Rocket Mass Heaters

In September, I had the opportunity to learn about Rocket Mass Heaters with Ashley Lubyk and Heather Noakes of Dirt Craft.  This workshop was hosted and organized in Invermere by Groundswell.

Ashley and Heather were super wonderful- informative, well spoken, and organized.  The Groundswell team were really welcoming and did a great job at making sure everything ran smoothly and that we had a good experience.

Note- I ran out of batteries the first evening (forgot the charger), so I only have a few pictures from the workshop.  So in this post, I had to source many pictures from the web.

Groundswell Community Greenhouse in Invermere

We started off learning about the theory of rocket mass heaters (RMH).  A rocket mass heater has two basic components: a combustion unit and thermal mass.  Ashley makes the point that when you are trying to heat yourself, a house, or cook something, you want to be efficient (clean burn) as well as effective (heat is actually transferred to whatever you want to heat).  That is why there are two components to a rocket mass heater.  The combustion unit is designed so that it runs a very hot combustion temperature (creating a clean, efficient burn), and the thermal mass (cob, water, or other mass) absorbs most of the heat from the flue gases.

Bare bones of RMH combustion unit.  Image from (minus markups)

In a rocket mass heater, the combustion unit is composed for a feed tube, a combustion chamber and a heat riser.  In a normal wood stove, you only have a combustion chamber (wood box) and a chimney.  Heat radiates from the wood box and makes the room warm.  However, because it is releasing heat to the room, it is lowering the combustion temperature of the fire, reducing efficiency and creating incomplete combustion products such as soot, ash and creosotes.  You are also loosing heat in the chimney.

Unlike a normal wood stove, the combustion chamber in a rocket mass heater is insulated, so heat is not extracted from this part of the heater. The RMH combustion unit is designed so that the combustion chamber can run at very high temperatures, resulting in a complete burn (so no soot, ash and creosotes are produced).

Combustion unit (on the right, with barrel over the heat riser), with laid out chimney pipe waiting to be covered by cob (the thermal mass for this RMH).  photo from, Michael Janzen's post

A side note on fire for those scientifically minded....  Isn't fire just burning wood?  Well yes and no.  To have a fire you need oxygen, heat and fuel.  The first thing you need is some kind of heat (a match, friction, something else burning, a magnifying glass) to raise the temperature of the wood. Then, the wood undergoes pyrolysis, which means at high temperatures the wood releases a bunch of gases, and turns to char (a carbon rich material).  The gases from the wood then combine with oxygen and produce a chemical reaction known as combustion, and releases heat and light.

In order to have a complete burn, you want to make a fire that consumes all the fuel, gases and minimizing waste products (creosotes, un-burnt fuel etc.).  Part of making sure you have a complete burn is having enough oxygen, another part is having the high temperature so that you can facilitate pyrolysis and the efficiency of the burn.

A complete RMH. Image from the dirt craft blog

Why build a Rocket Mass Heater?

  • efficient and effective
  • uses less wood
  • you can burn sticks and small woody biomass
  • maintain the heat in a building for a longer time (because of the thermal mass) with only a short firing period
  • very little to no smoke produced
  • easy and relatively inexpensive to build

What are some reasons not to build a Rocket Mass Heater?
  • Regulations- Depending on permitting processes in your municipality and house insurance, you may need a Wood Energy Technology Transfer (WETT) technologist to sign off on rocket stove, which is not as easy as a regulated wood stove.
  • if you are using a RMH for a cabin or a place that you are not there very often, it might take a while for the space to heat up.  The drum heats up quite quickly, but the thermal mass will take a while to heat up and radiate the heat into the room. The thermal mass may take 3-4 hours to warm up, and will radiate up to 2 days afterwards.

Burning rock on the Big Island of Hawaii

The rocket mass heater was invented through research and experimentation by Ianto Evans of Cob Cottage Company and Dr. Larry Winiarski from Aprovecho Research Center. The RMH was based on 10 Principles for Maximizing Combustion,  from 'Design Principles for Wood Burning Cook Stoves' by Larry Winiarski, Aprovecho Research Center, Partnership for Clean Indoor Air (PCIA), Shell Foundation, June 2005.  Incidentally, it is this research that has helped pioneer more effective, efficient and cleaner wood burning stoves in many areas where wood or charcoal is used for cooking.

Improved wood stoves in Bukati school, Western Kenya

The 10 Principles for Maximizing Combustion (in point form):

  1. insulate around the fire (hotter fire=less smoke=burning of more combustible gases)
  2. place a chimney right above the fire (to burn up the smoke and speed up the draft)
  3. heat and burn the tips of sticks (make flame, not smoke)
  4. amount of heat depends on # of sticks you put in the fire
  5. create a fast draft under the fire (air through the coals, not above the fire)
  6. too little draft= smoke and excess charcoal
  7. keep unrestricted airflow (maintain constant cross sectional area through the stove)
  8. use a grate under the fire for air
  9. insulate the heat flow path so that the gases heat the pot, not the stove
  10. maximize heat transfer to pot with properly sized gaps (to maximize heat transfer, and draft)

Rocket stoves for cooking at the communal kitchen in Adamama, Israel

Let's see how the 10 Principles for Maximizing Combustion, have been applied in a RMH. RMH work because a good fast draft is created through the whole stove. (One of the reasons RMH have the word 'rocket' in the name is because RMH literally sound like a rocket... because the draft is so fast). For the heater to do this with such a long flue pipe, it requires a little math and consideration to temperature loss through heater.  RMH work on the basic principle is that hot air rises and cool air sinks.

