WATER HEATER AND WATER HEATER COMPONENT CONSTRUCTION

Abstract: A water jacket assembly (50) for an instantaneous gas fired water heater (10), the assembly including pressed profiled plates (51) made of copper or copper coated steel, one plate being the inverted image of the other, said plates being placed together in pairs, the pairs of plates being arranged in a parallel array to form a heat exchanger, the heat exchanger being bordered by a water jacket (52) comprising overlapping side and end panels of copper or copper coated steel attached to the array of plates, the assembly (52) being fused together to define a combustion chamber (50A) with discrete combusted gases and water passages within said assembly.

Description: Water heater and water heater component construction Field of the invention This present invention relates to a fired instantaneous water heaters and other instantaneous types such as combination boilers, central room heaters, commercial and industrial water heaters and water processing systems to name some others. More particularly the present invention relates to a heat exchanger and a water jacket assembly for such water heaters. The invention also relates to a method of manufacturing such a water heater. These water heaters can be used to heat not just potable water, but mixtures of water and other additives whether potable or not. Such instantaneous fired water heaters provide hot water on demand. Background of the invention Approximately 70% of the world's manufactured water heaters are believed to be of the "instantaneous type" where hot water is provided on demand by use of means to instantaneously heat the water as it flows through the heater. This type of water heater is typically pressure dependent with limitations on the flow rate of hot water it can sustain. One attempt to remedy this has been to increase the cross sectional area of the passages through the heat exchanger. However this solution has led to increased susceptibility to scaling and its difficulties in the prior art. Also difficulties exist in constructing and assembling cost effective, long life heat exchangers for this type of water heater. This type of water heater also generally has combustion chamber surrounds which have a relatively high heat loss to the surroundings. The lack of cost effectiveness relating to construction and assembly result from the fact that prior art instantaneous water heater heat exchangers are manufactured from a fin and tube construction. This is a costly and time consuming construction method. Another problem of prior art instantaneous water heaters is that because of a single water path through the fin and tube construction such water heaters experience a significant pressure drop across the inlet and outlet to the heat exchanger. This has the effect on the flow rate of hot water from such heaters as being relatively low compared to mains pressure storage water heaters. The construction of prior art instantaneous water heater heat exchangers have been susceptible to scaling. In general terms scaling is known to occur under conditions of high temperature and high levels of dissolved solids in the water. Hot spots in heat exchangers, particularly a problem in fin and tube constructions, can result in a higher incidence of scaling. Instantaneous water heaters are common in colder climates as they can be used for the dual purpose of providing hot water of approximately 80°C for central room heating as well as hot potable water at approximately 50°C for cooking and washing. Such boilers serving these dual purposes are commonly known as combination boilers or combi-boilers. One of the particular difficulties with these systems is that there can be a significant thermal lag on the hot potable water circuit. This thermal lag almost always results in large quantities of water being wasted until the hot potable water arrives at a user's water spout. It is an object of the present invention to provide a water heater, water heater components and manufacturing methods of water heaters and components, to at least in part ameliorate at least one of the difficulties of the prior art. 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 channel therebetween to provide at least one liquid flow path inside of said element and a combustion heat transfer surface on the outside thereof, said element being characterised by said at least one flow path each consisting of a single path which extends across a portion of said element in one direction and across said portion in the opposite direction, in a serpentine manner. The flow path can extend partially or fully across the width of the element. The flow path when extending across the plate does so in preferably a zig zag or sinusoidal configuration. Further the serpentine manner is such that the flow path winds back over itself at least once, and preferably two to four times. The water flow path is preferably shaped like one of the following: generally helical; cork screw; square helical; or vortex shaped. The first plate can have a single continuous groove whereby when a flat second plate is joined thereto, said channel is formed. Alternatively said first and second plates each have a series of discrete dimples therein, whereby adjacent dimples on said first plate are connected by the series of adjacent and partially overlapping dimples on said second plate. Preferably the flow path requires liquid flowing therein to travel along one of said dimples in said first plate having a straight line pathway then flow through an approximately 90° direction change into said second plate then through an approximately 90° direction change to flow through an adjacent dimple in said second plate in a straight line pathway through said adjacent dimple. The dimples can provide a straight line path after transition from said first plate to said second plate whereby the maximum length of the straight line path is in the range of two to ten times the depth or height of the dimple. Preferably the first and second plates each have a flared end extending away from a joining plane of said first and second plates. The flared ends can extend along the side edges of said plate from said leading edge to said trailing edge. Also said flared ends can extend for a distance in the direction towards the centre of said plate along said leading and trailing edges. Preferably the element is formed from identical plates placed back to back. The heat exchanger element and or plates forming said heat exchanger element have a nestable shape. In one embodiment the heat exchanger element and plates from which it is formed has or have a shape which includes a main portion and at least two arms extending away from the main portion. The at least two arms can 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: a Y shape, U shape, C shape, E shape H shape, V shape or any other appropriate shape. Preferably the arms of the form a water jacket around a combustion chamber, while the main portion forms a heat exchanger unit. The exchanger element can also have a shape whereby the at least two arms extend in opposite directions to each other, such as in a T shape, with the cross bar of said T shape forming an end wall of said combustion chamber. Preferably the element formed includes at least one dimple formation thereon whereby when two or more of such elements are positioned side by side, said dimple formations are aligned to form a header which can receive a liquid and which will direct said liquid through each of said heat exchanger elements simultaneously. If desired the leading edge of the plates and or the element formed from the plates or is scalloped or curved so that most points along said leading edge have a minimum distance to the nearest channel such that said minimum distances are similar. The plates each have a formation to allow all plates to be nested together prior to holding or joining them together. The formation is preferably a flange on said first and second plates such that when placed back to back said flanges all extend in the same general direction. The flange is preferably at an angle to said plate. The flange can extend partially around the periphery of said plates or alternative a wholly around the periphery of said plates, where a combustion products path is provided. The heat exchanger elements can be formed from plates made from two or more plate segments which are bonded together to from a composite single plate. Such a composite single plate can have different materials in the different plate segments. The different materials can be chosen on the basis of heat resistance characteristics. If desired a series heat exchange elements can have a leading edge formed of a different material to that portion of the heat exchange element which contains said dimples or channels. The leading edges can have a shape which will help to maintain the temperature in the combustion chamber in order to promote combustion. Heat exchanger elements can include a by pass channel or dimple which connects an entry header to an exit header. Further if desired the water flow paths in the heat exchanger elements can cross over each other at predetermined points to enable water flowing therein to mix with, pass through, or pass over and under, each other. The invention also provides a water heater heat exchanger element being formed from a first plate and a second plate forming a channel therebetween to form a liquid flow path inside of said heat exchanger and a heat transfer surface on the outside of said heat exchanger element wherein the configuration of said liquid flow path and said heat transfer surface is varied across the width of said exchange element. The first plate can have a single continuous groove wherein when a flat second plate is joined thereto said liquid flow path is formed. Alternatively said first and second plate each have a series of discrete dimples whereby adjacent dimples on said first plate are connected by dimples on said second plate to form said liquid flow path. Preferably said flow path is of a zig zag or sinusoidal configuration. The liquid flow path can be configured to provide at near to a leading edge thereof longer straight line sections compared with the length of straight line sections in the vicinity of a trailing edge of said element. The flow path is preferably a single path which travels over all or part of said element in a serpentine manner. Preferably said path extends across part or all of said element at least two. The path can be such that the included angle between lengths of dimples or segments of channels on said first and/or second plate in the vicinity of a leading edge is varied by comparison to the included angle in the region of a trailing edge. Also if desired the amplitude of said zig zag or sinusoidal configuration can be varied in said flow path in the vicinity of said leading edge by comparison to the amplitude of said zig zag or sinusoidal configuration in the vicinity of said trailing edge. If a single path is not utilised the flow path can be divided into a multiple number of parallel flow paths connecting 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. Another feature that can be incorporated is that the thickness of said heat exchanger is varied from said leading edge to said trailing edge. Alternatively, the depth of said dimples on said plate or plates is varied from said leading edge to said trailing edge. Preferably when two or more elements are positioned adjacent to each other a combustion products flow path is formed between adjacent plates, said flow path, near to said leading edges being wider than at said trailing edges. Preferably the water heater heat exchanger element as described above has a greater than one pair of inlets and outlets so that said heat exchanger element can have more than one liquid circuit passing therethrough. Preferably when in use the hottest parts of the heat exchanger element receive a first circuit, while a second circuit is heated in a cooler part of the element. The water heater heat exchange element can have, in addition to a series of discrete dimples, a continuous peripheral path to serve a water jacket function. The invention also provides a heat exchanger formed from a plurality of heat exchanger elements as described above, said elements being like oriented in said heat exchanger and placed parallel. Preferably the outside surfaces of dimples of said first plate of one element, make contact with outside surfaces of dimples on said second plate of another element at discrete lines or points of contact. The discrete lines or points of contact preferably fused, soldered, brazed or otherwise connected or contacting each other, by such other means as mechanical clamping forces etc. The heat exchanger so formed is such that when in use, combustion products are forced around said channels and said discrete lines or points of contact forming a 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, 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 between the plates, the assembly being held together to define a combustion chamber with combustion product passages and water passages within said assembly. Preferably the water jacket assembly includes a heat exchange element or a heat exchanger as described above Preferably said water jacket assembly is formed from a plurality of plates including at least a U or Y shaped plate to form a heat exchanger as described above and at least a T shape plate to form a second heat exchanger element as described above 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 unit; 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 unit; or generally