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. 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, approximately 50°C (122°F), and steam within these gases condenses into liquid releasing the associated latent heat of evaporation, which further increases the efficiency in the manner of a condensing (gas) boiler.^[1]

History

The key principles of the rocket (cooking) stove were described in 1982.^[2] 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.^[3]

Benefits

Efficiency

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."

—Ianto Evans

Quotes from users

It is 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

As for our own bench, it 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.

—Tom and Calleagh

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

Issues

• No rocket mass stove or stove design has ever been safety certified by the UL. NOTE: The "UL" is Underwriters Laboratories and is only one of many United States "Nationally Recognised Testing Laboratories." UL typically tests and "lists" products. Rocket mass heaters tend not be testable "products" but rather widely varying and individualised or site-specific projects.
• Stoves are often self-built to varying dimensions to suit the location and requirements, 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 preheating of the exhaust tube, resulting in inefficient draft to pull the smoke through.
• Rocket mass heater exhaust is cool, around 35°C, which means in some conditions it is denser than the ambient air. Conventional chimneys may not be suitable for discharge without additional energy inputs. (RMH, p. 93) The cool heater exhaust leaves open the question of chimney deposits such as creosote, and reliable measures for cleaning such combustible deposits to reduce the risk of chimney fires+++.+ This can be a problem if the rocket mass heater is built incorrectly, the properly built rocket mass heater burns all combustibles in the burn chamber and barrel and the exhaust contains minimal or no combustible properties.
• Horizontal exhaust vents may not be compatible with local building codes
• The lack of a widely accepted, reproducible, and formal test standard brings into question the claims of efficiency of rocket mass heaters.

References

Paul Wheaton's 4 DVD set on Amazon : http://www.amazon.ca/Wood-Burning-Stoves-2-0-Permaculture/dp/B00FBXAFQA

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.
• Top Five Things to Do With Oil Barrels When There’s No More Oil To Fill Them – #4. Build a Rocket Stove
• Living with a rocket stove
• Photo gallery of a rocket stove installation