Background of the Invention The water jacket combustion chamber and heat exchanger assemblies disclosed in PCT/AU99/01106 whilst being an improvement over the then known prior art do have scope for improvement. The following is directed to some further improvements.

Summary of the Invention The present invention provides a water heater heat exchanger element being formed from first and second plates joined together to form at least one liquid inlet and at least one liquid outlet, at least one channel between said plates and interconnecting said at least one liquid inlet and said at least one outlet to provide at least one liquid flow path inside of said element and a combustion heat transfer surface on the outside thereof defining a combustion gas flow path that extends from a leading edge to a trailing edge of said element, said at least one liquid outlet being located remote from said leading edge, said element being further characterised by said at least one liquid flow path extending in said element to direct liquid flow from said trailing edge to said leading edge and then closely adjacent said leading edge, then to connect to said at least one outlet.

The at least one liquid flow path can extend across a portion of said element in a zig zag or sinusoidal configuration. Also the at least one liquid flow path can include multiple path ways from said trailing edge to said leading edge.

The plates can include grooves whereby when said plates are joined together, said at least one channel is formed.

Preferably said water heater heat exchange element includes a continuous peripheral path. The continuous peripheral path can contain and or guide combustion gases flowing near side peripheries of said elements when side by side and causes a substantial proportion of said combustion gases to exit said heat exchange elements at or near to said trailing edge. The continuous peripheral path can include at least one inwardly directed portion so as to position at least a part of said inwardly directed portion over said at least one outlet. The continuous peripheral path can include a plurality of inwardly directed portions so as to position at least a part of said inwardly directed portions over said at least one outlet, said plurality forming a zig zag or sinusoidal shaped portion.

The element can be formed from identical plates placed back to back. The heat exchanger and or plates forming said heat exchanger have a nestable shape.

The heat exchanger element can have a shape which includes a main portion and at least two arms extending away from the main portion. The two arms extend in the one of the following directions away from the main portion: parallel to each other; diverging away from each other; converging towards each other; or produces any one of the following shapes: as a Y shape, U shape, C shape, E shape H shape, V shape or any other appropriate shape.

The arms of the element when placed near to adjacent elements form a water jacket along at least two sides of a combustion chamber.

The front and rear of the combustion chamber can be formed from an insulating material. In which case inlets and outlets to the heat exchange elements may pass through the insulating material.

The heat exchanger element can have a shape whereby the two arms extend in opposite directions to each other, such as in a T shape. The cross bar of said T shape can form an end wall of said combustion chamber.

Preferably the at least one inlet is located in said arms at a location away from said main portion.

The at least one liquid flow path preferably starts in said arm (s) and exits said arm (s) to enter said main portion which includes said at least one outlet.

The element can include at least one formation to form said at least one inlet and said at least one outlet thereon whereby when two or more of such elements are position side by side, said dimple formations are aligned to form an inlet header which can receive a liquid and which will direct said liquid through each of said heat exchanger elements simultaneously.

The leading edge can be profiled to substantially follow the liquid flow path. This can be so that points along said leading edge have a minimum distance to the nearest channel such that said minimum distances are similar.

The invention also provides a water heater heat exchanger element being formed from a series of plates placed front to back or back to back to thereby form a plurality of channels to form a plurality of liquid flow paths inside of said heat exchanger element and a heat transfer surface on the outside of said heat exchanger element, said plates including a main portion and two extension portions, whereby said two extension portions when multiple plates are assembled will form parts of a water jacket around a combustion chamber which can direct combustion gases over said main portion.

The at least one inlet to said element can be located in one of said extension portions.

The at least one outlet from said element can be located in said main portion at a location away from a leading edge thereof.

Preferably said plates have a plurality of continuous grooves whereby when the plates are placed front to back or back to back multiple liquid flow paths are formed. The liquid flow path can have a single path which extends across all or part of said element in a serpentine manner. Alternatively the liquid flow path can divide into a multiple number of liquid flow paths from a channel across said elements in the vicinity of said trailing edge to a channel across said elements in the vicinity of said leading edge.

