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Wood burning

Director, Swiss Central Forestry Office, Solothurn, Switzerland

Wood still remains a major source of fuel for domestic heating and cooking. For the most part it is wastefully burned although it is comparatively simple to design equipment adapted to the specific combustion properties of wood, so as to get the most out of its calorific value. This has been done for other solid fuels as their relative scarcity and expense provide the incentive, and there is no technical reason why the same should not be done for wood. The author has prepared an information paper for FAO on the techniques of wood burning, and this will in due course be made available for distribution. A brief extract from this bulletin is paraphrased below.

THE hearth was always the family gathering place, and the open hearth fire, the oldest heating system, has survived to the present day, right into this technical age. The open fireplace is still a normal fixture in town houses and even apartment buildings in many countries, despite the installation of modern central heating systems, and in rural areas and in tropical countries having cold winters it is still often the only method of heating. Its value today in most urban centers lies rather in the note of intimacy it gives to the home rather than to its efficiency as a heating unit.

An advance on the open fire was to enclose the flame completely, the result being the stove or furnace. This was originally a thick walled structure made of clay and stones, with an opening through which the smoke reached the exit of the room. When such stoves gradually came into general use, the cost of operating them led to their improvement. They were then made with plates of stone or earthenware tiles, set on supports and fitted with a closing door. Next there was added an enclosed flue emerging through the roof. The addition of the chimney was a later stage when principles of combustion were better understood. Eventually there came the large modern stove with its long flues and smoke pipes.

The primitive stove had no grate. In short, the practice was to burn a pile of billets in an empty space enclosed by flat stones. Because of the large excess of air, combustion was poor, the temperature of combustion relatively low and it was practically impossible to regulate heat output. Attempts were made to replace the grateless stove by one with a wrought iron base or some other such apparatus, but without much success. Yet, because of the simplicity of wood combustion, quite good results can be obtained by improving the design of the fuel chamber, the flues and the air intake. Besides, a grateless stove is always better than one with a poor grate that is too large, badly shaped or creates too strong a draft. In many places it is still thought better to burn wood in a grateless stove, but this idea is wrong. The efficiency of a grateless stove is always inferior to that of one with a good grate. The problem, however, is to find the right dimensions for the grate and to regulate the air intake accordingly. In other words, it is a question of constructing to suit the specific properties of wood.

For some uses, the grateless stove is still justified today, for example, in ovens for baking bread and in glazed tile stoves. Such stoves made to modern designs are known to have good combustion, insignificant waste and a high capacity for heat accumulation.

Features of modern stoves

To obtain the best possible results from any stove, it is essential to suit its construction to the particular properties of the fuel which is to be burned. This principle holds true especially for wood whose properties differ markedly from those of coke, coal and other solid fuels. Unfortunately, too little importance is usually attached to the peculiarities of wood, which ignites and burns so easily - a pile of billets on the ground and in a moment a fire is glowing - that no special adaptations are considered necessary.

The purpose of heating technology is to devise systems by which the most complete combustion can occur, allowing regulation of the rate of heat emission and its conduction wherever desired.

The correct way to burn wood may be deduced from its particular physical and chemical properties, which are not dealt with here. It must be sufficient to say that three factors are of cardinal importance: the dimensions of the grate, the design of the fuel chamber, and the dividing of the air intake (primary air and secondary air). No set rules can be laid down in regard to dimensions. Each apparatus has to be designed for the particular use to which it will be put, but in a general way all wood stoves should have a fuel chamber, a grate and air intakes.

Fuel chamber

The purpose of the fuel chamber is not only to hold the necessary quantity of fuel but also to allow sufficient space for the flames to burn freely. It must therefore be large enough so that the gases will have time to burn completely before coming into contact with the heating surfaces which will cool them. The junction of the fuel chamber with the narrower grate is effected by sloping lateral walls which make the lower part funnel shaped. With this arrangement, wood charcoal accumulates on the grate and covers it completely and in a sufficiently thick layer. The fuel chamber must be slightly deeper than the billets are long.

The fire tubes and smoke flues which lead off from the fuel chamber must be placed in such a way that the heating surfaces are well exposed to the gases arising from combustion. It is a question of finding the proper solution from the standpoint of draft technology. The dimensions of the flues must be proportional to the gas content of the fuel and to the total heating surface. With wood, the flues must be lengthened by reducing their cross section near the outlet in proportion to the decrease in volume of the gases as they cool off.


The grate must be relatively small, in all cases smaller than the bottom of the fuel chamber so that the wood charcoal which forms during combustion attains a sufficient depth to cover the grate completely. The quantity of heat produced per square meter of grate must normally be as follows1:

1The "kcal" (kilogram calorie) of the metric system is equal to 3.968 British Thermal Units (B.T.U.)


90,000-200,000 kcal/m2/h

Kitchen ranges

200,000-250,000 kcal/m2/h


220,000-250,000 kcal/m2/h

Such heating obviously implies a draft in proportion. If the draft is too weak owing, for instance, to a very low or badly made flue, the admissible charge must be less. If the grate is too big, the temperature of the gases escaping through the flue is too high, which entails heavy losses in heat. On the other hand, too small a grate easily causes draft trouble and consequently incomplete combustion with the risk of tar deposit in the flue.

For a wood stove, the grate must be so made that the ratio of the interstices or slits to the total area is between 0.25 and 0.35. This ratio is much smaller than that for coke grates, because of the relatively low air consumption of the wood fire, and because the air intake must be arranged at two different levels.