Basic diagram of RMH combustion unit, photo from

The combustion unit is shaped like a J. Fire starts at the feed tube, goes through the combustion chamber and the hot air gets sucked up through the heat riser (because hot air rises).  The heat riser is usually covered by a steel drum.  The steel drum is very conductive and transfers heat from the hot air inside the stove to the outside air.  The air inside the drum thus loses some of its heat and drops to the bottom of the barrel.  The air will then flow through the rest of the chimney pipe until it reaches the outlet, releasing heat along the way into the thermal mass.

How do I size my system?

There are a few numbers and ratios that you need to keep in mind when building a rocket mass heater. Chimney or flue pipe usually comes in 6 inch, 7 inch and 8 inch diameters.  We built an 8 inch system for a 1900 sq ft greenhouse at Windberry Farm, Invermere. Rob Avis of Verge Permaculture has a 6 inch system in his 200 sq ft greenhouse in Calgary, and finds it is suitable for weather down to -40 C.   How big your system is will depend on how you use it, the size of your building, how insulated it is and what kinds of outdoor temperatures your are moderating.

General rules and ratios of RMH (extrapolated from the 10 Principles of Maximizing Combustion)

Keep unrestricted airflow:

For example, if you build a 8 inch system you have to remember that your feedtube, combustion chamber, heat riser and even the gap between the steel drum and you heat riser have to preserve that 8 inch diameter (and thus around 50 sq inch area) through out the system!  The one place you can afford to have a bit extra space is around the steel barrel as the air flows flows down from the heat riser to the flue pipes. Remember you want to maintain the airflow all through out your heater (from the 10 Principles of Maximizing Combustion).  

Area calculations for an 8 inch system with fire bricks

Maintain a good fast draft (by choose an appropriate length of flue pipe):

In a RMH, you want to capture the most amount of heat from you system, while maintaining your fast draft.  The air in the flue pipe might start at 150 C (300 F) at the exit of the steel barrel but will drop as it goes through the thermal mass.  The air needs to be at least 50 C (120F) before it exits up through a vertical chimney. If it is cooler than that, it will cause problems in the draft. Dirt craft has found that if you have a 6 inch system, then 25 to 30 ft of pipe is a long as you want to go.  For an 8 inch system, the maximum amount of pipe you want to have is 50ft. (Note, every elbow you put in = 3ft because the draft looses speed around corners.)  If you build your flue pipe any longer, then your RMH will have trouble creating a good draft because it is loosing too much heat to drive the process at the end.

Maintain a good fast draft (by having a chimney):

When the RMH is first lit, it usually has good draft.  As it warms up, the heat differential gets less and less within the RMH and so the draft gets weaker.  That is why you need outside air chimney that extends at least 3 ft from your roof (see Dirt Craft's article about Chimneys) for the hot air to rise up and continue to power the draft, even after the thermal mass has started to warm up.

Maintain a good fast draft (by having the correct proportions in your combustion unit):

The proportions of your feed tube to heat riser are important.  This makes it so that air will be sucked towards the heat riser, not back out into your face through the feed tube.  So if your feed tube has a height of l, then your heat riser should be 2 to 2.5 times l.  The combustion chamber should be as short as possible, given how much room you need for the placement of the steel drum.


Combustion unit: 

The feed tube and combustion chamber is usually made out of firebricks because they can withstand the high temperatures and they are reflective so keep the fire hot.  

Ashley making a mock up of RMH at Groundswell- base of combustion unit
Feed tube, combustion chamber and base of heat riser taking shape
Heat riser added 

The heat riser can be made out of fire brick or heavy steel pipeline material.  If you are using firebrick to make the heat riser, use splits so it doesn't become to big. You do want to insulate around the heat riser (we used clay slip and vermiculite) in between two steel pipes. 

Extra pipe around the heat riser, so we can insulate in between.  The steel barrel would sit on top of the heat riser (about 2 inches above the pipe).  You can support the steel barrel by constructing out of bricks, or cutting a second barrel to fit around the heat riser brick supports.
Base of the combustion unit we built at Windberry farms. Photo from Dirt Craft.

Combustion unit with steel barrel cut out to support the top barrel.  Photo from Dirt Craft
RMH (minus the cob) set up at Windberry Farm. There are a number of openings in the flue pipe left accessible in order to check on the system, clean it out and prime the system. Photo by Dirt Craft.
Finished RMH at Windberry Farm.  Photo from Dirt Craft.
There is a bunch more info about cob, plaster, tricks of laying everything out, and trouble shooting that I left out in this blog post- but that is the basic intro into RMH.

For more info- check out Ianto Evans books on Rocket Stoves and Rocket Mass Heaters and go to a Dirt Craft workshop!  Thanks so much Dirt Craft for a great workshop- and thanks to Groundswell for organizing!


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