horizontally oriented. If horizontally oriented 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 or are adapted to cause turbulent flow of combusted gases past the exterior; and or are such that their exterior also provides an escape path for condensate that in use collects on the external surfaces of the heat exchanger. The invention further provides a water heater having a heat exchanger as described above and or a water jacket assembly as described above. The water heater can include a storage means to receive hot water which would otherwise remain in said apparatus when a user has closed a valve preventing further hot water passing through said valve. The hot water in said storage means can be passed through said valve once said valve is re-opened. The present invention also provides a water heater system having at least two water flow paths, with both paths passing through a water/gas heat exchanger, which transfers heat from combustion products to water contained in said circuits, a first of said at least two paths including a serial connection to a radiator means and a serial connection to a water/water heat exchanger where water in said first path can transfer heat to or receive heat from water in said second path. Preferably said water in said first path is in a closed loop. Preferably the second of said at least two paths includes a cold water inlet. The cold water inlet can split into two water flow sub-paths, a first sub-path to deliver water to said water/water heat exchanger and a second sub-path to deliver water said water/gas heat exchanger. The second sub-path can merge with said first sub path for water to flow out of said system, when a valve on an outlet conduit from said system is in an open condition. When a valve on an outlet conduit from said system is in a closed condition water in said first and second sub-paths is circulated. The invention also provides a water jacket assembly for an instantaneous gas fired water heater, the assembly including pressed profiled plates, one plate being the inverted image of the other, said plates being placed together in pairs, the pairs of plates being arranged in a parallel to form a heat exchanger, the heat exchanger being bordered by a water jacket comprising overlapping side and end panels of copper or copper coated steel attached to the heat exchanger, the assembly being fused together to define a combustion chamber with discrete combusted gases and water passages within said assembly. It is preferable that the profiled plates of the heat exchanger are adapted to cause turbulent flow of water through the water passages, and turbulent flow of combusted gases past the exterior, the exterior also providing escape routes for condensate that in use collects on the external surfaces of the heat exchanger. It is preferable that the water jacket has a cold water inlet and a hot water outlet, at least one gas burner being positioned within the combustion chamber whereby cold water flows through the assembly to exit as hot water. Preferably, the at least one gas burner is positioned above the heat exchanger and the heater includes a fan that mixes gas with air and forces the gas/air mixture to the burner and, as combusted gases past the heat exchanger. The invention also provides a method of manufacturing a water jacket assembly including making profiled heat exchanger plates, placing pairs of plates together to form a heat exchanger element, placing a plurality of heat exchanger elements together to form a sandwich, said assembly having a combustion chamber and combustion products passages and water passages within said assembly. Preferably two type of heat exchanger elements are formed, end elements and intermediate elements with said end elements having a different water path to said intermediate elements. Preferably said heat exchanger plates are manufactured such that those portions not required on a blank for one plate are a part of the next successive plate stamped. The heat exchanger elements can be assembled as a set of parallel plates, which can be any one of the following: vertically oriented or horizontally oriented. If vertically oriented the elements can extend such that their leading edges run generally parallel to the width of the combustion or water heater into which the water jacket assembly will be installed. Alternatively vertically oriented the elements can extend such that their leading edges run generally parallel to the depth of the combustion or water heater into which the water jacket assembly will be installed. According to a further aspect of the present invention there is provided a method of manufacturing a water jacket assembly comprising pressing profiled heat exchanger plates, side panels and end panels out of copper or copper coated steel, placing two plates together, one being the inverted image of the other to form a pair of abutting plates, placing a plurality of pairs of heat exchanger plates together to form a sandwich, attaching the side panels to the sandwich and placing the end plates on each corner so that the side and end panels overlap, holding the assembly with a jig, and placing the assembly in an oven for a predetermined time to fuse the copper surfaces together to provide an integral assembly having a combustion chamber and discrete combusted gases and water passageways within said assembly. In accordance with one aspect of the present invention there is provided an instantaneous gas fired water heater comprising a heat exchanger through which water flows to be heated by a gas fired burner, the heat exchanger having a cold water inlet and a hot water outlet, an air tight chamber positioned downstream of the hot water outlet of the heat exchanger whereby when the demand for hot water ceases, the internal pressure within the hot water system causes hot water to flow from the heat exchanger into the air chamber to reduce the temperature of the heat exchanger. The air chamber preferably comprises a sealed tank that initially contains only air, the internal pressure of the system causing the tank to fill when no water is being drawn off and causing the tank to empty as hot water is drawn off, the tank refilling when demand for hot water ceases. According to a further aspect of the present invention there is provided a water jacket assembly comprising a plurality of heat exchanger plates pressed out of metal, the plates being placed together in inverted pairs in parallel, each plate having peripheral side portions that overlap as the plates are placed together, a pair of end panels being positioned at the ends of the heat exchanger plates with overlapping edges whereby the assembly can be placed in an oven on its end so that the weight of the assembly causes the plates and panels to fuse together to define discrete water and air passageways. Preferably the profile of the plates and panels defines a heat exchanger surrounded by a water jacket, the water jacket defining a combustion chamber within the assembly. Brief description of the drawings Embodiments, incorporating all aspects of the invention, will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a front elevation view of a water heater being one embodiment of the invention, with the front portion of its water jacket assembly removed; Figure 2 is a left side elevation of the heater of Figure 1 , Figure 3 is a right side elevation of the heater of Figure 1 ; Figure 4 is an exploded perspective view of components of a water jacket assembly; Figure 5 is a front elevation of a water jacket assembly used in the water heater of Figure 1 ; Figure 6 is a side elevation of the water jacket assembly of Figure 5; Figure 7 is a plan view of the water jacket assembly of Figure 5, with the heat exchanger 51 shown in schematic representation, for a detail of the plan view of the heat exchanger refer to figure 23; Figure 8 is a diagrammatic cross section through the plate 53 of Figure 9 along plane A-A; Figure 9 is a front elevation of the plate 53 of Figure 4; Figure 10 is a representation of the water flow path as viewed in front elevation formed from plates 53 and 54 of Figures 4, 9 and 1 1 ; Figure 11 is a rear view of plate 54 of Figure 4; Figure 12 is a diagrammatic cross section through the plate 54 of Figure 1 1 along plane BB; Figure 13 is a diagrammatic cross section through heat exchangers formed from pairs of plates 53 and 54; Figure 14 is an enlarged view of part of the cross section of Figure 8F; Figure 15 is a diagrammatic perspective view of a heat exchanger assembled from elements formed from pairs of plates 53 and 54 of Figures 4, 9 and 11 with combustion products and water path ways ; Figure 16 is a detail view of three of the dimples of plate 54 of Figure 11 ; Figure 17 is an enlarged view of two whole dimples of the plate 53 of Figure 9; Figure 18 illustrates the dimples of Figures 16 and 17 connected together back to back; Figure 19 illustrates portions of plates 53 and 54 (in spaced apart relation for clarity) indicating diagrammatically the flow path of liquid through the plates and passages formed when the plates are abutted face to face; Figure 20 illustrates a typical combustion gas flow path over the external surface of plate 53 of Figure 9; Figure 21 illustrates a typical combustion gas flow path over the external surfaces of plate 54 of Figure 11 ; Figure 22 illustrates the drawings of Figure 20 and 21 combined showing the convoluted combustion gas flow path between adjacent contacting elements formed from plates 53 and 54; Figure 23 illustrates a plan view of a heat exchanger made up of plates 53 and 54 of Figure 4; Figure 24 is a plan view of a single dimple; Figure 25 is a side elevation of the dimple of Figure 24; Figure 26 is a cross section through line BB of Figure 25; Figure 27 is a cross section through line CC of Figure 25; Figure 28 illustrates a front view of a plate of heat exchanger element being a further embodiment of the invention; Figure 28A illustrates an embodiment similar to that of Figure 28 except that the straight line path length is varied; Figure 29 illustrates a front view of a plate of a heat exchanger element of another embodiment having parallel flows therethrough; Figure 29A illustrates an embodiment similar to that of Figure 29 except that the straight line path length is varied; Figure 30 illustrates a side view of a heat exchanger assembly of another embodiment ; Figure 31 is a left side elevation of an instantaneous gas fired water heater of another embodiment of the invention; Figure 32 is a front elevation of the heater of Figure 31 ; Figure 33 is a right side elevation of the heater of Figure 31 ; Figure 34 is a side elevation of a plate that forms part of a water jacket assembly of a further embodiment of the invention applicable to the water heater of Figures 31 to 33; Figure 35 is a side elevation of an end plate forming part of the water jacket assembly of Figure 35; Figure 36 is a side elevation of a pair of end plates of Figure 35 to illustrate flow path; Figure 37 is an exploded perspective view showing the assembly of heat exchanger plates and end panels in the water heater of Figures 31 to 33; Figure 38 is a schematic cross section of stacked plates showing nesting feature; Figure 39 is a side elevation of an alternative form of heat exchanger plate; Figure 40 is a diagrammatic side elevation of the liquid flow path through a channel formed by back to back dimples on a pair of plates of Figure 39 placed together; Figure 41 is a side elevation of a heat exchanger element plate of another embodiment; Figure 41 A is a side elevation of an end heat exchanger element plate for use with the element formed from the plate of Figure 41 ; Figure 42 is a schematic circuit illustration showing use of the heat exchanger plate of Figure 41 ; Figure 43 is a schematic circuit illustration showing a different use of the heat exchanger plate of Figure 41 ; Figure 44 illustrates a front elevation of a further embodiment of the invention; Figure 45 illustrates a side apparatus view the apparatus of Figure 44; Figure 46 is a sectional view through the apparatus of Figure 44 through the lines AA of Figure 45; Figure 47 is a front elevation of a further embodiment of the invention showing water flow path; Figure 48 is the front elevation of Figure 47 showing combustion gasses flow path; Figure 49A is a perspective view of five plates which are used to manufacture the water jacket assembly 50C of Figures 47 and 48; Figure 49B is an exploded view of the water jacket assembly plates formed form the plates of Figure 49B and as used in the embodiment of Figure 47; Figure 49C is a schematic water flow path through the plates of Figures 49A and 49B; Figure 50 illustrates an accumulator or reservoir for use with the water heater of previous drawings; Figure 51 illustrates a schematic cross section of a part of a heat exchanger being another embodiment; Figure 52 illustrates a heat exchanger element with a cross over path; Figure 53 illustrates an end plate with an end plate with a bypass feature; and Figure 54 illustrates a cross section through another embodiment of a water heater having a water jacket assembly similar to that of Figures 44 to 46, but with a naturally aspirated burner system. Detailed description of the embodiments A domestic water heater 10 is illustrated in Figures 1 to 3, and is fuelled by gas and operates to provide an instantaneous flow of hot water. As shown in Figures 1 to 3, 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 13 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 50 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 50 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 10 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 10. 