The liquid flow paths in said heat exchanger element can cross over each other at one or more predetermined points to enable water flowing therein to mix with, pass through, or pass over and under, each other.

The water heater heat exchanger element preferably has more than one inlet and one outlet, with each inlet having communication to one outlet, so that said heat exchanger element can have more than one liquid circuit passing therethrough.

The water heater heat exchange element can have in addition to a series of discrete dimples or channels or grooves, a continuous peripheral path.

The elements can be like oriented in said heat exchanger and placed in parallel.

The outside surfaces of dimples or channels of one plate of one element make contact with outside surfaces of dimples on another plate of another element at discrete lines or points of contact. The discrete lines or points of contact can be one of the following: held; joined; joined by fusion; joined by brazing; joined by soldering; joined by diffusion bonding.

Preferably in use, combustion products are forced around said channels and said discrete lines or points of contact forming a multi convoluted combustion path through said heat exchanger.

The invention further provides a water jacket assembly for an instantaneous gas fired water heater, the assembly including plates having therein an array of dimples or channels, said plates being placed together in pairs, the pairs of plates being arranged in parallel to form a heat exchanger, the heat exchanger being bordered by a water jacket being formed from plates having therein channels or dimples to allow water to flow through said jacket, said jacket being joined to or integral with the heat exchanger, said heat exchanger and water jacket having passages interconnecting them to allow liquid to pass therebetween, the assembly being held together to define a combustion chamber with combustion product passages and water passages within said assembly. The heat exchange element can be one of those described above.

The water jacket assembly can be formed from a plurality of plates including at least a first plate to form a heat exchanger and at least a second plate to form a second heat exchanger, whereby each of said plurality of plates are joined back to back with like plates to form a plurality of intermediate and end heat exchange elements, said water jacket assembly being constructed by sandwiching. said intermediate heat exchangers between said end heat exchangers and holding them together.

The elements can be generally vertically oriented so that when said elements are assembled leading edges of said elements are generally aligned with the depth of said water jacket assembly; or generally vertically oriented so that when said elements are assembled the leading edges of said elements are generally aligned with the width of said water jacket assembly; or generally horizontally oriented.

Preferably adjacent ones of said elements include apertures therethrough to permit combustion products to flow between pairs of elements.

Preferably the plates of the heat exchanger are adapted to cause turbulent flow of water through the water passages, and further the plates of the heat exchanger are adapted to cause turbulent flow of combustion products past the exterior.

The plates of the heat exchanger can be such that their exterior surfaces also provide an escape path for condensate that forms in use.

The present invention also provides a water heater having a heat exchanger element as described above.

The present invention also provides a water heater having a water jacket assembly as described above.

Brief description of the drawings An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which : Figure 1 is an exploded view of a combination heat exchanger combustion chamber and water jacket assembly; Figures 2,3 and 4 illustrate a front view, side view and a mid line cross section of a water heater made from an assembly similar to that of Figure 1 with the plates being aligned in parallel from front to back rather than from left to right as in Figure 1; Figure 5 is a plan view of a plate embodying the present invention; Figure 6 is a front elevation of a heat exchanger element using another embodiment of the invention; Figure 7 is a portion cross section through BB of Figure 6; Figure 8 is a partial cross section through line AA of Figure 6; Figure 9 illustrates a perspective view of a combustion chamber heat exchanger and water jacket assembly showing end plate configuration; Figure 10 illustrates a front elevation another heat exchange element; Figure 10A illustrates a detail, being a plan view, longitudinal and lateral cross section of a dimple or channel portion of Figure 10; Figure 10B illustrates a detail, being a cross section of a circular locating dimple or formation of Figure 10; and Figure 11 illustrates a perspective view of an assembly of heat exchange elements and insulation panels.

