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New designs for fuelwood cooking stoves

Swiss Forestry Association, Solothurn, Switzerland

The author undertook a mission in Indonesia, under the sponsorship of FAO and the Swiss Government, to investigate, the improvement of fuelwood cooking stoves and secure economy in fuelwood consumption.

Fuelwood production and forestry economy

WOOD surpasses any other natural substance in the diversity of the uses to which it can be put: it can be used as a building material or as a substance to be worked, as a raw material in the chemical industry, and as a combustible.

Even when trees in their yearly growth supply wood quite naturally and abundantly, it is still essential that it should be utilized as intelligently as possible. Any overconsumption can involve disastrous consequences, which must be forestalled.

Wood is man's oldest source of heat, since it is more accessible than any other fuel and also simple to ignite. It is this very accessibility which accounts for its still being burnt in primitive fireplaces according to traditional procedures. The result can only lead to very high consumption, amounting to serious wastage.

FIGURE 1. - Construction of the FAO/Singer stove.

The situation indeed differs from country to country. In one the full possibilities of wood as a source of energy have not yet been realized; in another, on the contrary, consumption must be cut down so as not to overwork the forests. Elsewhere wood is faced with competition from other fuels. But everywhere, the problem to be solved is the same: how to use the heating potentialities of wood to better advantage.


Solid natural fuels, although all of the same origin, can nevertheless he clearly differentiated one from another by their external appearance and by their basic constituents and composition. These characteristics account for their different burning properties. If, therefore, one wants to take most advantage of the heat produced, the peculiarities of the various fuels and their different burning properties must be taken into account. In several countries some fundamental research work, accompanied by practical experiments, has been carried out. General conclusions have then been drawn for the manufacture and operation of fuelwood fires. The final result has been cooking stoves and heating devices which, while intelligently employing the principles of combustion, are also clean and easily operated.

FIGURE 2. - Plate old: (1), (2), (3) plates.

FIGURE 3. - Brick mold with, below, diagram of a sun dried (not burnt) brick 5 X 10.5 X 22.5 centimeters.

FIGURE: 4. Chimney of FAO/Singer stove. Note that the chimney must be independent of the materials for the, chimney are drainage pipe and bricks; total area of ventilation openings must of the chimney hole; and the length of stove surface upward, should be a minimum of 2.5 meters.

Wood burns easily and requires no special prior preparation as a fuel. This is exemplified by the more, or less typical cases of the stoves in use so far in Indonesia. But tests have proved that these stoves are not efficient users of fuelwood. Certainly the calorific power of wood can be much better utilized.

Because of its high content of volatile constituents, wood burns comparatively quickly, with a long flame, and requires very little oxygen to burn. These properties must be borne in mind in the construction of the hearths, if good results are to be achieved.

This has been done in the construction of the new stoves, which have the following characteristics:

1. a closed hearth with a limited intake of air for the combustion process;
2. flues for the flames and fumes;
3. a chimney.

The hearth

The hearth has these functions to fulfill: to hold the necessary amount of wood, to give the flame enough room to grow, to hold the live embers together in a tight mass, and to ensure concentrated burning. It is impossible to give exact dimensions which are generally applicable. In the models considered here, the hearth is 27 to 28 centimeters long, 16 to 18 centimeters wide at the bottom and 18 to 20 centimeters at the top. The internal height ought to be 20 centimeters. Above all it is essential that the hearth should be completely closed in. The stokehole is stopped with a stone, in the bottom of which there should be an opening of 6 X 4 centimeters to act as an air-intake.

The flues for the flames and the fumes

When a stove has several cooking holes, the heat must be directed to them, otherwise there is excessive wastage. This function is performed by a flue. This flue must be so designed that it concentrates the flames and fumes at the cooking holes before they are carried out of the stove. The fumes are directed by the flue so that they are most intense at these places; in this way, an effective use of the heat is ensured.

FIGURE 5. - Modified FAO stove. Higher type. 633/T. Scale 1: 10. All dimensions for figures 5-15 inclusive are in centimeters, unless otherwise stated.