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 1 to 3, 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 50. 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 50. The overall height of the water heater 10 can be decreased by selecting a burner, such as a mesh burner, which will operate with a smaller flame length. As shown in Figures 1 to 3, the cold water inlet 14 extends into the base of the water jacket assembly 50 (cut away for illustration purposes) on the left hand side thereof as viewed in Figure 2, with hot water exiting the water jacket assembly 50 via conduit 15 A from the right hand side, 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 Tl 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 10 along the left thereof side to the modulating gas valve 32 and into the fan 30 as shown in Figures 1 to 3. 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 10 as shown in Figure 1 to control operation of the heater 10. To operate, the water heater 10 has to be coupled to a source of gas, a source of cold water and a source of electricity. The water jacket assembly 50, is illustrated in detail with reference to Figures 4 to 23 and includes an external water jacket 52 that supports an internally located heat exchanger 51 that is in the form of a discrete number of pairs of rectangular plates 53, 54 (as illustrated in Figures 4 and 5) depending on the heat exchanger 51 requirements. The construction of the heat exchanger 51 will be discussed in more detail later. The water jacket assembly 50 in summary has the combustion chamber 50A, the water jacket 52 and heat exchanger 51. These components are constructed from 3 differently shaped plates. Two of a first shaped plate form the front plate 100F and rear plate 100R (see Figure 4 and the associated description). These first shaped plates are placed back to back to encase opposite sides of the assembly. Four of a second shaped plate form end plates 101 A, 101B, lOlC and 101D that, as shown in Figures 4, 5 and 6, envelop and overlap the ends and sides to define the water jacket 52. A plurality of pairs of a third plate defines the heat exchanger 51 , with only one pair of rectangular plates 53, 54, being illustrated for convenience and clarity of Figure 4. The plurality of plates 53 and 54 when mounted in spaced apart pairs constitute the heat exchanger 51. As shown in Figures 5 to 7, the sandwich of the plates which forms the heat exchanger 51 is located towards the base of the unit. The water jacket 52 has liquid passageways or channels that run across the bottom, then up to the top of half of the sides 200 then crosses to the other half of sides 200, then across the top of this other half, then down to the bottom. In the sides 201 and 202 of Figure 4 the water flow path begins at the bottom and flows initially in two directions to the middle of the side 201 and 202, whereupon it follows a single path to the top of the side 201 and 202, then back down to the middle where it will exit the respective side 201 and 202 to enter the adjacent respective side 202 or 201. However, in the sides 201 and 202 as illustrated in Figure 6, which sides are a little different to those in figure 4, the path in the sides 200 will terminate near the midpoint of the corners, whereupon the water enters at a midpoint of the sides 201 and 202 then splits into an upward and downward path to the go to the top and bottom respectively, then across the top and bottom of sides 201 and 202, then back to an exit midpoint opposite the entry midpoint. The exit midpoint on one side 201 or 202 feeds into an entry midpoint on the adjacent side 202 or 201 respectively. The space bounded by the water jacket 52 and the top of the heat exchanger 51 defines the combustion chamber 50A. The water jacket 52 is positioned externally of the heat exchanger 51 with the gas flames from the burner 20 being produced along the centre line of the water jacket assembly 50 within the combustion chamber 50A. This feature has the effect of drawing off heat from the gas flames to reduce sideways escape of heat and also reduce the temperature of the hot gases at the heat exchanger 51. As shown in Figure 5, the cold water enters the assembly 50 from one side at the base and exits the assembly on the opposite side towards the top of the heat exchanger 51. Initially, the water moves in two directions around the sides and ends of the water jacket 52 so that water flows through the whole of the water jacket before passing into the heat exchanger 51. This reduces the likelihood of the water jacket 52 being overheated and reduces waste of hot gases. By manufacturing the assembly from three plates that are simply reversed and or inverted, the whole assembly can be produced from a simple stamping operation. Furthermore, in this embodiment, the assembly is manufactured from stainless steel plates coated in copper and the components are assembled together by use of a jig (not shown) so that the component plates are in abutting contact with all the abutting surfaces being copper to copper. The assembly is placed in an oven for a predetermined period at a temperature to fuse the copper to provide an integral unit in which all the components are bonded together and the water and air passageways are defined accurately with no leakages. There is thus no need for welding, soldering, or other fasteners and this fusing of the copper coating ensures satisfactory operation over a long life. The design of a convoluted passage for water flow is also specifically designed to encourage turbulent flow to ensure that there are no stagnant water pockets or hot spots in the heat exchanger 51 or water jacket 52. Furthermore the external shape of the plates provides a convenient route for run-off of condensate that forms on the exterior of the heat exchanger elements. The water jacket assembly 50 has proved efficient and allows maximum transfer of heat from the gas flames to the water without excessive heat being lost to exhaust. While fusing is described with respect to copper other fusing metals can be used such as nickel or the like. Instead of fusing however, any means of holding the plates together which form the water jacker 52 and heat exchanger 51 can be utilised. A gas pressure sensor 84 is positioned at the gas entry of the modulating gas valve 32 to sense a drop in gas pressure to reduce the output of the water heater 10 should there be a shortage of gas pressure. Conventional domestic gas pressures are such that if too many appliances are used at once there is often a drop in the gas pressure. To ensure that a drop in gas pressure does not reduce the temperature of the hot water, the gas pressure sensor 84 causes the rate of flow of water to be reduced by means of valve 60 to compensate for a reduction in gas pressure so that the water heater 10 operates at the desired temperature albeit at a reduced output in terms of litres per minute. Alternatively if a water flow valve 60 is not used, by means of an appropriate signal from sensor 84 the controller 80 can be made to decrease the flow rate of air being mixed with the gas, and or the gas by the valve 32, to form the air/gas mixture, to maintain optimum combustion. Another feature of the gas valve 32 and controller 80 is the use of an oxygen sensor 71 A that detects the amount of oxygen in the flue gases. If the oxygen content of the flue gas is either too high or too low, a signal is fed back to the controller 80 to change the gas flow by valve 32 to ensure an optimum mixture. The computerised controller 80 monitors three temperatures, namely the temperature Tl which is the temperature at the inlet of the cold water, the temperature T2 being the temperature which is at the heat exchanger outlet; and the temperature T3 which is the temperature at the hot water outlet from the water heater 10. The third temperature monitor T3 is adjustable by a user or service person to adjust the desired output temperature. The controller 80 on sensing the three temperatures can then control the rate of water flow through the water heater 10 and also the gas input through the modulated gas valve 32 and the air input by varying the speed of the fan 30. The controller 80 varies these parameters to ensure maximum efficiency. A flow sensor 90 is positioned in the cold water inlet 14. The flow sensor 90 provides an electrical signal which is sent to the controller 80 to control the operation of the water heater 10 in relation to demand. It will also be understood that with this flow sensor 90 can produce a signal to give a visual indication, or be processed to give a visual indication, of the rate of flow which can be displayed at the water heater 10 and/or at remote locations. The flow control valve 60 as illustrated in Figure 1 is provided to compensate for too much demand for hot water, or to reduce flow, or if there is a danger of the water exceeding the maximum design temperature. In these cases the valve 60 can be made to open or close as the conditions require. To start up the heater, an electrically operated ignition system is (such as a spark igniter or glowing surface or HSI [hot surface ignition] or any suitable system) is utilised in the combustion chamber 50A and the control system 80 ensures that when a hot water tap is opened causing flow of water, there is first a pause to purge any combustible gases within the combustion chamber. Then depending on the ignition system used there may be a short pause during which the electrically operated ignition system activates and commences to ignite the air/gas mixture otherwise the air/gas mixture enters the combustion chamber and is ignited. In this embodiment a spark igniter is preferred as it has no lag time to operate. If combustion does not start the water heater 10 shuts down the gas flow and the whole process is repeated. The control system can be programmed so that if this fails a predetermined number of times then the water heater 10 shuts down and a warning light comes on warning the user of the system that a service call is required. To construct the water jacket 52 as illustrated in Figures 4 to 7, the water jacket 52 is made from a plurality of two types of plates. The first type of plate is that which makes plate 100F and 100R which has three sides, a middle side 200 forming the front and rear faces of the water jacket assembly 50 and the outer sides 201 and 202 at right angles to the side 200. When assembled, the side 201 on the front plate 100F is located adjacent to side 202 on rear plate 100R on the left side and vice a versa on the right side of the water jacket assembly 50 as illustrated in Figures 4 and 7. A series of channel formations, generally designated by numeral 204, are formed in the front plate 100F and are constructed so as to have depth into the page as illustrated in Figure 4. While in the rear plate 100R (which is identical to plate 100F) the channels 204 are formed so as to have depth out of the page of the drawings. The channels 204 are at the front and rear of the combustion chamber 50A and define the combustion chamber 50A therebetween. To complete the water jacket 52 four identical second plates illustrated in Figure 4 as plates 101 A, 101B, 101C and 101 D are oriented and positioned on the front and rear plates 100F and 100R so that their respective comers 205 and 206 are positioned onto front and rear plates 100F and 100R at corresponding respective corners 205R, 205L, 206L and 206R. The plates 101B and 101C as illustrated in Figure 4 have their channels, which are generally designated by the arrows 208, formed such that the depth of the channels away from the plate is out of the page in Figure 4. Whereas the plates 101A and 101 D have their channels generally designated by the arrows 208, such that the depth of these channels 208 are formed into the page. The channels 204 and 208 will form a sealed flow path whereby those portions of the channels 208 on all plates 101 A, 101B, 101C and 101 D in the top half U of the jacket 52 will form double depth channels with the channels 204 on the front and rear plates 100F and 100R. The double depth channels are in the vicinity of or adjacent to the combustion chamber 50A and are required to obtain increased flow rates of water through these portions of the channels to enable the withdrawing of a significant amount of heat being generated immediately around the combustion chamber 50A. In the lower half L of the plates 101A, 101B, 101C and 101 D a half width channel is formed between the channels 208 and the flat plate area 210 on the middle sides 200 of front and rear plates 101 F and 101R. Half depth channels are also formed on the left and right sides of the water jacket 52 by both the upper half U and lower half L of plates 101 A, 101B, 101C and 101 D. As illustrated in Figure 4 , and Figures 8 to 23, each pair of generally rectangular plates 53 and 54 are positioned in abutting contact to define a convoluted water path therebetween. The two plates 53 and 54 are identical. One plate has been inverted and placed back to back with the other so that the corners UL, LL, UR and LR on the plate 53 are placed adjacent to the respective corners LL, UL, LR and UR on the plate 54 (which is simply an inverted plate identical to plate 53). The expression 'back to back' in this specification and claims signifies the procedure of aligning plates, or the arrangement of aligned plates, whether identical or not, so that the concave sides of the dimples or channels formed in or on the plates are brought together to define a channel or flow path between the dimples or channels. Examples of back to back formations can