We are proud to install the Onil Stove when we go into indigenous homes with our stove program. The photos and words on this page are from the ONIL Stove web site.
How the stove works:
Theory of Operation Several factors work together making the HELPS stove efficient, safe and convenient. The cast concrete stove body is molded in fiberglass molds, providing a fast low cost way of producing a long lasting, low cost stove body. The body, however only provides structure for the stove. Two main factors make the stove fast heating and fuel efficient.
Complete combustion In order for all energy in the wood to be converted to hot gases, it must be completely burned including the oil vapor that normally would be emitted as smoke. Containing the fire in a insulated combustion chamber allows the fire to burn hot enough to consume the oil vapor. Smoke is wasted fuel. It is also a health hazard both inside and outside the house.
The photo on the left shows a stove housing with the clay combustion chamber but without the insulation.
On the left is a photo after the insulation has been added. Note the square opening of the combustion chamber and also the rectangular slot along the back that collects the hot gases and vents them to the outside through the chimney. As the gases pass back from the combustion chamber to the collection slot, they heat the plancha (metal stove top not shown in the middle photo).
On the right are the stoves with plancha added.
The insulation is pumice, a volcanic rock that is a very good high temperature insulation and quite cheap.
Burning the wood inside the insulated chamber creates a gas temperature that exceeds 1400 degree F. At these temperatures, the gas vapors (smoke) ignite, adding energy to the fire, and are eliminated from the exhaust.
Efficient transfer to the pot Once the energy from the wood has been efficiently transformed to hot gases, it must be efficiently transferred to the pot. Energy that heats the stove body and is exhausted to the chimney is energy not available for cooking. Wood ashes or pumice provides insulation that prevents the heat from being wasted heating the stove body. Hot gases that do not touch the plancha also waste their energy. The insulation fills the stove cavity to within 1 inch (2.5 cm) of the metal plancha. In this way, all the hot gases come in contact with the cooking surfaces thereby transferring their energy to the pot and leaving only enough heat in the exhaust gases to provide a draft up the chimney.
Wood management such as drying the wood and splitting it to proper size is taught by the HELPS stove technicians prior to the installation of the stoves.
Due to complete combustion, efficient heat transfer. and wood management, the wood consumption (and carrying) is reduced by 60-70%.
Carbon Monoxide and cooking stoves
Carbon monoxide poisoning is the number one cause of unintentional poisoning deaths in the world.
Carbon Monoxide (CO) is produced by the incomplete combustion of fossil fuels, including wood. Incomplete combustion results from insufficient air to fuel mixtures. CO has no smell, taste or color and it is impossible to know when there is a dangerous build-up of CO levels( without instrumentation).
Dangerous amounts of CO can accumulate when the fuel is not burned properly as in an open fire, or when rooms are poorly ventilated. Carbon Monoxide poisons by entering the lungs and displacing oxygen from the bloodstream. Interruption of the normal supply of oxygen puts at risk the functions of the heart, brain and other vital body functions.
In the United States different municipalities have different criteria on which to base estimates of air quality, most however are governed by the Environmental Protection Agency (EPA). Most US codes uses the term "good" to describe air with less than 9 parts per million (ppm) of carbon monoxide (CO), "fair" to describe air with between 9 and 15 ppm, and "poor" to describe air with greater than 15 ppm CO.
HELPS (An NGO in Guatemala) has measured CO concentrations of 160 ppm in homes in Guatemala where the cooking is done on open fires. After these homes receive the "ONIL" stoves, CO concentrations drop to 3-5 ppm.
Safety Benefits
The Dark Side of Open Fire Cooking Open fire cooking is the cultural way to cook in rural Guatemala. It is how their mothers cooked and their grandmothers before them. It is done without thinking about other alternatives. It also provides a nice place to sit and talk. It is also very dangerous! Cooking on open fires creates two major hazards.Burn injuries—Like this leg burned when the lady’s skirt caught fire.
Other causes of major burn injuries are:Face burns from falling into the fires or hair catching fire as the lady blows on the fire to keep it burning.
Eye injuries from a popping fire, usually caused from blowing into the fire. Eyes are also often damaged in a hair fire as well.
Hand burns created from a small child falling into the fire. The houses are small usually 10 by 12 feet with the open fire in the middle of the room. Children playing or learning to walk in these conditions makes it easy for accidents to happen.
The photo on the left shows a stove housing with the clay combustion chamber but without the insulation. 
On the left is a photo after the insulation has been added. Note the square opening of the combustion chamber and also the rectangular slot along the back that collects the hot gases and vents them to the outside through the chimney. As the gases pass back from the combustion chamber to the collection slot, they heat the plancha (metal stove top not shown in the middle photo). 
On the right are the stoves with plancha added.
Other causes of major burn injuries are:Face burns from falling into the fires or hair catching fire as the lady blows on the fire to keep it burning. 
Eye injuries from a popping fire, usually caused from blowing into the fire. Eyes are also often damaged in a hair fire as well.
Hand burns created from a small child falling into the fire. The houses are small usually 10 by 12 feet with the open fire in the middle of the room. Children playing or learning to walk in these conditions makes it easy for accidents to happen.