The chimney

The chimney must serve several purposes. To some extent it constitutes the motive power of the combustion process. Its most obvious function is to lead off the fumes to the outside air; but it must also set up the draught which causes the intake of air needed for combustion. Only a good chimney can ensure efficient and economic functioning. The draught is caused by the difference in temperature between the outside air and the fumes, which in turn gives rise to a difference in weight. However, as this motive power is weak, certain factors have to be taken into consideration in the construction of chimneys. The size of the cross section will depend on the size of the hearth and the height of the chimney. For kitchen stoves with chimneys up to 4 meters in height it is preferable to use a pipe of hard baked clay with a 12 - centimeter hole. For even higher chimneys a 10- centimeter hole is sufficient. The minimum height must be 2.5 meters.

As the chimney must be airtight, it should be assembled with care, the joints being mortared.

A cover, set in such a way that the outcoming smoke is not hindered, must be provided to prevent rain from penetrating into the chimney. The chimney should also he cleaned every six months. The simplest procedure is to do the from the top end; a suitably sized brush is fixed on the end of a bamboo stick and then pushed up and down in the chimney several times.

Dry wood should be used in stoves not only because it is more economical, but also because it is safer. When green wood is burnt, a tarry deposit forms on the inside of the chimney. This may catch alight in the stove when a strong fire is burning. To put out a fire in the chimney, one must immediately prevent the air from entering it. Any chimney bearing signs of tarring up should be replaced before it catches fire.

The chimney can also be built of brick, the joints still being made airtight. The cross section can measure, 15 x 15 centimeters or 20 x 20 centimeters. A chimney of this type is undoubtedly more costly than a clay one; on the other hand, it lasts longer, is safer, and therefore more satisfactory.

FIGURE 6. - FAO/Singer stove 1. Higher type. 633/T.

FIGURE 7. - Modified FAO wove. Type 1. 634/T. Scale 1: 10.

Materials and construction of the new stoves

The traditional type of stove lasts barely a year. It is for this reason that special attention has been given to the materials and a number of tests have been carried out to ensure that the new models have a longer life. As the basic material, fired clay, dried or moist, was chosen. Sawdust, rice husks, rice straw or ashes can be mixed in to impart greater strength to the clay. Three ways of construction were selected: with burnt bricks, sun-dried bricks or clay.

Stoves of burnt brick proved to be the best and most durable. The bricks are 22 centimeters long, 10 centimeters wide, and 5 centimeters high. They are made 50 percent of clay and 50 percent of sawdust, the whole being thoroughly mixed, then placed in wood molds of the correct size and finally oven-baked. The resulting bricks are easy to work with and fire-resistant. When laying them, it is essential that they should he moistened to make sure that the mortar takes. For mortar, a mix one part cement to four parts sand (ground from the bricks) is used. Clay, even when other materials are mixed in with it, is not suitable as mortar, because it contracts too much from the heat, thus causing the joints to crack and the bricks to become loose.

The most difficult part to make, however, is the plate with its three cooking holes and one chimney hole. The simplest solution is to make three plates for each oven. They have the same composition as the bricks and are formed in a mold. Before they are fired, they should be allowed to dry out somewhat until they are solid enough for the cooking hole to be cut. Figure 2 shows the mold and gives the measurements of one of these plates. The plate can also be in brick, though this is a little more difficult to make. The bricks must be well laid and mortared. A round wooden forming-block (20 centimeters in diameter) should be used to make sure that the holes are perfectly shaped. Quite apart from their durability, stoves made of brick have the added advantage that only three or four days must be allowed for drying out, before they can be used.

The second method is to employ only sun-dried bricks. These bricks, although made in exactly the same way, are allowed to dry in the sun. The stove is assembled as in the first method. The mortar, however, is clay mixed with 40 percent sawdust and 10 percent ashes. The mold and dimensions of the bricks will be found in Figure 3. It is recommended that the mold should be sprinkled with dry ashes before, the mix is tipped in.

FIGURE 8. - FAO/Singer stove I. Lower type. 634/T.

FIGURE 9. - Modified FAO stove. Type. 635/T. Scale 1: 10.