be seen in figures 4, 23, 30, 37 and 38. The plate 53 (and likewise 54) has an array of discrete dimples 220, which have their longitudinal axis at approximately 45 degrees to the direction of flow of combustion products. The dimples 220 are all of similar size, width, depth and length and are angled at approximately 45° to the longitudinal axis of the plates 53 and 54. There are also odd shaped dimples, some of which have other purposes as described below, while others just serve as interconnections between different paths on the plates. The dimples 220 in plate 53 have their depth out of the page of the illustration, whereas because the plate 54 has been inverted, the dimples 220 of plate 54 have their depth into the page of Figure 4. When the plates 53 and 54 are joined together as illustrated in Figures 5, 13, 15 and 23 a channel or flow path is formed by the dimples 220 on plate 53 and dimples 220 on plate 54. Any two adjacent dimples 220 on plate 53 are interconnected by a corresponding dimple 220 on plate 54 so that water can flow from a first dimple in plate 53 along that dimple into the connecting dimple on plate 54, then along that dimple and out of plate 54 and back into plate 53 but into the next adjacent dimple to the first dimple mentioned on plate 53. In this way, as illustrated in Figure 10 there is formed a flow path which has a zig zag or sinusoidal configuration when viewed from the front of the heat exchanger 51. This is better illustrated with respect to Figure 16 to Figure 19. Water, after passing through the jacket 52, enters the heat exchanger 51 made up of a series of heat exchanger elements as illustrated in Figure 14, such that the dimple formation 230 and 231 mates respectively with dimple formation 231 and 230 on the plate 54. When the plates are placed adjacent to each other and fused or otherwise held together in a leak proof manner, as in Figures 13, Figure 15 and Figure 23, an upper header 232 is formed (from dimple 231 on plate 53 and dimple 230 on plate 54) and a lower header 233 is formed across the heat exchange (made up of dimple 230 on plate 53 and dimple 231 on plate 54). Water leaves the water jacket 52 via port 240 in Figure 4 and enters the lower header 233. From the lower header 233, water will flow through each of the heat exchangers (made up of pairs of rectangular plates 53 and 54) and as illustrated in Figure 10 will follow a zig zag or sinusoidal path when viewed in front elevation. The water will flow across the width of each heat exchanger element firstly along, flow path 242 near to the trailing edge of the heat exchangers; then into path 243 where it travels back across the width to the right hand side of the heat exchanger elements; then back along flow path 244 across the width to the end of the heat exchanger element opposite to the headers 233 and 232; and finally along flow path 245 back to the upper header 232. As illustrated in Figure 14, the dimples 220 form a channel through the both the plates 53 and 54 with the last portion of the flow path being illustrated. The water flows from the interior 220A" and crosses over to the interior 220A' and then into the interior of the differently shaped dimples 231 and 230 which form upper header 232 (see Figure 10). Whereas the combustion products travel through the passages 220B into the page of the illustration. This flow path (which is further described below) is repeated to make up each of the paths 242, 243, 244 and 245. As illustrated in Figure 15 hot combustion gases X flow downward in Figure 15 and pass over the leading edges 260 of each of the heat exchangers formed from pairs of plates 53 and 54. The combustion products flow through the heat exchanger 51 with a flow path as described in more detail with respect to Figures 20 to 23, until they pass over the trailing edges 261 of each of the elements formed from the plates 53 and 54. Simultaneously the water enters the heat exchanger 51 through entry 56 being on the outermost side of lower header 233. The water then flows through the paths 242, 243, 244 and 245 in each of the heat exchangers making its way via a zig zag/helical or sinusoidal/helical serpentine path up to the upper header 232 where it will exit the heat exchanger 51 by the outlet 57, which is on the same plate and heat exchanger that inlet 56 is formed on. If desired the inlet and outlet 56 and 57 can be located on opposite sides, and if the number of paths were to be varied, could even be located on opposite ends of the heat exchanger 51. The following will describe in more detail the water flow path through paths 242, 243, 244 and 245. As illustrated in Figure 16, Figure 17 and Figure 18, the water will flow in the direction of arrows 250 in the dimples 220 in plate 53, whilst in plate 54 the water will flow in the direction of arrows 251 as illustrated in Figure 16. The combination of these two flow paths are shown in Figure 18 (as if the plates 53 and 54 were transparent) and in the exploded three dimensional view of Figure 19. It can be seen that the flow path is such that after travelling through one of the dimples 220 in the plate 53, the water must change direction by approximately 90° to its direction of flow in the dimple 220 in plate 53 to enter the dimple 220 in plate 54. Once in plate 54, the flow will have to change direction again by approximately 90° for the flow to travel along the dimple 220 in the plate 54 and so on creating a complex convoluted flow path which can be described as generally helical, or corkscrew, or possibly even of a square or rectangular helical shape or other three dimensional flow path. The dimples 220 have a length as illustrated in Figure 24 to 27 of approximately 24mm, however, the straight line path length of the dimple 220 when a water path is formed is of only approximately 15mm because after a maximum of 15mm travel, water will have to change direction when flowing in the dimple 220. The depth of the dimple 220 out of the plate on each plate, is approximately 3.5 to 4.5mm. It has been found that by sizing the straight line length of travel in the dimples 220 to being in the ratio of approximately 2 to 10 times the depth of the dimple 220 there results a high level of mixing of the water flowing through the flow path thus ensuring even distribution of heat and prevention of hot spots to minimise the risks of scaling occurring in the heat exchanger 51. Once water has passed through the heat exchanger elements, the upper header 232 is connected to the outlet 15 of previous Figures to deliver water to the user or for the temperature to be sensed and output flow monitored. When adjacent heat exchanger elements are assembled as illustrated in Figure 13, Figure 15 and Figure 23, a convoluted combustion gas flow path is formed over and around the external surfaces of the dimples 220 as illustrated in Figures 20 to 22. As seen in Figures 20 to 22, the fan 30 will cause combustion products to pass over the leading edge of the heat exchanger elements and if the adjacent elements are arranged such that the external extremity of the outside surface of dimple 220 contacts the extremity of the external surface of an adjacent dimple 220 (on an adjacent heat exchanger element) then combustion products will flow around respective dimples 220 as illustrated in Figure 20. If a gap were provided between adjacent heat exchangers, then combustion products would flow not only around individual elements 220 but also between dimples 220 on adjacent plates effectively flowing over as well as around the dimples 220. Illustrated in Figure 21 is a similar flow path over the plate 54 which helps to provide an even more complex flow path between adjacent heat exchanger elements when plate 53 on one element contacts plate 54 on an adjacent element to produce a flow path as illustrated in Figure 22, which is highly a convoluted flow path and as illustrated is the path that can be present between adjacent elements formed from plates 53 and 54. The combination of the convolute and tortuous combustion products' flow path, together with the serpentine and zig zag helical water flow path, results in the heat exchanger 51 being extremely efficient to the point where, by the time the combustion products have passed from the leading edges 260 to the trailing edges 261 , the combustion products will have been cooled to a temperature whereby they may condense on the heat exchanger 51 in the vicinity of the trailing edge 261. Further, by drawing off the hot water for domestic use from the upper header 232 after the water has passed along the element near to the leading edge 260, the water will be at the highest temperature possible. The plates 53 and 54, as illustrated in Figure 4 have a flared end or flange F (which is also visible in Figure 14) that serves two purposes. The flange F forms a double width flange on the heat exchanger element when the two plates 53 and 54 are placed back to back as illustrated in Figure 14. This double width flange helps to provide greater surface area through which heat can be exchanged between the heat exchanger 51 and the water jacket 52. As the flange F extends a part of the way across the leading and trailing edges 260 and 261 as illustrated in Figure 4, the portion of the flange F which does this effectively acts like a dam or flow director which is useful to direct the flow of combustion products over the external surfaces of the dimples 220. Illustrated in Figure 28 is a heat exchanger element 300 which is configured differently to the heat exchanger element made from plates 53 and 54 of the previous Figures. In this heat exchanger element 300, a lower header 233 is provided and an upper header 232 is also provided in a similar manner to that of Figures 4, 9, 10 and 11. Also, the flow path segments 342, 343, 344 and 345 double back across the heat exchanger 300 in a serpentine fashion as in the previous embodiment. The difference between the heat exchanger element 300 of Figure 28 is that a zig zag/helical water flow path segments 342, 343, 344 and 345 provided, is such that the included angle 301 between the straight line sections or dimples 220 in path 342 is a larger angle compared to the included angle 302 in the path 243 above or the included angle 303 in the path 344, or the angle 304 in the path 345 closest to the leading edge 260. By varying the included angles 301, 302 and 303 in respective passes across the heat exchanger 300, even greater efficiency can be provided ensuring that the water passing, for example, through the path 345 near to the leading edge has a longer path to travel and possibly passing through the leading edge segment at a slower rate than through the flow path segment 342 at the trailing edge segment. This can help heat transfer process such that it may cause the combustion products to condense. The plates used to make the heat exchange element as illustrated in Figure 28 are identical. Illustrated in Figure 28A is a heat exchanger element 300 which is similarly configured to the embodiment as illustrated in Figure 28 and like part have been like numbered.. The difference between the heat exchanger element 300A of Figure 28A and 300 of Figure 28 is that at the trailing edge there is provided a flow path which begins from the lower header 233 and has full depth (that is on each of the plates dimples or a channel of equal depth are formed so that when they are placed together, a flow path results of a depth equal to twice the depth of one dimple or channel ). The first flow path 342A is generally of a straight configuration and runs across the full width of the heat exchanger 300A. While a full depth flow path is described, it may be necessary to either vary the depth or size it appropriately to ensure that a combustion products' flow path will be formed which will not be constricted to the point of preventing the escape of combustion products. Upon reaching the side opposite to the header 233, a zig zag/helical water flow path is provided, wherein the included angle 301 between the straight line sections is a relatively large angle compared to the included angle 302 in the row above or the included angle 303 in the row closest to the leading edge 260. The flow path segments 342A, 343A, 344A and 345A double back across the heat exchanger 300A in a serpentine fashion as in the previous embodiment. By varying the included angles 301, 302 and 303 in respective passes across the heat exchanger 300A, better efficiency can result, ensuring that the water passing, for example, through the leading edge has a longer path to travel and thereby remains in the combustion products' flow path longer than through the flow path segment 342 at the trailing edge segment. This can help the heat transfer process by producing greater mixing and turbulence such that it may cause the combustion products to condense. Further The angles need not be such that angles 301 > 302 >303, as circumstances may arise where a different relationship is found to be advantageous given the design criteria and conditions to be worked with. The effective cross-sectional area of the flow paths 342A, 343A, 344A and 345A can also be varied according to the requirements of operation of the heat exchanger. Also if desired, the amplitudes A, B and C of paths 343A, 344A and 345A might also be varied to optimise the efficiency and heat transfer. Illustrated in Figure 29 is a heat exchanger element of a further configuration useable with the present invention. The heater exchanger element 400 has an upper header 232 and a lower header 233, however, in this embodiment a lower header channel 442 is provided running from lower header 233 substantially parallel to the trailing