The present invention relates to a fireplace, comprising: a grate, a housing at least partially enclosing the grate, and an outlet for flue gases connecting via at least one flap to the housing. The invention also relates to a method for operating such a fireplace.

The use of fireplaces for heating purposes has been known since antiquity. In the present-day application of fireplaces for heating purposes it is possible to envisage gas- fired fireplaces and solid fuel devices (such as for instance wood-fired fireplaces) of very diverse types. Fireplaces include, non limitatively: open hearths, gas-permeable shielded fireplaces, fireplaces closed with glass, stove heaters and so forth. A drawback of existing fireplaces is that they can have an environmental impact in densely populated areas because of the contamination of the combustion gases with dust particles. Furthermore, the energy efficiency of fireplaces is generally low (open fireplaces normally have an efficiency in the order of magnitude of -5 to 15% and built- in/insert fireplaces and freestanding stoves have an efficiency in the order of magnitude of 70 - 85%). There is therefore a trend toward regulation which only allows fireplaces which comply with determined efficiency standards and emission values.

The present invention has for its object to provide a fireplace, and to provide a method for operating such a fireplace, with which the efficiency of a fireplace and the emission of dust from a fireplace can be reduced in an efficient manner.

The present invention provides for this purpose a fireplace of the type stated in the preamble wherein the housing is also provided with an additional flue gas passage running from a side of the flap located at the grate to a side of the flap located at the outlet for flue gases, and wherein the fireplace is provided with a filter element lying between the grate and the flap. Such an additional flue gas passage thus forms a so- called bypass for the direct discharge of the flue gases from the housing to the outlet. It thus becomes possible, depending on the conditions of the fireplace, to influence the route covered by the flue gases. Also important here is that the flue gases must also always pass through the filter element irrespective of the position of the flap, this evidently being advantageous in respect of emission reduction of the fireplace. A further advantage of the filter is that the flap is shielded from the grate by the filter element, whereby the heat load on the flap remains limited, with all the advantages this entails (less wear, better operation, less contamination of flap and so improved closure and so forth). The operation of the fireplace depends on, among other factors, the temperature of the fireplace. Important for the present invention is the insight that, immediately following lighting of a fireplace which has cooled to ambient temperature, this fireplace will display a flow behaviour other than after the fireplace has been burning for some time and, together with a chimney, has been heated to a much higher temperature. Flaps are commonly known with which an outlet for flue gases can be closed, but these are intended to prevent the fireplace functioning as undesirable ventilation when it is not burning. The special feature of the flap according to the present invention is precisely that, wholly contrary to the existing manner of flap regulation of fireplaces, it can be closed while the fireplace is burning well, and that it is simultaneously protected from too great a (heat) load. The additional flue gas outlet then begins to function as a so- called bypass, whereby the flue gases must cover a longer path. Only when the flow behaviour of the flue gases allows this (when the fireplace "draws" sufficiently because it has been burning for some time at a determined level) is it possible to now opt for discharge of the flue gases, in addition to passage through the filter element, through the additional flue gas outlet, since this path does after all produce more resistance than the path directly from the grate to the outlet. The advantage of allowing flue gases to escape via the additional flue gas outlet is that, due to the longer path length and also an intensive contact with the walls of the additional flue gas outlet, more heat can be generated to the fireplace (or at least the housing thereof)- The efficiency of the fireplace can thus be increased by now streaming the fireplace on the outside with for instance air or by discharging the heat efficiently from the housing in other manner; the flue gases will after all generate more heat. It is also noted that the fireplace can be adapted as desired to burn solid fuel and/or to burn gases.

The filter element can advantageously be manufactured from ceramic foam, although other filter materials can also be envisaged. Favourable results can be expected with a filter at least partially manufactured from a metal foam. It is also advantageous if a side of the filter element is directed toward the grate. For good operation it is further desirable that the additional flue gas passage connects on one side between the filter element and the flap and on the other side debouches behind the flap into the outlet for flue gases. For replacement and cleaning of the filter element it is simple if the filter element is mounted releasably in the fireplace. The advantage of the ceramic foam material is that this can function as a self-cleaning filter because it can be heated such that the captured soot particles burn in the filter. Heat can also be radiated back to the grate and/or outside the grate with the ceramic foam material, this having an advantageous effect on the emission of the fireplace as well as on the efficiency with which the fireplace can be operated. It is also noted that the ceramic foam material can be provided with a catalyst so as to have a still more effective cleaning action.

In a preferred variant of the fireplace the additional flue gas passage comprises a chamber with a flow surface area which is greater than the flow surface area of the parts of the additional flue gas passage which connect to the chamber. As the flue gases flow through the additional flue gas passage the flow speed of the flue gases will decrease upon entering the chamber, with the result that as a consequence of the mass inertia flue gas particles will be separated out of the flue gases. It is particularly advantageous if the flow direction of the flue gases simultaneously also undergoes a change of direction in the chamber; in addition to the change in speed, centripetal forces can then also contribute toward separation of flue gases and particles (such as particularly dust particles and fine dust particles).

For compact installation of the additional flue gas passage, at least two successive flue gas passage segments running parallel to each other are desirably provided. In this way • the additional flue gas passage can for instance be wholly incorporated in the housing at least partially enclosing the grate.

In yet another preferred variant the additional flue gas passage takes a dual form. An even greater heat-exchanging surface can thus be created over which the flue gases can generate heat. Another advantage is that a symmetrical discharge of flue gases thus becomes possible, even after the flap has been closed; this contributes toward a good combustion in the fireplace. In a favourable variant of the fireplace the additional dual flue gas passages are therefore incorporated in two opposite sides of the housing at least partially enclosing the grate.

