It is known that wood-burning stoves draw the combustion air through the room in which they stand and thus help to cool the room down, thereby reducing the total efficiency of the heating. It is also known that the efficiency of these stoves is further reduced because the combustion air is cold and a relative large heat loss takes place through the flue gases.

To counter the cooling effect on the room, it is known to take the combustion air through a duct that is separated from the inside air. This can be done through a vertical duct, in connection with the chimney, or through a separate, horizontal duct. When the combustion air is supplied in this way, it is generally done in heat exchange with an adjacent flue gas duct so that the combustion air is preheated, and at the same time the heat loss through the flue gases is reduced. For this process, it is known to use mechanical circulation. If the supply of combustion air takes place without any heat exchange with the flue or the flue gases, it is seldom necessary to use mechanical ventilation.

The object of the present invention is to permit a vertical heat exchange of combustion air and flue gases, without mechanical ventilation, in order to make use of the temperature of the flue gases. According to the invention, this is achieved because the said device for the admission of outside air has its inlet facing downwards and this inlet is located above the position for the heat exchange.

The standard, relatively primitive ventilation of a fuel-burning stove, where the combustion air is taken into the stove from below and the flue gases pass out and up through a chimney, is based on the draught effect that is produced. When the heating starts, the volume of the air increases, its specific weight decreases correspondingly, and it is displaced into the chimney due to the effect of the heavier, cold supply air. The chimney is gradually heated and the flue gases thus maintain their temperature until they reach the outlet above the roof. There, the flue gases will be lighter than the ambient air and will be displaced some distance by this air until they reach ambient air

temperature. This draught process increases and decreases at the same rate as the combustion process.

When there is a vertical supply of combustion air, heat-insulated relative to the flue gases, the process is in principle the same as that described above. That the supply of air takes place from the roof, does not change the fact that the heating does not take place until the air enters the fireplace of the stove. The conditions as regards draught would be the same as those mentioned above.

When there is a vertical supply of combustion air which passes in heat exchange with the flue gases, with inlet openings for the combustion air facing upwards, the draught conditions, at the start when the air is cold, will be the same as those described above.

However, as the inlet duct becomes warm, the volume of the combustion air will increase and its specific weight will decrease. Therefore, the effect will be almost the same as in the flue gas duct, as the cold, heavy outside air will slow down the inlet velocity and circulation will only be possible through intense firing, which increases the temperature inside the heater substantially. The poor circulation will result in inefficient combustion. After the loading of fuel, when the combustion air has the same temperature as the flue gases, it will be extremely difficult to get the process going again.

This is where the invention comes in. As the temperature increases in the heater, the flue gases will rise as before. This happens at the same time as heat is given off to the combustion air through the admission device or inlet pipe. This heating causes an expansion of the air and a weight reduction. The heavy air in the downward facing inlet pipe, due to the outside temperature and atmospheric pressure, will prevent the return of air and will ensure that all expansion takes place downwards, i. e. , inside the combustion chamber. At the same time, the weight reduction will cause new, cold air to fall down continuously and maintain a positive pressure relative to the stove and smoke flue. This ensures an exceptionally good and even air supply to the combustion process, without a fan.

Additional features of the invention are disclosed in the dependent patent claims.

Advantages and features of the invention will be explained in more detail in the following description of the invention, with reference to the drawings, wherein:

Figure 1 is a purely schematic illustration of the inventive idea; Figure 2 shows a favourable embodiment of a heater in which the invention is used; Figure 3 is a schematic section taken along the line III-III is Figure 2; and Figure 4 is an enlarged section of the heater in Figure 3.

Figure 1 shows a heater comprising a combustion chamber 1 with a fuel-supporting grate 2, a vertical flue 3 with an inlet 4 and an outlet 5, and a device for vertical natural admission of outside air to the combustion chamber 1, which device includes a supply air duct 6 with an inlet 7 and an outlet 8, inside the combustion chamber 1, below the grate 2. The combustion chamber 1 has a non-illustrated fuel loading opening and is above the heat exchange zone in the flue 3.