edge 261 whilst an upper header channel 445 runs back to upper header 232 in a direction substantially parallel to the leading edge 260 of the heat exchanger. Between the lower header channel 442 and upper header channel 445 are a plurality of flow paths 443. In this embodiment, water enters the heat exchanger element 400 from the water jacket 52 via lower header 233 and fills the lower header channel 442. As combustion products are flowing from the top of the page through to the bottom, the most efficient direction of movement of liquid in the heat exchanger element 400 is in the opposite direction and thus by the arrangement illustrated in Figure 29, water travels through the flow paths 443 from lower header channel 442 up to upper header channel 445 and out of the heat exchanger element 400 via upper header 232. It will be noted in the Figure 29 that the flow path has a constant zig zag or sinusoidal path wherein the amplitude D of the zig zag/helical flow path measured is constant as is the included angle 401 between connecting dimples on the two plates Illustrated in Figure 29A is a heat exchanger element of a further configuration which is similar to that of Figure 29, with like parts being like numbered. The differences between the elements of Figures 29 and 29A is that in Figure 29A the flow path has a decreasing zig zag or sinusoidal path wherein the amplitude of the zig zag/helical flow path measured in the region D closest to the trailing edge, E in the middle and F at the leading edge are decreasing in size across the height of the heat exchanger element 400A. Also the included angle 401 and other included angles 402 and 403 are varied to minimise back pressure and/or resistance to flow through the heat exchanger 400. While the amplitudes D, E and F and angles 401 , 402 and 403 are illustrated as decreasing in size it is sufficient that they can be varied to produce results to suit the requirements demanded or desired from the heat exchanger 51. As illustrated in Figure 30, is another variation which can be applied to the heat exchanger elements and thus the heat exchanger 51 of Figures 4, 8 to 23, 28 and 29. In Figure 30 it can be seen that in the flow path 245, 345 or 445 of the previous embodiments, that the depth G of the flow path in the trailing edge 261 portions is greater than the depths H, I and J which progressively get smaller upwardly to the leading edge 260. When heat exchanger elements are placed adjacently together, what results, as illustrated in side view in Figure 30 is a combustion gas pathway 350 which decreases in volume from the leading edge 260 to the trailing edge 261. As illustrated in Figure 30, it can be seen that the cross sectional area of the flow path depth closest to the leading edge is generally smaller than the rest of the flow paths. The flow path depth can be sized so as to ensure that water passing therethrough will be sufficiently turbulent to increase the probability that thermal boundary layer mixing will occur. While the depths G, H I and J are decreasing in size in a particular direction, it may be advantageous to have the variation occur in some other pattern depending upon the requirements demanded or desired form the heat exchanger 51 By producing higher turbulence within water flow paths, the probability of scaling will be reduced. By also providing a tortuous or convoluted path such as a zig zag/helical path as previously described in the area closest to the leading edge, a high level of turbulence can be created. Whereas in the channels closest to the trailing edge, (with depths I, H and G) the risk of scaling is less likely in view of the lower temperatures found at these locations and thus if these dimples and the formed channels are longer or of larger cross sectional area, this may be of little consequence as the risk of scaling is diminished. By having longer or larger cross sectional area of channels on the cool side of the heat exchanger elements this is an opportunity to reduce the pressure drop, whereas in the prior art the fin and tube construction generally has the same size tube all the way through. Illustrated in Figure 31 to 33 is an instantaneous gas water heater similar to that of Figures 1 to 3, except that the combustion chamber 50A, water jacket 52 and heat exchanger 51 are formed in a different manner. Like parts between Figures 31 to 33 and 1 to 3 have been like numbered and their function and purpose need not be described further as reference can be made to earlier description. The water heater 10A of Figures 31 to 33 differs from the water heater 10 of Figures 1 to 3 in that a cold water bypass conduit 81 in the form of a tube 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 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 zig zag/helical flow path is tortuous or convoluted 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 10A 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. In the embodiment illustrated in Figures 31 to 33 the water jacket assembly 50' is constructed from a plurality of two different plates 550 of Figure 34 and 560 from Figure 35. The plate 550 of Figure 34 that makes up the central portion of the water jacket assembly 50' is of a generally Y shaped appearance. Each plate 550 includes a central 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 37 where only 3 pairs are illustrated for convenience) which form the central portion of a water jacket assembly 50'. Illustrated in Figure 35 is the plate 560 which is of a generally T shaped appearance. The plate 560 differs from the plate 550 in that at the base of the T in plate 560 there are no dimples and thus no channels formed in the middle portion, unlike in the base of the Y of the plate 550. This is because this flat region on the plate 560 is adjacent the base 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 37, four plates 560 are arranged placed in pairs and oriented back to back to form the end heat exchanger elements 580 of Figure 38. 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 an unbroken channel 244 and 244A which runs around the outer periphery of the plates 550 and 560. This unbroken channel 244 and 244A forms a water jacket type flow path. In plate 550, water fills lower header 233 and travels through the first row of dimples and the channel formed therefrom, upwards toward the upper header 232, then to the right of the plate in a "left and right' circuitous path 243 back down the left hand half of the plate 550 until it connects with the base of continuous path 244 on the outer periphery of plate 550. Once in the path 244 the water travels to the top of the arm 552 then back down arm 552 through path 245 and adjacent the leading edge 260 to upper header 232. A mirror image of this path is followed on the other half of the plate 550. By contrast the plate 560 of Figure 35, when two plates 560 are placed back to back (as illustrated in Figure 36), a lower header 233 is formed with water exiting to the left side of the lower header 233 following a continuous path 244A around the left hand side outer periphery of the plate 560 to the top of plate 560. At this point the flow path 245A takes the water in a downward direction until flow path 243A which takes the water in an upward direction and so on through downward path 242A, upward path 241 A then back down through downward path 240A to upper header 232. These serpentine flow path formations in the plates 560 and 550 result in an effectiveness which is added to by the convoluted flow path of similar shape to the previously described paths of previously described embodiments. As shown in Figure 38 two plates 550 are reversed which will allow them to be pressed and fused together to form joined pairs with flow paths therethrough. The plates 550 are made such that the dimple portions are identical, but during the manufacturing process flanges 600 (which will be described in more detail later) are oriented in different directions (thereby making the plates no longer identical) so as to form a front and back or a left and right side plate, which will be placed back to back to form a heat exchanger element. The pairs of Y shaped plates 550 are arranged in a vertical orientation and in parallel to form a combustion chamber 50A between the arms of the Y shape which form the front wall 100F and the rear wall 100R of the water jacket 52. The water jacket assembly 50' is completed by placing 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 52 thus forming the combustion chamber by completing the water jacket 52 therearound. The water jacket assembly 50' is thus defined as a block containing a rectangular combustion chamber 50A bounded by a water jacket 52 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 the 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 positions cut away from the base of the Y of the previous plates in the stamping or guillotining process. After the plates 550 and 560 have the dimples (as illustrated in Figures 34 and 35) formed in individual plates 560 and 550 by a stamping process, the flanges 600 can be bent so as to form an angled flange which is best illustrated in the schematic cross section of Figure 38. As can be seen from Figure 38 each plate 550 and 560 has a flange 600 which nest together. The angled flanges 600 are all oriented in the same direction irrespective of which plate is considered. This flange 600, provides the nesting ability of the plates 550 and 560, and thus helps to form the water jacket assembly 50' of Figure 37, with a relatively strong periphery due to the overlapping of these flanges 600. The angled flanges are arranged to overlap each other as the plates 550 and 560 are placed together in a sandwich. The angled flanges 600 have a slight taper of approximately 5 or 10 degrees so that the plates 550 and 560 can be loosely placed together in a wedged arrangement. By locating a bead of copper or nickel or other suitable material between adjacent flanges or using a copper or nickel coated stainless steel and heating and applying pressure simultaneously, fusion of the plates 550 and 560 will result in a water jacket assembly 50' as illustrated in Figures 31 to 33. 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 (Figure 37) having metal gauze 612 to isolate the comparatively cold surfaces of the heat exchanger 51 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 main body portion 51 of the panel would 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. This is best illustrated with respect to Figure 47, 49A and 49B and its associated description below. 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 one of 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: as a T shape, Y shape, U shape, C shape, E shape, H shape, V shape or any other appropriate shape. In the embodiment shown in Figures 39 and 40 a water jacket assembly 170 is constructed in a similar manner to the assembly described with reference to the embodiments above. However, in this case the plates 150 are of a U shape with a rectangular base structure 151 with a pair of upstanding arms 152, 153. As in the previous embodiment and the embodiments described below, the plate 150 illustrated and the heat exchange element made therefrom form intermediate elements. The end plates will be similarly structured to the plates 150 except that the base 151 will have no dimples, as dimples and channels are somewhat redundant on the end plates in those regions other than the water jacket region, due to the effectiveness of the water jacket assembly formed therefrom. The base structure effectively defines the body of the heat exchanger whilst the arms 152, 153 define the water jacket and the periphery of the combustion chamber in the same manner as the embodiment of Figure 31 to 38. The plates are superimposed in the same manner as the previous embodiment and similar manufacturing techniques can be used to ensure that the assembly is produced, or if desired without the need for brazing or welding, by mechanically clamping the plates and thus the heat exchange elements together. A cold water inlet 233 is positioned centrally at the base of the unit whilst the hot water outlet 232 is positioned also along the centre line of the top part of the rectangular heat exchanger element base structure 151. The combustion chamber is of rectangular configuration and houses the burner in the same manner as the embodiments described above. One of the main problems with instantaneous hot water systems is the breakdown of the heat exchangers caused through either components overheating or more particularly by the build up of scale. Many water supplies contain high concentrations of dissolved solids and once water is heated to a temperature which changes the water's chemistry the probability will be increased that scale deposits will form on the interior surfaces of the passageways of the heat exchanger until those passageways become clogged and fail. One means of reducing the likelihood of build up of scale deposits on the interior surfaces of the heat exchanger is to ensure that the water is heated evenly by preventing hot spots on the heat exchanger. One way to do this is to provide turbulent flow of water through the heat exchanger. The turbulence of the flow reduces the likelihood of scale deposits and tends to sweep particulate material out through the system. Consequently, the profile of the water passageways in the heat exchangers is sufficient to optimise the possibility of turbulence and to increase thermal boundary layer mixing which also enhances heat transfer and reduces the probability of scaling. Another means thought to be able to reduce the likelihood