocket stove technology. They have made ov ...
*
Paul Wheaton Paul Wheaton is an American permaculture author,Stollar, R.L“Paul Wheaton and Permaculture”Eugene Daily News. Retrieved January 4th 2013.Fredrickson, Erika"Grow Wild" Missoula Independent. Retrieved May 26th, 2020. Master gardener program, ma ...
*
Ianto Evans Ianto Evans is a Welsh applied ecologist, landscape architect, inventor, writer, social critic, and teacher. He is known for his work building, writing and teaching about natural building, cob and high-efficiency solid-fuel stoves, ovens and ...


References


External links


Ernie and Erica Wisner's site

Rocket Stove Mass Heater Tour and How-To
- includes video samples of 12 heaters and a 10-minute how-to {{Authority control Cooking appliances Residential heating appliances Permaculture