FIGURE 10. - FAO/Singer stove II. Higher type. 635/T.

FIGURE 11. - Modified FAO stove. Type II. 636/T. Scale 1:10.

FIGURE 12. - FAO/Singer stove H. Lower type. 636/T.

The advantage of such a method is that more often than not the materials will not have to be bought. On the other hand, the stove will not last as long as one built with burnt brick. Furthermore, because drying out takes longer, more time must be allowed to elapse before the stove can be used.

A very simple method, which employs clay, is the third. It is, however, also the most unsatisfactory in its operation and the shortest lived. Moreover, it is a method which can be employed only for low stoves. Therefore it should only be used in the absence of burnt or sun-dried bricks. The basic material used is clay, to which rice straw and ashes have been added. A mix of clay and sawdust should on no account be used, for these materials contract differently in drying out, so that cracks would appear. Not more than 15 percent of water should be added to ensure that the clay is malleable enough for the mold.

First one constructs a base which will have the same ground area as the floor of the stove. To make the hearth space and the flue holes, forming-blocks should be used, such as the stems of banana trees (Figure 1).

The blocks should not be removed until the clay is sufficiently dry. The inside of the stove is then shaped in the form shown in the diagram.

Whatever method is chosen, the chimney must be made from fire clay or burnt bricks. In Figure 4 the construction and layout of the chimney together with the way of passing it through the roof are all illustrated.

FIGURE 13. - Modified FAO stove. Type, R.H.C. 639/T. Scale 1: 10.

Functioning of the stoves

Among the new models are included two with three holes and one with two holes; an improved model with one hole was also constructed. How they function and their exact measurements are shown in Figures 5-15.
Models 1 and 2 are fundamentally similar to stoves already designed in Switzerland. All that had to be done was to change the dimensions so as to adapt them completely to local conditions. These two models have a good hearth, closed at the front by a flat stone with a hole in it to act as an air-intake, and at the top by the cooking pots. The flue for the flames and fumes forms a continuation of the hearth. The flue narrows where it connects with the chimney. In the first model it is bent, and in the second it is straight; this is the only difference. Both models may be made high (ideal height 70 centimeters) or low (height 30 centimeters). Because of its construction and cost, model 2 is the most advantageous.

FIGURE 14. - FAO/Singer stove III.

After examining the existing models and the culinary habits of the people, it seemed desirable to design yet a third model with two holes, for the benefit of small families. Its characteristics are the same as those of the other two models: a closed hearth, a flue and a chimney. The hearth is built in line with the cooking holes; but it can also be placed at right angles to them as in the other two models. AS for the stoves with one hole (formed quite simply of three stones), they cannot be basically changed. One must be content with improving what is already there. In principle their construction is the same, but bricks are used. The front is closed in above the stokehole, which means that the heat is well concentrated on the pot. The model is illustrated in Figure 15.

Testing the stoves

The tests were intended to determine the cooking efficiency and the technical output coefficient as well as the burning properties of certain woods.

They were carried out in various ways; but an effort was made to run parallel series of tests so as to be able to compare the results obtained from the different stoves. All three models were tested as well as a one hole model. For the sake of comparison several one-hole models, burning wood and sawdust, and already on the market, were bought and tested. The models in question were of fire clay. The results were fairly satisfactory, but the material used in their manufacture failed to meet requirements; these tests were therefore abandoned.

FIGURE 15. - FAO one-hole type stove.

Several kinds of wood were tested. The wood consumed was weighed exactly, and for each test 2 kilos of logs and 100 grams of kindling wood were used. Test samples of each kind were taken and the calorific power measured in the laboratory.