of scaling is to split the water flow path in the heat exchanger into two separate water passageways. Such a splitting of the water path circuit can be useful in combination or co bi- boilers or heaters which provide water at approximately 80°C for radiator usage which may be used in central room heating and at 50°C for potable water uses. Such a splitting of the water flow path is illustrated in Figure 42 where a heat exchanger 51 made from plates 550A and 550B (see discussion with respect to Figure 41 and 41 A below) has two water flow paths therethrough. The first water flow path is labelled with numeral 701 and the second with label 703. The water flow path 701 is a closed loop while water flow path 703 contains an open loop. In water flow path 703, cold water 709 enters the flow path 703 and is divided into two conduits 711 and 713 at junction 710. When a hot water tap connected to hot water outlet 719 is open cold water 709 will flow into both conduits 711 and 713. Conduit 71 1 passes water into the heat exchanger 51 to those areas near to the trailing edge 261. When the hot water tap is open, water will flow through conduit 713 into potable water passages 717 of a water to water heat exchanger 716. The heat exchanger 716 also has heating water passages 717A through it for the water in circuit 701 to heat the water in passages 717, and visa versa as will be described below. The water which may be heated in coil 717 passes via conduit 714 to meet up with water coming from the heat exchanger 51. The mixing of the two flows takes place at junction 718 so that the combined hot water can flow out of the water heater via outlet 719. Cold water bypasses, such as those described previously with respect to other embodiments, can be included together with temperature sensing, etc to ensure that the exiting water is cooled, if necessary, to the desired output temperature and to provide the best possible flow rate of hot water. The water, which flows through conduit 711 into the heat exchanger 51 , passes through those parts of the heat exchanger 51 heated by combustion products that are initially cooled by the water flow path 701 in the hottest parts of the heat exchanger 51. The closed loop water flow path 701 contains water preferably with glycol (or similar additive) for the purpose of reducing the likelihood of scaling. The water flow path 701 includes a series connection to a radiator 705, a pump 707 and heat exchanger 716. In operation, the water flow path 701 will deliver water along conduit 701B to the outer side of the heat exchanger 51. The heat exchanger 51 water flow path passes from the entry point up to the top of the water jacket assembly, then down the side of the combustion chamber through the water jacket, then through the hottest part of the heat exchanger 51 near to the leading edge 260, then up the other side of the water jacket and then delivers water at a desired temperature to the radiator 705, which can be used for central room heating radiator. All the time the combustion chamber is functioning, the pump 707 is circulating water through the water flow path 701. After the water has passed through the radiator 705 (and the radiator is functioning) the water temperature will have dropped. The temperature of the water that exits the radiator 705 will more than likely be above the water temperature of the water in the conduit 713 passing through heat exchanger 716. If the water temperature in passages 717A is higher than water in passages 717 then water in passages 717 will be heated by water in passages 717A. If valve 719 were to be closed however, without pump assistance, water would not circulate in water flow path 703. Thus, a pump 712 is provided to circulate water through the flow path 703, conduits 711 , 713 and 714 and passages 717. The pump 712 can be made to automatically switch on once no flow is detected in hot water outlet 719. In this way, if the temperature in 717A were less than 717, the water in the water flow path 703 (which is temporarily in a closed condition) will continue circulating and dissipating heat to the water from heat exchanger 717 into 717A. This process will keep water in a relatively hot condition, that is approximately 50°C in the closed loop. This also has the effect that the heat exchanger elements made up of plates 550A will be protected from overheating in view of the heat being drawn from the central portion of the heat exchanger 51. Another effect is that a ready supply of hot water will be available thereby decreasing the time lag to receive hot water, if the closed loop circuit 701 is in operation, that is, if the central heating option is in use. If the central heating option were to be not in use, the circuit 701 can be used to assist in the transferral of heat to the open loop circuit 703 thereby providing water at a relatively high flow rate but also, it is expected, to be with less thermal lag than by comparison with prior art systems. Illustrated in Figure 43 is a circuit very similar to that illustrated in Figure 42 and like parts have been like numbered. It is expected that the embodiment of Figure 43 will be able to provide hot water with less thermal lag than prior art systems. In this embodiment the heat exchanger 51 heats the cold water in the first instance. The heat exchanger 51 is in a series connection with passages 717 of heat exchanger 716, which is the potable water side of heat exchanger 716. In the situation where water (containing glycol) passes through the passages 717A of heat exchanger 716 is cooler than the water passing through passages 717, a bypass capability is provided by the opening and/or closing of valves 717B and/or 717C so that the bypass line 718A can connect directly from the start of conduit 713 to the hot water outlet. The same circuit can be used to ensure that the water jacket assembly 50 can be used not only as a hot water supply but also a heating mechanism whereby the closed circuit is used to feed a series of radiators positioned around a dwelling for the purpose of central room heating. Figure 41 and 41 A illustrate the profiles of a plate 550A and 550B to construct a water jacket assembly that can be used in the Figure 42 or 43 dual circuit approach. The end plates 550B are of a similar construction to the plates 560 of Figure 35. The panel has four apertures 795, 796, 797, 798 to form 4 headers, and are arranged vertically and in line centrally of the panel. The lower aperture 795 is the first water heating line (equivalent to item 703A of Figure 42) , the next aperture 796 constitutes the cold water input (equal to item 711 in Figure 42). The second water heating line which is the closed circuit is illustrated as the aperture 797 connects to 701B of Figure 42 and the hot water outlet aperture 798 is the third highest aperture and connects to 701 A of Figure 42. Whilst the plate 550A of Figure 41 only has inlets and outlets for two circuits illustrated, it will be readily understood that additional outlets and inlets can be provided on the heat exchanger plates and heat exchanger formed therefrom so that other water circuits can be connected. By this means hot water for a variety of purposes simultaneously is capable of being produced at relatively low cost as a result of the means of construction of the water jacket assembly. Illustrated in Figures 44 to 46 is a water heater 10B similar in construction to that of Figures 31 to 33 and like parts have been like numbered and their function and purpose need not be described further as reference can be made to earlier description. The difference between these two embodiments is that the plates 550B and 560B even though still of a generally Y and T shape are configured and 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 44 the flow path through the water jacket assembly 50B is illustrated. In Figure 44 it can be seen that the two flow paths are the mirror image of each other. Illustrated in Figures 47 to 49 is a further embodiment of the invention. Figure 47 is a front elevation of a water heater IOC having a water jacket assembly 50C, which differs from water jacket assemblies 50A and 50B of previous Figures. Illustrated in Figure 49A and 49B are the five component plates and the four plates assembled from the component plates, which when paired form heat exchanger elements, which when sandwiched form water jacket assembly 50C having combustion chamber 50A, a heat exchanger and a water jacket. A first plate is end plate 380A which effectively forms the ends or side portions of each of the plates illustrated in Figure 49 in exploded view. The second plate is 380D from which plates 380B and 380E are formed by the punching of an aperture therethrough. The third plate is 380C. The burner plate 380 is made from two end plates 380A and a central plate 380B. The plate 380B has a central circular aperture 381 therein. As the view in Figure 49 is from the top, the underside is not visible. However, once plates 380A has been joined to 380B they form a single plate which can be joined to a like plate (not illustrated) having a nesting flange similar to the flange 600 (but in the opposite direction relative to the dimple formations) such that the dimples on the surfaces of the back to back plates join together to form channels between the respective plates. In this way, a single heat exchanger element can be formed. Further, if desired, the plate 380A can be of a different metal to the plate 380B to take account of heat resistance of the metals used if such a requirement is necessary. The aperture 381 through the plate 380 will form a through hole in the corresponding heat exchanger whereby this element is used as the upper most element in the water jacket assembly 50C. This element in use has a fan shroud 382 passing through the aperture 381 and seals therewith as illustrated in Figures 47 and 48. The burner 20 as illustrated in Figure 47 is attached to the shroud 382 and is of a circular type that can radiate flame 360° therearound, or alternatively the burner 20 will send flame to the left and right sides of combustion chamber 50A. The plates 383 which form the combustion chamber 50A are constructed in a similar manner to the plate 380 except that the plate 380B is replaced by a channel plate 380C, with each plate 383 having two of these. The channel plates 380C conduct water from the left end plate 380A over to the right end plate 380A on plate 383. A stack of pairs of plates 383 form heat exchanger elements, as indicated in Figure 47 between the top plate 380 and a separation plate 390. Two of the separation plates 390 form an end heat exchanger element for the combustion chamber 50A. This end heat exchanger element is constructed from two plates made up of end plates 380A and a central plate 380D which in the central portion thereof has a flat closed section. If desired, the plate 380D could have a series of dimples across the flat closed section for the purpose of forming a channel through the central portion of the plate 380D. However, as this would increase the number of plates required, this may have an undesirable effect of increasing the overall cost to manufacture a water jacket assembly 50C. The plates 392 form flue elements (so described because they provide therein a flue by which the combustion products may escape). The plates 392 are made from two end plates 380A and a central plate 380E, which has an aperture 381A, which is preferably larger than aperture 381 of plate 380. Most plates 380A have an aperture 232A, which when adjacent plates are positioned back to back to form elements and the elements are sandwiched to form the water jacket assembly 50C, will form a left and right side header 390A and 390B when all plates and elements are assembled. It will be noted that the plate 380 has its right side aperture 232A blanked off so that water will not pass through the upper most plate 380. Similarly the lower most plate 392 is also blanked off to prevent water passing therethrough. The right side header 390B is thus formed between the uppermost plate 380 and lower most plate 392. It will be noted that plate 390 is blanked on the left side thereof so as to divide the left side header 390A into an upper and lower chamber, the purpose of which will be described below. It will be noted that all but the uppermost and lowermost of the end plates 380A have two apertures 388 thereon. These apertures 388 provide a passage 388A for the combustion products to move outwardly across the heat exchangers from the centre as illustrated in Figure 48 and out to the left and right sides. All the apertures 388 together and aligned form a left and right hand side downward passage as illustrated in Figure 48 whereby the exhaust gases will move past the separation plate 390 and then begin an inwardly directed motion across the flue plates 392 passing out of the central aperture 381 A and out of the flue 38 IB which is formed at the base of the heat water jacket assembly 50C. Figure 47 illustrates the water flow path through the heat exchanger elements which make up the water jacket assembly 50A, while Figure 48 illustrates the hot combustion gas flow path . The left and right headers 390 A and 390B and left and right side vertically oriented combustion paths 388A cannot be viewed easily from a single front elevation as they are located one in front of the other as Figures 49A and 49B illustrate. While the above illustrates and describes the plates 380, 383, 390 and 392 as being an assembly of plates, they could each be made from a single integral plate. As can be seen from Figure 47 the direction of the liquid flow path is upward from the cold water supply to fill a lower