Detailed description of the embodiments Illustrated in Figure 1 is an exploded view of the water jacket heat exchanger and combustion chamber assembly 50'which is constructed from a plurality of two different plates 550 and 560 as is also described in PCT/AU99/01106. A series of heat exchange elements 570 are formed from like plates 550 which make up the central portion of the water jacket assembly 50'. Each plate 550 and heat exchange element formed therefrom is of a generally Y shaped appearance. Each plate 550 includes a main body portion 551 with a pair of upwardly extending arms 552 and 553 that define a bight or space 54 therebetween.

When a plurality of plates 550 are placed back to back in pairs, a series of heat exchanger elements 570 are formed (as illustrated in Figure 1 where only 3 pairs are illustrated for convenience) which form the central portion of a water jacket assembly 50'.

Illustrated in Figure 1 is the plate 560 which is of a generally T shaped appearance. The plate 560 differs from the plate 550 in that in the lower portion 5511 of the T in plate 560 there are no dimples or channels and thus no channels are formed in the middle portion, unlike in the main body portion 551 of the Y of the plate 550. The lower portion 5511 on the plate 560 is adjacent the base 551 of the Y of plate which will extract heat from the combustion products thus making the base of the T of plate 560 redundant in the heat scavenging process.

Thus to complete the water jacket assembly 50'of Figure 1, four plates 560 are arranged placed in pairs oriented back to back ('back to back'being where the concave surfaces are made to face each other) to form the end heat exchanger elements 580. These elements 580 have a different structure and configuration to the elements 570 due to the differences in structure and configuration of the plates 550 and 560, as is described below.

It will be noted that the plates 550 and 560 have a continuous channel 244 and 244A which runs around the outer periphery of the plates 550 and 560. This continuous channel 244 and 244A forms a water jacket around the elements and serves to contain or guide combustion gases through the assembly 501.

For a detailed description of the flow path of water in assembly 50'please refer to PCT/AU99/01106.

The pairs of Y shaped plates 550 are arranged in a vertical orientation and in parallel to form a combustion chamber A between the arms 552 and 553 of the Y shape, which form the front wall 100F and the rear wall 10OR of the water jacket 52. The water jacket assembly 50'is completed by placing the two pairs of plates 560, one pair on each end of the assembly, to close off the water passageways and to create end walls 100LS and 100 RS of the water jacket 50' thus forming the combustion chamber by completing the water jacket therearound.

The water jacket assembly 50'is thus defined as a block containing a rectangular combustion chamber 50A bounded by a water jacket defined by the walls 100LS, 100RS, 100F and 100R formed from the arms 552 and 553 of Y shaped plates 550 and the ends by pairs of end plates 560. The base portion 551 of the Y shape contains a rectangular block that constitutes the heat exchanger 51.

One of the advantages of the Y and T shaped plates 550 and 560 is that a stamping, forming and separating process can be performed with a minimum of waste of material. The Y shape in particular has the advantage that the arms of the Y are formed from those portions cut away from the base of the Y of the previous plates in the stamping or guillotining process.

The gas burners of the heater are located in the combustion chamber 50A and the interior surface can be lined with a box-like structure 610 having metal gauze 612 to isolate the comparatively cold surfaces of the heat exchanger from the very hot surface of the gas burner.

The gauze has the effect of ensuring good combustion at the gas burner which could be detrimentally affected by the cold surface of the heat exchanger 51.

In the embodiment described above the plates 550 and 560 are preferably pressed out of stainless steel or copper coated stainless steel. In another option, each panel 550 and 560 can be made of composite materials so that the hotter part of the panel, namely the upper portion including the arms 520 and 530 can be made of a material, such as those stainless steels or titanium and its alloys specifically designed to resist high radiant and convectional temperatures whilst the lower portion 551 of the panel could be manufactured of a material that does not need to withstand particularly high temperatures such as 316 stainless steel. This feature allows efficient use of materials and reduces overall costs. The panel could be stamped/pressed in two halves which are then brazed or fused together.