Preference should be given to a flat grate. It should be made in such a way that the ash can be easily removed and the air have access at all points. For large stoves, the grate bars should be indented or grooved on the lower surface to increase their cooling capacity.

The grate should be as deep as the billets used.

Air intake

In a wood stove the combustive air currents must be introduced at two levels:

primary - under the grate
secondary - in the flame

The primary air current reaches the wood through slits in the grate. Since it is only a part of the total quantity needed for combustion, gases are given off more slowly and combustion can be regulated more easily. Primary air, however, must be introduced in sufficient amount because, if the production of gases and consequently development of heat are kept too much in check, combustion will be incomplete.

Secondary air must be preheated, then released evenly into the combustion gases above the layer of fuel so as to ignite the as yet unburnt gases. In small household stoves, secondary air, like primary air, can be drawn in through the ash box. The flues in which it is heated convey it into the fuel chamber where it emerges through regularly spaced openings. In large stoves, on the contrary, primary and secondary air are introduced and regulated separately.

There should be a specific quantitative ratio between these two air intakes. The total area of secondary air openings is one-quarter to one-third of the area of the grate slits through which primary air penetrates.


A modern heating system must not only ensure complete combustion without condensation and as high as possible a heating efficiency, but must also meet standards of convenience and cleanliness.

Left to Right. The diagrams illustrate three methods of combustion: ascending, horizontal and descending or reversed. P signifies primary air; S. secondary air.

To meet these requirements there is a choice of several combustion methods, depending on the kind and purpose of heating:

Ascending combustion

With this method, the simplest and oldest, the flame rises and the fire spreads throughout the layer of fuel so that the total load burns simultaneously. Hence it is not possible to obtain a continuously burning fire.

With the narrow grate and the sloping slides forming a funnel, a good layer of live charcoal is always obtained which ensures slow, easily regulated combustion. The live charcoal, closely piled, keeps burning for a long time and after a prolonged interval can still ignite a fresh charge of wood. The secondary air current which passes through the lateral flues emerges into the fuel chamber above the fuel, and mixes with the gases. The slanting sides of the specially made fire-proof stones used for the inlets and outlets for the secondary air give the funnel shape to the fuel chamber.

Horizontal combustion

The fuel chamber serves primarily as a container for the fuel while the flames mount in a second chamber. The openings between the fuel chamber and this second chamber must not be too large, so that the zone of combustion can be kept as low as possible. On the other hand, they must not be too small, otherwise there would be insufficient draft. The size of these openings must be exactly proportioned to the use conditions. The secondary air is introduced at the mouth of the fuel chamber, that is, at the narrowest part where the flames are compressed.

Primary and secondary air penetrate through the ash box door. They may enter through one and the same opening or through separate openings from the beginning. The feeding door, which should not have any rosette disc for air admission, must be airtight and sealed as closely as possible. In fact, if the air penetrates above the zone of combustion all the wood supply may ignite, which is contrary to the principle of this type of combustion.

During combustion, gases gather in the upper part of the fuel supply. If the charger door is opened before all the load has burned, the penetration of air produces a small explosion. To remedy this drawback, a bypass valve is fitted to the charger door and blocks it. Consequently, this door cannot be opened until the valve is released. In this way, explosions and the escape of smoke are avoided when the stove is recharged. In kindling, the bypass valve lets in a direct draft. It is closed when a bed of live charcoal has formed or when charging is finished.

Descending or reversed combustion

This system also ensures continuous combustion with emission of steady heat.

As the name indicates, the particular feature of this system is combustion directed downwards. Over the grate there is only the firebox with the fuel supply. The flames mount under the grate which is made of fireproof bars with narrow interstices. During combustion only the layer of fuel immediately on top of the grate burns. The flames pass through the interstices and expand in the chamber underneath where the smoke flues begin. The introduction of the combustive air is also reversed, that is, the primary air is introduced above the grate while the secondary air arrives underneath. This system guarantees good combustion because the gases pass first through the fireproof grate which is very hot. It follows that less secondary air is needed and any excess of primary air can serve as secondary air.

As in the preceding system, a bypass valve operating above the fuel supply will have to be provided. If continuous fire, that is, slow combustion, is desired, obviously the fire and ash-box doors will have to be airtight and the valve closed.

A stove of this type can only use fuel burning with a long flame, and is therefore well adapted to wood. Like an modern stoves which get a high heating capacity from the fuel used, descending combustion units require a chimney or flue in perfect condition to provide the proper draft.

Translated from an original French text.


The International Union for the Protection of Nature has, over the past few years, with the collaboration of national educational authorities and with financial aid from UNESCO published leaflets for use by teachers and children in elementary schools in many-parts of the world.

These local adaptations of a standard prototype lesson, and a leaflet in the form of a diploma bearing the name and address of each child, are now in use in countries as widely divergent as Mexico, Italy, Greece French Cameroons and Madagascar.

The so-galled diploma is of especial interest, in its appeal-to the child and by the reminder that it constitutes. Headed by a design incorporating a map of the territory on which the child can mark his home-town, and suitably framed by drawings portraying the national resources of his country, there follows space for the name and home address of the "graduate." Below this, the rest of the space is taken up with the text of a few golden rules governing the principles of conservation.

At the request of the International Boy Scouts Association, FAO is helping to write a handbook on conservation for use by the Boy Scouts Movement, and based also on the idea of a prototype which can be adapted to local conditions and needs throughout the world.

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