portion of left side header 390A. Once lower portion of header 390A is pressurised water flows from the left to right, across the flue plates 392, then up into right side header 390B. From header 390B the water travels from the right hand side of the plates 383, 390 and 380 into the upper portion of left side header 390A. The upper portion is a separate chamber to the lower portion in the region of the flue plates 392 with the hot water exiting at exit 15 as in the previous embodiments. A typical water flow path such as formed in elements formed by plates 392 is schematically shown at the bottom of Figure 49. As can be seen the shape of the path is zig-zag (and helical) as in the previously described embodiments. The zig- zag/helical path travels across half the depth and ultimately across the full width of the plates in a serpentine fashion. The difference with the heat exchangers formed by plate 383 is that the channel plate 380C has a straight line flow path straight across from the left hand side plate 380. The water jacket assembly 50C differs in that it is made up of 5 plates which are oriented in a generally horizontal arrangement. The top most plate as illustrated in Figure 37 substantially fully closed except for a circular aperture 381 in the middle thereof. Two such plates are placed back to back to form the upper element through which the fan shroud 382 passes and seals therewith. The burner 20 in this instance, is attached to the shroud 382, and is of a circular type that will radiate flame in 360 degrees therearound. The combustion products, under the influence of the fan travel outwardly across heat exchangers formed from plates 384 as illustrated in Figure 48. The plates 384 have a relatively large cavity 386 which provides the formation of the combustion chamber 50A when several elements are stacked on top of each other. The combustion products move outwardly until the downward exhaust passages 388A are reached. Then the passages 388A take the combustion products downwardly through the water jacket assembly until the middle heat exchanger 390 has been passed. This ensures that all combustion products must exhaust form the combustion chamber in a horizontal direction. Once past the element 390 has been passed, the combustion products can then travel between the heat exchanger elements 392 in the portion of the water jacket assembly below element 390. The water flow path is illustrated in Figure 36 and flows from the left hand side of the water jacket assembly 50C to the right hand side. The channel or water path formed by the dimples in the plates 381 , 392, 390 and 384 has a continuous path around each element and a zig zag or sinusoidal path through the middle of the plate. The zig zag or sinusoidal path crosses the elements in a serpentine manner doubling back across the elements to increase the efficiency thereof. Illustrated in Figure 50 is a water heater having a storage vessel or accumulator 450 to receive hot water once a hot water tap has been closed. The accumulator 450 is of a generally circular construction but can be any appropriate shape. The accumulator 450 receives hot water from the water heater by suitable control systems and valves provided in the heater. This hot water, once the hot water is closed, would otherwise remain in the water jacket assemblies described above. Diverting the hot water to accumulator 450 ensures that stagnant hot water, which is prone to scale because of the risk of the temperature increasing, will not remain in the water jacket assembly. The accumulator 450 is able to be used as a storage of hot water to minimise the lag time to deliver hot water to the hot water tap. In some countries this lag time has to be minimised so as to preserve water. The accumulator 450 has a flexible membrane 451 , whereby hot water is received in the chamber 452 on the water heater side of membrane 451 whereas the chamber 453 will have air therein or will vent to atmosphere allowing the membrane 451 to minimise the volume of chamber 453 when hot water fills chamber 452. The accumulator need only be of a volume sufficient to hold all the hot water in the water jacket assembly with a 10% to 15% extra volume as a safety factor. Illustrated in Figure 51 is a schematic representation of a heat exchanger 430 made up of elements 431 formed from plates 432 which can be like any of the previously described embodiments. The heat exchange 430 is made from a series of adjacent elements 431 made from plates 432 which are preferably identical but need not necessarily be so. The improvement in Figure 51 is the provision of a leading edge plate 433 which is joined to what would have been the leading edges 434 of elements 431. The joining can be by fusing, soldering, bronzing, welding or any appropriate means which provides good surface contact for heat transfer purposes between plates 433 and plates 432. The leading edge plates 433 are manufactured from a metal having high temperature resistance qualities such as appropriate stainless steel or titanium and/or its appropriate alloys. The leading edges 435 of the plates 433 are at right angles to each other. The leading edges 435 are angled in this way to support the combustion process preventing quenching of the heat coming from the combustion chamber which would normally tend to denigrate the combustion process. Notwithstanding this, the shape of the leading edges of the plates 433 can be any appropriate shape to protect the plate 433 in this arrangement and to help support the combustion process. The leading edges 435 and plates 433 can help to reduce C0 2 emissions from the combustion chamber by helping to maintain heat therein to support the combustion process. In Figure 52 is a heat exchanger with the flow path shown, for illustration purposes only, such as it would be if the plates were manufactured from perspex or some clear material. The shape of the plate is of a Y configuration similar to some of the previous embodiments and the flow path is formed in much the same way and has much the same characteristics. The plate of Figure 52 and the heat exchanger formed therefrom has a difference with the elements of previous embodiments in that at two points in the flow path an opportunity for crossing over is provided. These cross overs are at points 527A and 527B. In the flow path water travels from the lower header 233, out to the left and right hand sides thereof, then upwardly and back towards the centre to approach the first cross over at 527B. Depending upon water pressure and other physical characteristics the two flows may either cross over each other; pass through each other; or generally fill the central dimples of the cross over and then from there radiate outwardly in the right and left hand direction. It is possible that the water may not actually cross over but rather may keep to the respective halves of the plate. After passing the first cross over 527B water will radiate towards the left and right sides then up around the outside then up the arms 552 and 553 of the Y shape back, then down towards the central portion of the plate. The path then approaches the cross over 327A from the left and right sides. At cross over 527A, again, depending upon physical characteristics the water may or may not cross over; may or may not pass over; under or through. However water will continue to flow from the central dimples of the cross over in an outward direction along the predefined paths making its way back along the leading edge into the upper header 232. In this embodiment it can be seen that the leading edge 260 has an appropriately scalloped or shaped leading edge so that the material of the leading edge is such that along any point thereon the distance to the closest channel or dimple is roughly the same across the same of the whole leading edge. This ensures that hot spots which might otherwise have formed because of being a further distance away from the channel or dimples will not be created. The construction and flow path illustrated in Figure 52 is thought to be advantageous in that if any one portion of the flow path were to be effected by scale, water could still flow through at least half the combustion chamber. Illustrated in Figure 53 is an end heat exchanger which could be utilised with the embodiment of Figure 52. Whilst there are no cross overs in this element and it is constructed in a similar manner to the plate 560 of Figure 35 (and like numbers have been used for like parts). A special feature of the plate of Figure 53 is that a bypass passage 529A is provided between the lower header 233 and the upper header 232. The use of such a bypass passage can be advantageous in that a lesser pressure drop can be sustained. Illustrated in Figure 54 is a water heater 10D which uses a water jacket assembly 50D similar to water jacket assembly 50B of Figures 44 to 47. Like parts have been like numbered and their function and purpose need not be described further as reference can be made to earlier description. The difference between the water heater 10D and 10B of Figures 44 to 47, is that he water jacket assembly 50D is utilised with a burner 20 which is naturally aspirated and is oriented so that the combustion products flow in an upward direction. As can be seen in Figure 54, the water jacket assembly 50D is rotated through 180 degrees by comparison to water jacker assembly 50B of Figures 44 to 47. Each of the embodiments previously described above do or can use a cold water bypass which helps to increase the pressure at the output as well as to control temperature. By providing electronically adjustable valves on such bypasses (although no valve is envisaged for the bypass 529A of Figure 53) the bypass can be adjustable to cope with conditions as sensed by the control unit 80 which may need to increase or decrease the flow through the bypass thereby achieving both hot water and a higher pressure at output. One of the features of the water heaters of the present invention is that several prior art instantaneous water heaters tend to have a serially connected water passage through the heat exchanger. One of the distinct advantages of the present invention is the ability to "gang up" or "connect in parallel" heat exchanger elements made up of similarly shaped plates which can result in a lower pressure drop from the inlet to the outlet of the heat exchanger in view of many parallel flows occurring simultaneously through the heat exchanger elements. The above description and method of manufacturing a heat exchanger, water jacket and combustion chamber assembly is such that one of the advantages is that the construction and features allow for a modular and scalable unit. The term scalable is used in the sense of being able to scale the size up or down according to the amount of Mega Joules utilised or required from the hot water system. For example, in a hot water system that would utilise approximately 200 Mega Joules of energy per hour, the combustion chamber/water jacket/heater exchanger assembly of Figure 37 which will have effectively two end elements and three heat exchanger elements would be of a size generally suitable for example for say 60 to 90 Mega Joules of heat throughput. To increase this to a 200 Mega Joule unit some twelve heat exchanger elements made up of some 24 plates would be utilised for the heat exchanger portion and four plates to form two end heat exchanger elements to encase and form the combustion chamber and ends could be provided. Of course, a burner size would have to be increased to allow for the greater width and to ensure that combustion flow passes across the whole width of the heat exchanger portion or the combustion chamber width. It will be readily understood that while the description of the above embodiments is generally directed to an externally mounted, domestic gas fired instantaneous, fully condensing, fan forced downwardly drafted water heater, that the inventions described herein are applicable not only to this specific type of water heater but also to instantaneous water heaters that may have different characteristics such as: • naturally aspirated or natural draft burner systems • systems which can be mounted or located either internally or externally of a building • with or without fully condensing heat exchanger operation • commercial, industrial or domestic systems • orientation of combustion chamber and combustion gas flow path can be in any one of the following directions: upwardly; downwardly; side ways directed; directed at an angle to the horizontal and or vertical. While the above descriptions of the embodiments generally utilises identical plates to form heat exchanger elements, it will also be understood that non identical plates can be utilised. By the use of non identical plates different shaped flow paths can be produced to those described above which in the main tend to be somewhat regular and in some cases symmetrical in front elevation. However, to produce a flow path such as a saw tooth shape made up of one vertical path length connected by an angled path length, will require the use of non identical plates if dimples are to be formed on both plates. Clearly, if a flow path is formed from a plate having a continuous channel therein and is joined to a flat plate, the shape of the path of the continuous channel can be any desired shape. 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. For example fusing is the preferred process described above to join and hold plates and adjacent plates together. This can be substituted by any joining or holding process such as suitable adhesives, mechanical clamping systems with appropriate sealing mechanisms; welding etc.

