A mass heater made by Organic Arts
FA info icon.svg Angle down icon.svg Project data
Authors Leigh Blackall
Location Canberra, Australia
Instance of Rocket stove
OKH Manifest Download

Building a rocket stove mass heater (including research).

Advantages[edit | edit source]

  • Heat your home with much less wood than an open fire (it has been claimed 5 to 10 times less).
  • Exhaust is mainly steam and CO2, with much less of the impurities normally contained in smoke. (A little smoke is emitted at the beginning.)
  • The heat from one fire can last for days thanks to the thermal mass.
  • You can build one in a day and half.
  • Cost is very low - it is possible to build one for less than $20, plus natural materials collected locally (mainly earth).

Design principles[edit | edit source]

A stovepipe or chimney is still recommended to vent the air to the outside to minimize indoor air pollution. To gain as efficient a heat transfer as possible, use a long thin pipe or heat exchanger, to extract maximum energy from the flue gas. The challenge here is that increasing the resistance offered by the pipe will interfere with operation of the furnace. Increasing the height of the vent increases the chimney effect and may counteract this.

Ernie and Erica Wisner's 6-inch Annex RMH plans, available here: https://permies.com/wiki/64029/Rocket-Mass-Heater-Plans-Annex
Image from Ernie and Erica Wisner's 6-inch Annex RMH plans, available here: https://permies.com/wiki/64029/Rocket-Mass-Heater-Plans-Annex

Questions and Considerations[edit | edit source]

  • It seems that when initially starting the fire, smoke might not draw through the system until the burning chamber is heated and drawing enough air so that smoke is taken out the exhaust. If this is a common problem, what additions could be added to the design so that smoke does not fill the room in which the heater is situated?
    • One user has found that blowing an electric fan at high speed into the burn tunnel solves the smoke-back problem; another rocket mass heater was built with a small USB-powered fan inside the chimney to assist when needed, such as on cold plug days.
  • When feeding a stove pipe horizontally out of a wall it is often put into a T with a bottom that can open to allow cleaning the chimney.
  • Some users will light a single sheet of newspaper crumpled into a ball in the T outside to start the air flow going the right way.
  • Can the mass-covered flu work vertically so that the hot gasses can be drawn downstairs and then exhausted back upstairs?
  • In the pipe that flows through the mass, any attempts to draw hot air downwards tends to air lock the flow. If at all possible, always stay level or sloping up
  • How long does the stove top last given the amount of heat being put there? Is maintainence built into the standard design?
  • Cleanouts are often included in the design of the exhaust pipe in the mass to allow removal of ash build up or a sucked in paper ball.

Understanding the Market/Legal Permitting Considerations[edit | edit source]

  • Permits and local regulations need to be considered which often require partical output measurements along with efficiency ratings. This will be difficult for a self built RSMH as it will always be different.
  • Two code-compliant, certified pre-fab models exist for the rocket core: the Liberator in the USA is code-approved in all 50 states and EPA-certified, and the Gamera has EU certifications (more info needed)
  • As of 2022, RSMH are frequently Do-It-Yourself and so are not commonly seen as sellable commodities and do not have standards in place.
  • Fuel efficiency and possible reduction in emissions could make this heater a preferred heating method in many regions

Safety[edit | edit source]

Carbon monoxide is known as the silent killer, responsible for approximately one million deaths each year worldwide. Carbon monoxide poisoning is a serious threat when burning wood in an enclosed space without proper venting. Burning less wood than a conventional wood stove alreayd mitigates this risk to some degree. Additionally, some steps you can take to make your rocket mass heater safer include:

  • Always install a carbon monoxide detector and regularly change the batteries.
  • Always have a certified professional stove installer review your installation.
  • Install an external air source vent and completely seal the stove from the interior of your structure. This may include adding a door to seal the fuel chamber.
  • To prevent the risk of 'smoke-back' out the external air source vent, the [entrance to the] external air source vent must be as low as possible, optimally 3 feet or more below the fuel chamber in order to reduce the risk of 'smoke-back' out the external air source vent and carbon monoxide being drawn back into your structure.
  • The [exit of the] exhaust vent of your stove must be at least 3 feet above your roof in order to reduce the risk of carbon monoxide being drawn back into your structure.
  • If you are having a problem with 'smoke-back' into the interior of your structure, your stove design is not safe and may be a serious risk to you and your family. Leave the area immediately and replace your stove as soon as possible.
  • The amount of weight (or mass, if you prefer) involved in a thermal-mass stove may be unsafe for existing flooring, and do-it-yourself builders may be unfamiliar with safe and appropriate measures for such construction.
  • When in doubt, be safe, not sorry.
  • The greatest risk to a wood-burning heater is that of chimney fire; the Rocket Mass Heater design mitigates this to a great extent by a) burning the smoke as well as the wood itself, in a sense pre-burning the chimney fire's creosote fuel constantly rather than allowing it to build up and then burn and b) using much less wood overall than a conventional wood stove. Nevertheless, chimney fires are a danger and chimneys should be inspected yearly before operation.

Rocket Mass Heater proponent Paul Wheaton advocates the continued use of a J-Tube style design, rather than the more complicated batch-box systems, reasoning that J-Tubes are more tolerant of error in design, construction, and operation, making them more easily made by the inexperienced builder.

Next steps[edit | edit source]

Contact details[edit | edit source]

Leigh Blackall in Canberra, Australia is working on this and would like to hear from you: leighblackall@gmail.com

See also[edit | edit source]

External links[edit | edit source]

FA info icon.svg Angle down icon.svg Page data
Keywords cobb, heating, permaculture techniques, rocket stoves
SDG SDG11 Sustainable cities and communities
Authors Leigh Blackall, JoshuaDM, Beaumdavidson, CWG
License CC-BY-SA-3.0
Language English (en)
Translations Czech, Korean, Dutch, Chinese, Bosnian
Related 5 subpages, 20 pages link here
Impact 3,057 page views
Created April 13, 2009 by Leigh Blackall
Modified April 7, 2024 by Kathy Nativi
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