While the embodiment above describes generally Y shape and T shape plates and heat exchanger elements, it will be understood that any shape which has a main portion and at least two arms extending there from can be utilised. The two arms can extend in the following directions away from the main portion: parallel to each other; in opposite directions to each other; diverging away from each other; converging towards each other; or can extend so as to produce any one of the following shapes: T shape, Y shape, U shape, C shape, E shape, H shape, V shape or any other appropriate shape.

A domestic water heater 10B is illustrated in Figures 2 to 4, and is fuelled by gas and operates to provide an instantaneous flow of hot water.

As shown in Figures 2 to 4, the water heater 10 is housed in a rectangular enclosure 11 that is designed to be mounted flush against an external wall. The heater needs to be coupled to a supply of gas and it is understood that the heater can be adapted to work on a variety of commercially available gases. The combustion of the air gas mixture forms combustion products which are vented to the atmosphere via a small aperture 12 at the front of the heater.

Alternatively, the heater can be installed internally with exhaust gases being vented to the atmosphere via a small flue that would extend either through the wall cavity or up through the ceiling.

In summary, the water heater 10 has a burner 20 positioned above a water jacket assembly 50B so that heat and combustion products from the gas burner 20 pass through a heat exchanger 51 that forms part of the water jacket assembly 50B to heat up a supply of cold water that is arranged to flow through the heat exchanger to exit the heat exchanger as hot water.

Whilst having a single burner 20 will be the cheaper construction, if desired to allow the water heater 10B to cope with turn down situations, multiple burners (with appropriate controls) can be utilised so as to be able to effectively shut off parts of the burner thereby allowing optimising of the burners output depending upon needs.

A control mechanism 32 controls the amount of gas delivered from conduit 32A and which will ultimately be burned by the burners 20. The amount of gas burned is dependent on the flow of water and the temperature requested, ie on demand. The burning capacity of the gas burners is enhanced by the provision of a blower or fan 30 that mixes gas with air before prior to arriving at the burners 20 to ensure use of the most efficient air fuel mixture.

The fan 30 also operates to downwardly force the combustion products and hot air generated by the burners 20 in a generally vertical direction through the heat exchanger 51. The high efficiency of the heat exchanger 51 is such that it can produce condensation which drips down into a collection tray 71 mounted at the base of the enclosure 11. The condensate is directed out of the enclosure 11 by means of a discharge conduit 72 into a sewerage drain.

The burner 20 is positioned across the top of the heater 10B. The burner 20 is fed an air gas mixture from a mixing chamber 31, which receives gas and air via a modulating gas valve 32 and the electrically driven fan 30, which mixes the gas with the air prior to feeding the air/gas mixture to the burner 20. The burner 20 is in the form of one or more ceramic plates 35 having a series of small apertures (not shown) extending therethrough. Whilst a ceramic plate burner construction is described in the embodiment of Figures 2 to 4, any burner or multiples or combinations of burners can be used, such as mesh burners, plate burners, metal screen and mesh burners, carbon fibre burners etc.

The apertures in the burner provide a very large number of small flames that project downwardly (as a result of the air/gas mixture flow caused by fan 30) towards the water jacket assembly 50B. In order to ensure that carbon monoxide is kept to a minimum the flames terminate in the combustion chamber 50A at a position that is above the leading edges 260 of the heat exchanger 51. The heat exchanger 51 is positioned in the lower half of the water jacket assembly 50B. The overall height of the water heater 10B can be decreased by selecting a burner, such as a mesh burner, which will operate with a smaller flame length.

As shown in Figures 2 to 4, the cold water inlet 14 extends into the base of the water jacket assembly 50B (seen in cross section in Figure 4) via header 233 in the centre thereof as viewed in Figure 2, with hot water exiting the water jacket assembly 50 via header 232 by conduit 15A also from the centre thereof towards the top of the heat exchanger 51 at the hot water outlet 15.