Claims: Claims 1. A water heater heat exchanger element being formed from first and second plates joined together to form at least one channel therebetween to provide at least one liquid flow path inside of said element and a combustion heat transfer surface on the outside thereof, said element being characterised by said at least one flow path each consisting of a single path which extends across a portion of said element in one direction and across said portion in the opposite direction, in a serpentine manner. 2. A water heater heat exchanger element as claimed in claim 1 wherein said flow path extends across the full width of said element. 3. A water heater heat exchanger element as claimed in any one of claims 1 or 2 wherein said flow path extends across said plate in a zig zag or sinusoidal configuration. 4. A water heater heat exchanger element as claimed in any one of claims 1 to 3 wherein said first plate has a single continuous groove whereby when a flat second plate is joined thereto, said channel is formed. 5. A water heater heat exchanger element as claimed in any one of claims 1 to 4 wherein said first and second plates each have a series of discrete dimples therein, whereby adjacent dimples on said first plate are connected by the series of adjacent and partially overlapping dimples on said second plate to form said channel. 6. A water heater heat exchanger element as claimed in any one of claims 1 to 5 wherein said serpentine manner is such that the flow crosses the element at least twice. 7. A water heater heat exchanger element as claimed in any one of claims 1 to 6 wherein said water flow path is shaped like one of the following: generally helical; cork screw; square helical; or vortex shaped. 8. A water heater heat exchanger element as claimed in any one of claims 5 to 7 wherein said flow path requires liquid flowing therein to travel along one of said dimples in said first plate having a straight line pathway then flow through an approximately 90° direction change into said second plate then through an approximately 90° direction change to flow through an adjacent dimple in said second plate in a straight line pathway through said adjacent dimple. 9. A water heater heat exchanger element as claimed in any one of claims 1 to 8 wherein said dimples provide a straight line path after transition from said first plate to said second plate whereby the maximum length of the straight line path is in the range of three to seven times the depth or height of the dimple. 10. A water heater heat exchanger element as claimed in any one of the preceding claims wherein said first and second plates each have a flared end extending away from a joining plane of said first and second plates. 1 1. A water heater heat exchanger element as claimed in of claim 10 wherein said flared ends extend along the side edges of said plate from a leading edge to a trailing edge. 12. A water heater heat exchanger element as claimed in claim 10 or 11 wherein said flared ends extend for a distance in the direction towards the centre of said plate along leading and trailing edges. 13. A water heater heat exchanger element as claimed in any one of the preceding claims wherein said element is formed from identical plates placed back to back. 14. A water heater heat exchanger element as claimed in any one of the preceding claims wherein said heat exchanger and or plates forming said heat exchanger have a nestable shape. 15. A water heater heat exchanger element as claimed in any one of the preceding claims wherein said heat exchanger element has a shape which includes a main portion and at least two arms extending away from the main portion. 16. A water heater heat exchanger element as claimed in claim 15 wherein 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 17. A water heater heat exchanger element as claimed in claim 16 wherein the arms of the element when placed near to adjacent elements form a water jacket around a combustion chamber. 18. A water heater heat exchanger element as claimed in any one of claims 15 wherein said heat exchanger element has a shape whereby the tow arms extend in opposite directions to each other, such as in a T shape. 19. A water heater heat exchanger element as claimed in claim 18 wherein the cross bar of said T shape forms an end wall of said combustion chamber. 20. A water heater heat exchanger element as claimed in any one of the preceding claims wherein said element includes at least one dimple formation thereon whereby when two or more of such elements are position side by side, said dimple formations are aligned to form a header which can receive a liquid and which will direct said liquid through each of said heat exchanger elements simultaneously. 21. A water heater heat exchanger element as claimed in any one of the preceding claims wherein the leading edge is profiled to substantially follow the flow path. 22. A water heater heat exchanger element as claimed in claim 21 wherein points along said leading edge have a minimum distance to the nearest channel such that said minimum distances are similar. 23. A water heater heat exchanger element as claimed in any one of the preceding claims, wherein said plates each have a formation to allow all plates to be rested together prior to fusing. 24. A water heater heat exchanger element as claimed in claims 23 to 26 wherein said formation is a flange on said first and second plates such that when placed back to back said flanges all extend in the same general direction. 25. A water heater heat exchanger element as claimed in claim 24 wherein said flange is at an angle to said plate. 26. A water heater heat exchanger element as claimed in claims 24 or 25 wherein said flange extends partially around the periphery of said plates. 27. A water heater heat exchanger element as claimed in claims 24 or 25 wherein said flange extends wholly around the periphery of said plates. 28. A water heater heat exchanger element being formed from a first plate and a second plate forming a channel therebetween to form a liquid flow path inside of said heat exchanger and a heat transfer surface on the outside of said heat exchanger wherein the configuration of said liquid flow path and said heat transfer surface is varied across the width of said exchange element. 29. A water heater heat exchanger element as claimed in claim 28 wherein said first plate has a single continuous groove whereby when a flat second plate is joined thereto said flow path is formed. 30. A water heater heat exchanger element as claimed in claim 28 wherein said first and second plate each have a series of discrete dimples whereby adjacent dimples on said first plate are connected by dimples on said second plate to form said liquid flow path. 31. A water heater heat exchanger element as claimed in any one of claims 28 to 30 wherein said flow path is of one of the following: a zig zag configuration; or a sinusoidal configuration. 32. A water heater heat exchanger element as claimed in any one of the claims 28 to 31 wherein said flow path is configured to provide at or near to a leading edge of said element, a different length of straight line sections compared with the length of straight line section in the vicinity of a trailing edge of said element. 33. A water heater heat exchanger element as claimed in any one of claims 28 to 32 wherein said flow path has a single path which extends across all or part of said element in a serpentine manner. 34. A water heater heat exchanger element as claimed in any one of claims 28 to 33 wherein said flow path extends across part or all of said element at least two times. 35. A water heater heat exchanger element as claimed in any one of claims 28 to 34 wherein the included angle between lengths of dimples or segments of channels on said first and/or second plate in the vicinity of a leading edge is varied by comparison to the included angle in the region of a trailing edge. 36. A water heater heat exchanger element as claimed in any one of claims 31 to 34 wherein the amplitude of said zig zag or sinusoidal configuration is varied in said flow path in the vicinity of said leading edge by comparison to the amplitude of said zig zag or sinusoidal configuration in the vicinity of said trailing edge. 37. A water heater heat exchanger element as claimed in any one of claims 28 to 32 wherein said flow path divides into a multiple number of parallel liquid flow paths connecting 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. 38. A water heater heat exchanger element as claimed in any one of the preceding claims wherein the thickness of said heat exchanger element increases from said leading edge to said trailing edge. 39. A water heater heat exchanger element as claimed in any one of the preceding claims wherein the depth of said dimples on said plate or plates increases from said leading edge to said trailing edge. 40. A water heater heat exchanger element as claimed in any one of the claims 38 or 39 wherein when two or more elements are positioned adjacent to each other a combustion products' flow path is formed between adjacent plates, said combustion products' flow path, near to said leading edges being of a different cross sectional area than at said trailing edges. 41. A water heater heat exchanger element as claimed in any one of claims 1 to 40, wherein the element is formed from plates made from two or more plate segments which are bonded together to form a composite single plate. 42. A water heater heat exchanger element as claimed in claim 41, wherein said plate can have segments are made from different materials. 43. A water heater heat exchanger element as claimed in any one of claims 1 to 42, wherein said heat exchange elements have a leading edge formed of a different material to the rest of the heat exchange element which contains said dimples or channels. 44. A water heater heat exchanger element as claimed in claim 43 wherein said leading edges have a shape or result in the effect of promoting combustion in the combustion chamber. 45. A water heater heat exchanger element as claimed in any one of claims 1 to 44, wherein said heat exchanger element includes a by pass channel which connects an entry header to an exit header. 46. A water heater heat exchanger element as claimed in any one of claims 1 to 45, wherein the water flow paths in said heat exchanger element cross over each other at predetermined points to enable water flowing therein to mix with, pass through, or pass over and under, each other. 47. A water heater heat exchanger element as claimed in any one of claims 1 to 46, wherein said water heater heat exchanger element 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. 48. A water heater heat exchanger element as claimed in claim 47, wherein when in use the hottest parts of the heat exchanger element receives a first circuit, while a second circuit is heated in a cooler part of the element. 49. A water heater heat exchanger element as claimed in any one of claims 1 to 48, wherein said water heater heat exchange element has in addition to a series of discrete dimples, a continuous peripheral path to serve a water jacket function. 50. A heat exchanger formed from a plurality of heat exchanger elements as claimed in any one of claims 1 to 49, said elements being like oriented in said heat exchanger and placed in parallel. 51. A heat exchanger as claimed in claim 50 wherein the outside surfaces of dimples of said first plate of one element make contact with outside surfaces of dimples on said second plate at discrete lines or points of contact. 52. A heat exchanger as claimed in claim 51 wherein said discrete lines or points of contact are one of the following: held; joined; joined by fusion; joined by brazing; joined by soldering; joined by diffusion bonding. 53. A heat exchanger as claimed in claim 52 wherein 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. 54. A water jacket assembly for an instantaneous gas fired water heater, the assembly including plates having therein an array of dimples, 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. 55. A water jacket assembly as claimed in claim 54 including a heat exchange element as claimed in any one of claims 1 to 49. 56. A water jacket assembly as claimed in claim 54 including a heat exchanger as claimed in any one of claims 50 to 53. 57. A water jacket assembly as claimed in claim 54 wherein said water jacket assembly is formed from a plurality of plates including at least a first plate to form a heat exchanger as claimed in any one of claims 15 to 17 and at least a second plate to form a second heat exchanger as claimed in any one of claims 18 or 19 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. 58. A water jacket assembly as claimed in any one of claims 55 to 57 wherein said elements are 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. 59. A water jacket assembly as claimed in any one of claims 55 to 57 wherein said elements are 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. 60. A water jacket assembly as claimed in any one of claims 55 to 57 wherein said elements are generally horizontally oriented. 61. A water jacket assembly as claimed in claim 60 wherein said elements include apertures therethrough to permit combustion products to flow between pairs of elements. 62. A water jacket assembly as claimed in any one of claim 54 to 61 wherein the plates of the heat exchanger are adapted to cause turbulent flow of water through the water passages. 63. A water jacket assembly as claimed in any one of claims 54 to 62 wherein the plates of the heat exchanger are adapted to cause turbulent flow of combusted gases past the exterior. 64. A water jacket assembly as claimed in any one of claims 54 to 63 wherein the plates of the heat exchanger are such that their exterior surfaces also provide an escape path for condensate that forms in use. 65. A water heater having a heat exchanger element as claimed in any one of claims 1 to 49. 66. A water heater having a water jacket assembly as claimed in any one of claims 54 to 65. 67. A water heater as claimed in claim 65 or 66 further water including a storage means to receive hot water which would otherwise remain in said apparatus when a user has closed a valve preventing further hot water passing through said valve. 68. A water heater as claimed in claim 67 wherein hot water in said storage means is passed through said valve once said valve is re-opened. 69. A water heater system having at least two water flow paths, with both paths passing through a water/gas heat exchanger, which transfers heat from combustion products to water contained in said circuits, a first of said at least two paths including a serial connection to a radiator means and a serial connection to a water/water heat exchanger where water in said first path can transfer heat to or receive heat from water in said second path. Preferably said water in said first path is in a closed loop. 70. A water heater system as claimed in claim 69, wherein said second of said at least two paths includes a cold water inlet. 71. A water heater system as claimed in claim 70, wherein said cold water inlet is split into two water flow sub- paths, a first sub-path to deliver water to said water/water heat exchanger and a second sub-path to deliver water said water/gas heat exchanger. 72. A water heater system as claimed in claim 71 , wherein said second sub-path can merge with said first sub path for water to flow out of said system, when a valve on an outlet conduit from said system is in an open condition. 73. A water heater system as claimed in claim 71 , wherein when a valve on an outlet conduit from said system is in a closed condition water in said first and second sub-paths is circulated. 74. A method of manufacturing a water jacket assembly including making profiled heat exchanger plates, placing pairs of plates together to form a heat exchanger element, placing a plurality of heat exchanger plate elements together to form a sandwich, said assembly having a combustion chamber and combustion products passages and water passages within said assembly. 75. A method as claimed in claim 74, wherein two type of heat exchanger elements are formed, end elements and intermediate elements with said end elements having a different water path to said intermediate elements. 76. A method as claimed in claim 74 or 75, wherein said heat exchanger plates are manufactured such that those portions not required on a blank for one plate are a part of the next successive plate stamped. 77. A method as claimed in claimed in any one of claims 74 to 76, wherein said heat exchanger elements can be assembled in parallel, which can be any one of the following: vertically oriented the elements can extend such that their leading edges run generally parallel to the width of the combustion or water heater into which the water jacket assembly will be installed; vertically oriented the elements can extend such that their leading edges run generally parallel to the depth of the combustion or water heater into which the water jacket assembly will be installed; or horizontally oriented.

Inventors: ADAM QUENTIN ARTHUR CARL, CAMP MARTIN HENRY, MANLEY DAVID WILLIAM, MASTALERZ KRZYSZTOS, ADAM, QUENTIN, ARTHUR, CARL, CAMP, MARTIN, HENRY, MANLEY, DAVID, WILLIAM, MASTALERZ, KRZYSZTOS
Applicants: ADAM QUENTIN ARTHUR CARL, CAMP MARTIN HENRY, MANLEY DAVID WILLIAM, MASTALERZ KRZYSZTOS, SOUTHCORP AUSTRALIA PTY LTD, ADAM, QUENTIN, ARTHUR, CARL, CAMP, MARTIN, HENRY, MANLEY, DAVID, WILLIAM, MASTALERZ, KRZYSZTOS, SOUTHCORP AUSTRALIA PTY. LTD.
Application year: 1999
Main classification: F24H1
IPC codes: F24H1, F24H9, F28D9, F28F3