Rocket stoves & mass heaters: introduction

What are rocket stoves?

Rocket stoves are stoves designed to burn wood, but also to operate at high enough temperatures to superheat the wood to release gases that are also burnt, increasing efficiency. The flammable gases released are hydrogen and methane, which are burnt, along with carbon monoxide, carbon dioxide and steam, which are vented.

camping rocket stove (notice the flue pipe leaving the tent, and the little back boiler).

Camping rocket stove (notice the flue pipe leaving the tent, and the little back boiler). ‘A warm tent, 8 liters of hot water and an omelette on the hob. Life’s good.’

There’s no smoke with a rocket stove. With a wood stove you burn plant matter, which releases gases – as smoke, which is vented, not burnt. Modern stoves with baffles hold the smoke and gases longer, and are therefore more efficient – but not as efficient as a rocket stove.

A rocket stove is usually a simple L-shape, comprising combustion chamber / flue and a ‘magazine’ to load wood in at the front – usually but not always horizontally. Air is drawn in past the wood, so that combustion occurs at the bottom of the insulated flue (which is often double-skinned, infilled with wood ash or vermiculite). The insulation and the airflow means that the heat is concentrated in the combustion chamber / bottom of the flue, and extremely high temperatures can be achieved.

Rocket stoves are more prevalent in poorer countries such as Somalia, Ethiopia and Uganda, and have been used successfully in refugee camps in Rwanda. They can be made from various materials, including bricks, clay and recycled oil drums or cooking oil cans.

simple rocket stove made from two cans

Simple rocket stove made from two cans.

From simple, single-burner outdoor cooking facilities, you can utilise the same principle for indoor ovens, water boilers, multiple burner stoves or mass heaters, where the heat from the flue is used to heat up a mass of earth, cob or some other material that will store the heat. This way you can make a heated cob bench next to the stove, for example. Throw on a few cushions and we’re talking very, very cosy. For an indoor heater, there needs to be a flue to vent gases outside, with a vertical chimney to take carbon monoxide up and away from windows etc, where it may re-enter the house.

There are presses that can make little round briquettes from sawdust that fit exactly into the magazine of a rocket stove. They have a hole through the middle that allows the air to flow through. They are pushed further into the stove by the next briquette as they burn down.

The principle of superheating wood to release gases, then burning the gases, is the basis of a wood gasifier, which produces gas to be used as a fuel. Wood is burnt in an anaerobic environment (pyrolysis), and the gases released can be used immediately or stored for use elsewhere. Cars can be adapted to run on wood gas.

cross-section of a rocket mass heater. source: EdgeCAD

Cross-section of a rocket mass heater. source: Drawfire.

What are the benefits of rocket stoves?

  • lower emissions of carbon and pollutants per unit of heat delivered
  • they use wood more efficiently; you can bring a pot of water to the boil twice as fast as an open fire or wood stove, using less than half the wood
  • the smoke leaving the flue from a wood stove is hot, but the exhaust from a rocket stove is around normal room temperature; heat is radiated into the room or into the food being cooked, not lost via the flue
  • you don’t have to spend so much time chopping firewood
  • cheap to construct, from recycled materials
  • can reduce deforestation – a serious problem in sub-Saharan Africa due to firewood harvesting for cooking fires
  • in countries where cooking is over an open fire, there has been a high incidence of pneumonia among children and adults alike, due to smoke inhalation; using a rocket stove instead removes this problem, as there’s no smoke
rocket mass heater with heated cob bench - just has to be the cosiest way to heat your space

Rocket mass heater with heated cob bench – just has to be the cosiest way to heat your space.

What can I do?

You can buy a stove, or you can make one. You can search online for instructional videos, or occasionally there are rocket stove building courses.

Possibly the simplest stove you can make is with 3 cans (e.g. olive oil cans) – a large can for the exterior, a smaller one for the interior chamber, and a third, horizontal one for the feed magazine. Put wood ash between the interior and exterior cans for insulation, and you have a basic stove. You could also make a cob/clay stove by forming it around two pieces of pipe in an L-shape, and then taking the pipes out. Sturdier rocket stoves can be fabricated from metal, which is a bit more involved, and you’ll need welding skills.

It is possible to incorporate the rocket stove principles described above into a rocket mass heater that can be installed in a house and used to heat water and/or a mass such as ceramic, fire bricks or a cob bench that will store and radiate heat like a large-scale storage heater without the need for electricity. These are very efficient as heat is stored by the mass rather than wasted from the chimney as with conventional fireplaces. They are used successfully in homes around the world, and in some very cold climates too. There are portable designs available for use in tents, yurts, tipis etc for cooking and heating.

building a clay rocket stove, moulded around 2 pipes at right angles, which will be removed later

Building a clay rocket stove, moulded around 2 pipes at right angles, which will be removed later.

If you want to install a rocket mass heater in your home, you need to think about building regs, otherwise there may be safety issues, and your house insurance will be invalidated if your installation isn’t signed off properly. Some people in remote, self-built, low-impact accommodation often ignore building regs altogether, are confident in their ability to install a heater safely, and don’t go in for house insurance for a property that they can fix or even completely rebuild themselves if necessary. If this isn’t you, then our advice is: be nice to your building control officers. They could block your installation if they want to, because equipment should really be lab tested, which is expensive. The building control officer may not know about that, but they’ll know about document J, covering flues and surrounds. Flues are not allowed to be horizontal, but you could argue that the the horizontal run is in fact part of the heater, and then hook up to a vertical chimney after the horizontal run. There has to be a good hearth around the heater too, to protect flammable materials from falling embers. It really depends on what your local officer is like, but if you approach them in a non-antagonistic way and ask for their help with an interesting, environmentally-friendly (but safe) project, you’re likely to get a more favourable response.

view of a home-made rocket stove from above, showing the insulation and the sticks feeding the flames

View of a home-made rocket stove from above, showing the insulation and the sticks feeding the flames.

You can use rocket stoves on the patio, or for camping. They can be used as cooking stoves, bread ovens and/or water heaters, and rocket heaters can be used indoors as a sealed unit with a flue. Even in a northern climate, located in the centre of the home, they can perform all these functions with a couple of hours burning in the morning, and a couple in the evening.

If you tweak the design so that the fuel (sticks) is inserted vertically (see diagram), then gravity will feed them into the fire, and you can see when you need to add more.

Rocket stoves are very good for camping and there are designs using an enclosed rocket stove that can provide a hot plate cooking surface, water heating and little side flue that can be used in your tent.

a very small, portable rocket stove for camping

A very small, portable rocket stove for camping.

They are quite simple to light. Start a small fire at the bottom of the flue using balls of paper and kindling, and then feed in dry wood. Often you have to tend the fire more often than a conventional wood stove, especially when cooking, due to the small amounts of wood involved. If you leave it for more than 15 minutes at a time, it could go out. However you will have cooked your meal, heated some water or mass to radiate heat and all using a lot less fuel than a conventional wood stove.

 

Thanks to Ed Morriss of Drawfire for information.


We'd love to hear your comments, tips and advice on this topic, and if you post a query, we'll try to get a specialist in our network to answer it for you.