In every case the tests began with a cold stove. To determine the proportion of the heat available for cooking, a pot, 20 centimeters in diameter and containing 2 liters of water, was placed over each hole and the amount of heat absorbed was measured. The results obtained were as follows:




23.4 to 28.2


26.6 to 28.7


20.6 to 23.5


10 to 13

To find out how each stove burned, a sample of gas was taken at the outlet every five minutes. The carbonic acid, carbon monoxide and hydrogen content was measured by means of the Orsat apparatus. On each occasion and simultaneously, a measurement was made of the heat of the fumes and of the amount of draught in the chimney. The technical output is given in the following list:




81.1 to 83.1


77 to 80.9


72.3 to 7 7.2

As well as these full-scale tests, simple output tests were made with each stove to discover its cooking efficiency (the data are contained in FAO/ETAP Report No. 1315). From the results obtained, models 1 and 2 may be said to be more or less equal. But model 2 offers certain advantages: a more regular distribution of the heat over the three cooking holes; shorter cooking time; simpler construction and lower price - as has already been remarked. For these reasons model 2 is recommended as the best.

The results from model 3 were, inevitably, not so good. Nonetheless, for small families this model represents a great improvement on those at present in use.

FIGURE 16. - Use and maintenance of the FAO/Singer stove.

(1) It is economical on fuelwood, shortens cooking time, reduces the risk of fire, and keeps kitchen tree from smoke.

(2) To clean, first close all openings and remove ashes from the stove.

(3) To lay the fire, first make a base of wood wool or paper with small pieces of wood, putting larger wood, chopped to size, on top.

(4) To increase the heat, close the fire-hole, with its door; to lower the, heat, halt close the hole in the door; to extinguish the fire, close the door completely.

(5) The pan should fit the stove pit closely. The base of the cooking utensil should be 3 to 5 centimeters below the surface of the pit.

(6) Regulating the stove Pit. An unused pit should always be covered. For pans smaller than the pit, rings/plate-washers can be used.

(7) The stove should be cleaned regularly and all damaged places patched immediately.

(8) Maintenance of the chimney. Remove the chimney cap, sweep it clean, remove all the soot. The chimney should be cleaned at least once, every six months.

WARNING. The, use of wet or damp wood causes damage to the chimney (accumulation of pitch, leading to risk of fire).

- Pitch-contaminated chimneys should be replaced by new ones.
- In case of fire, the smoke passage leading to the chimney should be blocked.

In short it can be said that there are now stoves available which, as well as being easy to make, use the fuel to the best advantage. In addition, the risk of fire has been considerably reduced; the smoke fumes no longer cause annoyance and cooking is noticeably more hygienic.

Cooking tests

In addition to the trials which were described above, practical cooking tests were also made. Women from the home economics school at Bandung volunteered to help with the tests under the guidance of the Pasar Minggu Rome Economics Institute, to ensure that the food would be prepared and cooked exactly in the Javanese manner. Both the traditional type of cooking pot and modern pots were used. Tests were carried out, using the following four types of stoves:

Traditional stove (3 holes) and pots
Model 2 (high) with modem pots
Model 2 (low) with traditional pots
Model 3 (high) with modern pots

Model 2 (high)

Model 2 (low)

Model 3 (high)

Traditional Model

New type pots

Old type pots

New type pots

Old type pots

Cooking time min.

Wood kg.

Cooking time min.

Wood kg.

Cooking time min.

Wood kg.

Cooking time min.

Wood kg.

























The same food was prepared on all stoves. This represents the normal menu of an Indonesian family of six:



tomato sauce

baked beans


Midday and evening meals


vegetables in sauce

baked beans and salt fish


vegetable soup


Altogether, 2,500 grams of rice, 1,680 grams of vegetables, 140 grams of small fish, 140 grams of oil and 12 bananas, costing in all 34 rupees, were used. Above are the cooking times and quantities of wood used for each stove.

These practical tests therefore demonstrate that there is a worth-while saving in both time and wood with the new stoves. It is important that pots which go down 3 or 5 centimeters into the stove should be used. The use and maintenance of the stoves is illustrated in Figures 16 and 17.

The Director-General of FAO, Dr. B. R. Sen, was received in a personal audience by Pope John XXIII on 28 July 1961. Dr. Sen expressed thanks for a reference to the good work of FAO made in the Pope's Encyclical "Mater et Magistra." Also pictured is (right) Msgr. Ligutti, Observer of the Holy See to FAO.

Courtesy, Pontificia Fotografia Felici, Roma

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