A water flow meter 90 monitors flow of water at the cold water inlet 14. A first temperature sensor T1 is positioned on the cold water inlet and a second temperature sensor T2 is positioned on the hot water outlet 15 from the heat exchanger 51. A third temperature sensor T3 is positioned on a water flow control valve 60 which is coupled both to the cold water inlet 14 and the hot water outlet 16.

The supply of gas flows up conduit 32A from the base of the water heater 10B along the left thereof side to the modulating gas valve 32 and into the fan 30 as shown in Figures 2 to 4.

The hot water outlet 16 from the water valve 60 has a first outlet 17 that is designed to provide water up to a temperature of 80°C and a second lower temperature outlet 18 that dispenses water up to a temperature of 50°C via a flow sensor 19 if the water heater 10 is to provide hot water for radiator use as well as potable water. Water heaters of this kind generally have safety controls to prevent scalding when water of 80°C can be produced. When flow is detected in outlet 18 by flow sensor 19, the electronic control system 80 automatically limits the maximum available temperature to 50°C.

The combustion products generated by burner 20 pass through the heat exchanger 51 and exit the water heater 10 at the base of the heat exchanger 51 via the rectangular outlet 12 in the front face 13. These exhausted combustion products exit at a temperature that is close to the temperature of the cold water entering at inlet 14, thus the loss of the heat to the surroundings is kept to a minimum.

The electronic control system 80 is mounted near the top of the water heater 10B as shown in Figure 2 to control operation of the heater 10B. To operate, the water heater 10B has to be coupled to a source of gas, a source of cold water and a source of electricity.

A cold water bypass conduit 81 is provided in the form of a tube which directly communicates with the cold water inlet 14 and first and second water outlets 17, 18. There is a pressure drop across the heat exchanger 51 where the pressure at the inlet 14 is greater than the pressure at outlets 17 and 18. This is a result of loss in pressure caused by the flow path being tortuous or convoluted in nature through the heat exchanger 51. Cold water bypasses the heat exchanger 51 through the bypass conduit 81 and mixes directly with hot water prior to exit 18 from the water heater 10B thereby increasing the outflow pressure. Mixing cold water with the hot water at the outlet 18 will of course mean that the outlet water temperature is decreased.

This can be simply compensated by increasing the heating temperature at the gas burners 20 so that the overall effect is hot water leaving the water heater at a desired temperature but having a greater pressure.

Flow valve 82 on the cold water bypass conduit 81 determines the amount of cold water supplied to the hot water outlets. It is understood that the greater cold water pressure, the greater will be the hot water delivery pressure at the outlet 18. The valve 82 can be manually adjusted by a technician at the time of installation of the water heater or during maintenance visits.

Alternatively, the valve 82 can be automatically adjusted in response to fluctuating inlet water pressure measured by a sensor (not shown) at the inlet.

The heat exchanger, water jacket and combustion chamber assembly of Figures 2 to 4 is constructed from plates 550B and 560B having a Y and T shape configuration, but are arranged so as to be parallel to the width of the water heater 10B. This configuration results in the use of a lesser number of actual plates, albeit bigger ones, but this is helpful for dimensional stability and integrity of the water jacket assembly when in use. In Figure 2 the flow path through plates 560 of the water jacket assembly 50B is illustrated. In Figure 2 it can be seen that the two flow paths are the mirror image of each other.

Illustrated in Figure 5 is an element 800 which can be used in a water heater combustion chamber, water jacket and heat exchanger assembly (similar to that of Figures 1 to 4) such as that in Figure 9. The element 800, formed from plates placed back-to-back ('back to back' being where the concave surfaces are made to face each other), will form a series of parallel heat exchange elements and with pairs (see Figures 9) of end elements 801 will form a heat exchanger, water jacket and combustion chamber as illustrated in Figure 9. The end elements 801 need not be an identical pair, as will be described later. One improvement in the element 800 over the above described elements is that inlet 802 and outlet 804 have elongated cross- sectional areas by comparison with the round inlets and outlets 14,15,233 and 232 of Figures 1 to4.