The supply air duct 6 inlet 7 faces downwards, as shown in Figure 1. That means to say that at the top of the supply duct 6 a bend is formed in which the heavy, cold outside air will form a barrier to the rising, heated supply air. The supply air in the supply air duct 6 is heated by the rising flue gases in the flue 3. The result is that the supply air will not be able to move in the upward direction and pass out through the inlet 7, but will be forced to move down through the duct 6 and out through the outlet 8 into the combustion chamber 1. In other words, all air expansion will take place downwards, inside the combustion chamber. New, cold air will fall down continuously and maintain a positive pressure relative to the combustion chamber and flue. This ensures an exceptionally good and even air supply to the combustion process, without a fan. To prevent the air passing back through the door opening (opening of the non-illustrated loading opening), it is advantageous to install a non-return valve (not shown) in the inlet pipe 6.

The advantages obtained by means of the invention can be summarised in the following way, which should not be understood as limiting: The system works without a fan and can in principle be used anywhere and in all types of stoves, both new and old.

The supply of combustion air is almost proportional to requirement.

The degree of heating does not have a negative effect on the circulation process.

The circulation takes place under a relative negative pressure, so that there is no danger of emissions of hazardous gases into the room through leakage.

The combustion air does not take air from the room in which the stove is located.

Elevated combustion temperature gives a better draught.

The system can easily be installed in existing stoves.

The draught will pick up as soon as fuel has been loaded.

A vast improvement of the fire-lighting process is obtained.

Heat production can take place by simple draught adjustment.

As the process takes place with a constant relative negative pressure between inlet and outlet, i. e. , between the air supply and the flue, the flue can be placed outermost, without any danger of leakage into the room, when there is a need to do so. This facilitates simple installation in existing heating appliances.

The process can take up a maximum amount of exhaust heat, whilst the final preheating can take place inside the stove. This will result in optimal efficiency of the combustion process.

Figures 2 and 3 show heater that is highly suitable as a so-called"all-night burner".

The stove 9 has a preheating chamber 10 with a grate 11 for supporting wood 12.

Under the grate 11 there is an ash pit 13. In this case, the combustion air is supplied though a duct 14. The duct 14 opens into the preheating chamber 10 at the top, as shown in Figure 3, and combustion takes place at the bottom, in the primary combustion chamber 15 around the grate 11. The flue gases pass out in ducts or secondary combustion chambers 15a which extend upwards along the walls of the combustion chamber 10, in which suitable openings 19 are made for combustion air, so that this may also enter the secondary combustion chambers 15a and from there pass out into the chimney 17. As in the embodiment in Figure 1, the combustion air supply duct 14 is run down through the chimney 17 and opens into the combustion chamber at 23. In this case, the duct 14 inlet 18 also faces downwards and is located above the heat exchange zone in the chimney 17.

Thus, the stove 9 receives a supply of preheated air from the top. Further heating takes place along the sides of the preheating chamber, as the air falls into the chamber. The air will therefore have a very high temperature when the fire is lit in the bottom, which also gives very good combustion. Because of the downward air flow in the preheating chamber 10, the flames will go downwards. A vertical spreading of the combustion will therefore take place at a limited rate on down-adjustment of the combustion air, which

is especially advantageous at night. Figure 3 shows extra passages 19 for combustion air into the ducts or secondary combustion chambers 15a, so that unburned gas can be burned there. As the wood 12 lies on the grate 11, which may preferably be in the form of a coarse steel mesh, the ash will fall down into the tray in the ash pit 13 as the ash is formed, and therefore the ash will not hamper the combustion process. New wood will fall down as the old is burned.

The door 20, which closes the loading opening, is connected in a manner not shown in any detail to an escape valve 21 between the preheating chamber 10 and the flue 16.

When the door 20 is opened, this escape valve 21 is also opened, thereby preventing smoke from entering the room.

A non-return valve 22 and a draught regulator 23 are indicated in the supply duct 14.

The non-return valve 22 will prevent the air from passing back at the door opening.

The process in the heater in Figures 2 and 3 is further clarified in Figure 4: Hot air enters at the top and spreads out in the preheating chamber. However, not much will pass through the tightly packed wood 12, but rather at the sides facing the secondary combustion chambers 15a, where space has been made therefor by means of flat vertical spacers. Here, the air will be heated intensely before it passes into the fire at the bottom of the stacked wood 12. As the wood is also preheated, combustion will take place at a high temperature. However, experience has shown that there are flue gases that are still not lit because of a shortage of air. To ensure these gases are burned, small holes 19 are made in the side leading into the secondary combustion chambers 15a which will thus support the combustion of the gases that enter unburned. As mentioned, very little air will pass through the wood, but rather at the side and is supplied in the bottom. It is therefore not expected that the combustion will develop upwards, but will remain close to the bottom surface.