In order to enable cleaning of the additional flue gas passage and - if present - particularly the chamber in which contamination collects, the additional flue gas passage is desirably provided with at least one releasable side for the purpose of making the additional flue gas passage externally accessible.

The flap is preferably displaceable between an opened position, in which the grate connects directly to the outlet via the filter element, and a closed position in which the grate connects to the outlet only via the filter element and the additional flue gas passage. The flap can be provided for this purpose with a manual control (for instance a handle), but it is also possible to provide the flap with an electronic drive. Such an electronic drive can connect to an intelligent control which, using a temperature- sensitive sensor connecting to the intelligent control, can determine the moment at which the position of the flap can be changed.

The invention also provides a method for operating a fireplace according to any of the foregoing claims, comprising the processing steps of: A) lighting the fireplace, which is not yet at operating temperature, in a situation where the flap leaves clear the direct connection, via the filter element, between the grate and the outlet for flue gases, B) heating the grate, the housing at least partially enclosing the grate and the outlet to a determined minimum temperature level by burning in the fireplace, and C) closing the flap between the grate and the outlet for flue gases, whereby the flue gases are forced to exit, via the filter element, through the additional flue gas passage. The possibility can also be envisaged of the flap being placed as required in one or more intermediate positions in order to carry a part or all of flue gases coming from the fireplace through the additional flue gas passage. When at least a part of the flue gases flow through the additional flue gas passage, it is advantageous if the flow speed of the flue gases, after flowing through a first flue gas passage segment, decreases in a chamber forming part of the flue gas passage and, after leaving the chamber, increases again in a second flue gas passage segment. Contaminating solid particles can thus be captured. In order to prevent accumulation of excessive contamination it is desirable to clean, at intervals, the chamber through which the flue gases flow at relatively low flow speed. When the fireplace cools to a determined value, for instance after the fireplace has been extinguished, it is desirable to open the flap between the grate and the outlet for flue gases. The flap can then only be closed again once there is no further generation of flue gas. For further discussion of advantages of the method according to the present invention, reference is made to the above discussed advantages of the fireplace according to the invention.

The invention will be further elucidated on the basis of the non-limitative exemplary embodiments shown in the following figures. Herein: figure 1 shows a section through a fireplace according to the present invention; and figure 2 is a detail sectional view of a part of the fireplace shown in figure 1 in a situation where the flue gases can be discharged with limited resistance.

Figure 1 shows a fireplace 1 with a grate 2 which is partly enclosed by a housing 3. Connecting to housing 3 is an outlet 4 through which the combustion gases can escape. Situated between grate 2 and outlet 4 is a plate 5 with openings, these openings being covered by ceramic foam material 6. The ceramic foam material 6 functions both as flame baffle plate, filter (of, among other materials, soot) and as radiation panel. The combustion gases will flow from grate 2 through ceramic foam material 6, during which passage some of the dust particles will remain behind in ceramic foam material 6. The ceramic foam material 6 is here also heated, and part of this heat will be irradiated downward, whereby soot captured in the ceramic foam material 6 will be wholly or partially combusted. The feed of combustion air takes place via feed openings 7.

After the combustion gases have flowed as according to arrows Pi through the ceramic foam material 6, they are urged sideways by a closed outlet flap 8, this being indicated by means of arrows P ₂ . The combustion gases then flow into a first indraught 9 of two additional flue gas passages 10. The combustion gases here flow partially downward into deceleration chambers 11 where the flow speed of the combustion gases will decrease relative to the flow speed in first indraught 9. The flow of combustion gases will also change direction as according to arrows P ₃ . The decrease in the flow speed and the reversal of the flow direction (P ₃ ) will result in the particles in the combustion gases which have been able to pass through ceramic foam material 6 being (partially) separated from the flue gases and remaining behind in deceleration chambers 11. The flue gases will then flow upward out of deceleration chambers 11 via a second indraught 12 (as according to arrows P ₄ ) and leave fireplace 1 via outlet 4 (see arrow P ₅ )- The additional flue gas passages 10 are protected on the side directed toward grate 2 by insulation panels 13 (manufactured for instance from vermiculite), while on the side remote from grate 2 the additional flue gas passages 10 can relinquish heat to the vicinity such that the efficiency of fireplace 1 hereby increases considerably relative to a fireplace without additional flue gas passages 10.

A drawback of the operation of fireplace 1 as described with reference to figure 1 is that the additional flue gas passages 10 obstruct the discharge of flue gases because they result in an increased air resistance. As soon as the fireplace is burning well and has heated up, for instance several minutes after fireplace 1 has been lit, this need no longer represent a problem; fireplace 1 then draws so well that the flue gases will flow without difficulty through the additional flue gas passages 10.

However, as long as fireplace 1 is not yet burning well and is not yet at operating temperature, flue gases will not yet flow unforced through the additional flue gas passages 10. During lighting of fireplace 1 it will therefore be necessary to opt for full or partial opening of flap 8. This is shown in figure 2 in the detail view of part of fireplace 1.

In figure 2 the flap is partially opened, whereby the flue gases flowing as according to arrows Pi through ceramic foam material 6 now flow, without having to flow through the additional flue gas passages 10, directly as according to arrows P _δ to outlet 4 and there leave fireplace 1 (see arrow P ₅ ). As soon as the fireplace is burning sufficiently well and therefore draws sufficiently, flap 8 can be closed in order to force the flue gases to flow through the additional flue gas passages 10.

Figure 2 also shows in more detail that first indraughts 9 of the additional flue gas passages 10 have releasable wall parts 15, whereby the additional flue gas passages 10 can be cleaned. Also shown is that ceramic foam material 6 is suspended in a releasable bracket 16, whereby ceramic foam material 6 can be replaced.