In the embodiment of Figure 5 water enters the water jacket portion 806 via inlet 802 and travels via a continuous channel 803 along the outside periphery of the element 800 to the opposite end 812 which is opposite to the location of the combustion chamber area 810. The channel then follows a sinusoidal/helical path through to the centre of the plate where a crossover formation 814 is located. As the element 800 is symmetrical, that is the flow paths on one half is the mirror image of those on the other half, water enters the crossover formation 814 from both the left and right sides. Thus water entering crossover portion 814, whether from the left or right, will travel out of the crossover portion 814 via channels 816 on the left and 818 on the right.

As can be seen from Figure 5 the water will then follow a sinusoidal/helical path sideways away from the centre then upwards and then sideways into the centre, then upwards and sideways away from the centre, then upwards and then sideways towards the centre then upwards and then sideways across the leading edge of the element 800 and then downwards to the outlet 804. The outlet 804 is located on the plate 800 approximately two flow paths away from the leading edge 820. This results in the water departing from the outlet 804 being marginally cooler than water passing through the flow path at or closest to the leading edge 820.

It will be noted that both the leading edge 820 and the trailing edge 822 are scalloped or profiled as described in co-pending application PCT/AU99/01106.

An advantage of utilising an elongated outlet is that its longitudinal axis can be oriented in the direction of exhaust gas flow over the external portions of the element 800. This orientation will ensure that exhaust gases passing around the relatively narrow width will not need to accelerate to the same degree as exhaust gases flow around a circular shaped outlet (such as in Figure 1 to 4) which has the same cross sectional area as outlet 804. This means that there will not be the same degree of heat transfer from the combustion gases to the water passing out of the outlet 804 by comparison to a circular outlet, even if the outlets were located in the same position on the heat exchanger element.

Another advantage of the use of an elongated inlet and outlet 802 and 804 is that the peripheral area where adjacent plates are mated (and secured or brazed) will provide a length of mating surface which would be longer than the circumference of a circle having the same cross- sectional area. By positioning the outlet 804 away from the leading edge 820 will also help to decrease the rate of heat transfer to the outlet 804 by positioning it in a cooler region of the heat exchange element.

Another advantage of positioning the inlet 802 on the arm of the element 800 is that by its elongated nature there results, by comparison to the heat exchanger elements of Figures 2 to 4, a plate 800 of lesser overall width W, for a comparable heat transfer capability.

Illustrated in Figure 6 is an element 860 similar to that of Figure 5 except that the heat exchanger portion between the leading edge 820 and trailing edge 822 has been replaced by a multi-path crossover heat exchanger formation.

The element 860 of Figure 6 has a continuous channel 850 that extends across the trailing edge 822. The opposite ends of channel 850 open into right and left hand side channels 803.

Extending away from the channel 850 are a series of continuous channels 851 which open into a continuous channel 852 which extends across the leading edge 820. At the right and left hand side of element 860 the channel 852 moves away from leading edge 820 and progresses to the elongated outlet 804 which is located closer to trailing edge 822 then leading edge 820.

While only the front plate of element 860 is visible, it will be noted that the rear plate is identical to the front plate, with the front and rear plates being positioned back to back ('back to back'being where the concave surfaces are made to face each other). This will provide a lattice shaped multi-path multi cross over access for water flowing from channel 850 to channel 852.

As can be seen from the partial cross sections of Figures 7 and 8 the continuous channels 850 and 852 have less depth than the channels 851 so that the adjacent elements 860 can abut each other. Alternatively in the case where the depth of portions of the channels 850,852 and the channels 851 are all the same in the region MW of Figure 6 these channels or portions of channels need to be less than the depth of the formations and channels outside this region.

It is envisaged that the difference in depth can be such that those channels of reduced depth are of the order of 1/4 to 1/2 the depth of the remaining channels. This has the advantage of increasing the speed of water flow in the channel 852 (compared with the non reduced depth channels) thus helping to minimise the probability of scale deposition.

