A rocket stove mass heater or rocket mass heater, is an innovative and efficient space heating system developed from the rocket stove, a type of hyper-efficient wood-burning stove, named in the 70’s, but dating back millennia in concept, and the masonry heater. Wood is gravity fed into a ‘J shaped’ combustion chamber, from where the hot gases enter a heavily insulated metal or fire-brick 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 typically forms 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.
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 (approx 50°C) and steam within these gases condenses into liquid releasing the associated latent heat of evaporation, which increases the efficiency further in the manner of a condensing (gas) boiler.
The key principles of the rocket (cooking) stove were described in 1982. Ianto Evans of the Cob Cottage Company then described how the same combustion principles could be used to heat a building in his 2006 book, Rocket Mass Heaters based on research and experience in many countries over a 30 year period.
From Rocket mass heaters: Superefficient woodstoves YOU can build (Ianto Evans and Leslie Jackson, Cob Cottage Company, 0-9663738-3-9):
“In my own cottage I burn only about two-thirds of a cord of (fir and alder) firewood a year, while my neighbors average 3-5 cords. You can usually tell when any of the neighbors are around by the cloud of smoke coming out of their chimneys. By contrast, we burn so clean that visitors coming into my house want to know how come it’s so snug without the stove burning. Imagine their surprise when they learn that in fact, it is burning merrily.”
“It’s extremely efficient, reaching 90 percent combustion, and almost all the heat is then stored in the cob mass bench, to be slowly released over days!” -Ianto Evans
“[The bench] takes about four hours to get totally warm. From a 4-6 hour burn time once a day or every other day, we can maintain a comfortable temperature in the house of about 65 [degrees] F, even on cold days.”
“On days with no sun we run our stove two to three hours in the evening, burning about a five gallon bucket full of wood. For regular winter temperatures of 35 to 50 [degrees] F, this keeps our house at a comfortable 60 [degrees] to 65 [degrees]F.” - Bernhard Masterson
“Ianto and I measured 1000 [degrees] C (1800 [degrees] F) in the combustion chamber and 32 [degrees] C (90 [degrees] F) in the top of the chimney - the rest of the heat was kept inside the house.” - Flemming Abrahamsson
No rocket mass stove or stove design has ever been safety certified by the UL.
Stoves are often self-built to varying dimensions to suit the location and requirement using a variety of materials.
Initial lighting of the fire may produce smoke that is often not drawn into the exhaust system until the burning chamber is heated and drawing air.
A common problem with some designs is ‘smoke-back’ where smoke from the fire is released back into the interior rather than outside. Smoke-back could indicate a serious risk of carbon monoxide poisoning. This issue could be a result of poor individual stove design, or inadequate pre-heating of the exhaust tuberesulting in inefficient draft to pull the smoke through.
Rocket Mass Heater exhaust is cool, at around 90 degrees F, which means in some conditions it is denser than air. Conventional chimneys may not be suitable for discharge without additional energy inputs.
Horizontal exhaust vents may not be compatible with local building code.
Rocket Mass Heater - a clean burning, high performance burner usually put together using a majority of recycled and/or natural materials, such as cob. Owners claim an 80-90% reduction in wood usage compared to heating the same space with a metal wood stove.
As opposed to the masonry heater which requires some advanced masonry skills, the rocket mass heater can be built by any enthusiast with basic masonry and engineering abilities.
This image of a tree appeared on the bottom of a barrel that I was burning the paint off of last week. The barrel was used to build the rocket mass heater that you see in the second photo. I love that several beautiful things came from such an ungraceful object.