If required a cross over formation 814 (indicated in phantom in Figure 6) can be provided to allow communication from the front and rear end elements.

The continuous channels 803 illustrated of Figures 5 and 6 serve a water jacket function, however they also serve to contain, confine, and/or guide combustion gases so that they exit the elements 800 and 860 via trailing edges 822.

Illustrated in Figure 9 is a water jacket, combustion chamber and heat exchange assembly which has sixteen heat exchange elements 800 or 860 of Figures 5 or 6 respectively and a front and rear element 801. The front and rear elements 801 have elongated formations 862 to connect to the inlets 802 on elements 800 or 860 and elongated formation 864 so as to communicate with the cross over formations 814 of element 800 of Figure 6 (or a similar cross over formation in element 860).

The front element 801 has a single cold water connection 868 which is fed by a passage to an inlet distributor 864. The inlet distributor 864 feeds water in five directions, namely a horizontally directed left passage 863, a horizontally directed right passage 865, a left side vertical passage 867, a right side vertical passage 869 and from front element 801 to rear element 801 via the passage 814. The rear element has a single outlet 870, which takes hot water from both the left side outlet 804 and the right side outlet 804. Thus the rear element 801 is not identical to front element 801.

The cross over 814 of Figures 5 & 6 also a provides passage from the front end element through to the rear element thereby allowing water to enter inlet headers from both ends. The inlet headers being formed from all the interconnected inlets 802 on respective sides of the elements 800.

If desired, the formation 814 can be by-passed by the channels which run across the elements 800,860 or 890 and be simply a passage from front end elements 801 to the rear end element 801. Such a formation is illustrated in Figure 10 at 814A where it can be seen that the channels that run across the element in its vicinity do not join thereto.

In Figure 10 like features have been like numbered with previously described embodiments. In the embodiment of Figure 10, the element 890 is made from identical plates back to back ('back to back'being where the concave sides of the plates are made to face each other), with the front of the plate on the right side having two partially spherical locating formations 891 and 892 which project in a forward direction, and two partially spherical locating formation 893 and 894 which project rearwardly. The formations 892 and 894 on respective plates nest with each other when placed back to back so as to align the plates to form an element 890. The formations 891 and 893 on respective plates nest with each other when adjacent elements are abutted so as to align all the elements 890 forming the heat exchanger.

It will be noted from Figure 10 that the continuous channels 803 have a zig zag or "chicane"region 897 below the inlets 802 and above the outlets 804. The continuous channels 803 take an inwardly directed path immediately prior to the"chicane"region 897 thereby positioning the"chicane"region 897 over the outlets 804. The continuous channels 803 then take an outward course for a short segment and then back inwardly over the outlets 804. Thus two portions of the continuous channels 803 in the"chicane"regions 897 are positioned over the outlets 802. By this means the combustion gases flowing from the leading edge 820 to trailing edge 822 in the vicinity of the region 897 will be directed towards the internal regions of the heat exchanger. This will help to reduce the amount of heat absorbed by the outlets 804, by acting as a deflector. This effect on the combustion gas flow path could also in part be produce by only a single inwardly directed portion of the flow path. However it is thought that in view of the speed of flow of the combustion gases in this region, two inwardly directed portions (which form the zig zag or"chicane") on each side such as illustrated in Figure 10 additionally assists the gas flow to take an inwardly directed flow path.

As illustrated in Figure 11, the end elements 801 of Figure 9 can be replaced by insulating plates 801A such as ceramic or foam ceramic elements. In this case the assembled elements 800, 860 or 890 will have two cold water inlets 14 and two cold water outlets (not visible) which pass through the front and rear insulating plates 801A respectively.

It will be understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

The foregoing describes embodiments of the present invention and modifications, obvious to those skilled in the art can be made thereto, without departing from the scope of the present invention.