GB2390133A

GB2390133A - Instantaneous water heater can

Info

Publication number: GB2390133A
Application number: GB0214534A
Authority: GB
Inventors: Dominic George Webber; Jocelin Langford; Robin Craig Cocker; Peter Georg Laitenberger
Original assignee: Aqualisa Products Ltd
Current assignee: Aqualisa Products Ltd
Priority date: 2002-06-24
Filing date: 2002-06-24
Publication date: 2003-12-31
Anticipated expiration: 2022-06-24
Also published as: GB2390144B; GB2390133B; GB0314022D0; GB2390144A; GB0214534D0

Abstract

The heater, that may be used in a shower, has an outer housing 14, a tubular chimney 16, and at least one heater element 18. The heater element is coiled around the chimney with the coils co-axial with the longitudinal axis of both the chimney and outer housing. The end portions of the heater element exit through a side wall 40 of the outer housing in a perpendicular direction to the longitudinal axis of the coils, chimney, and housing. An end wall 145 of the housing may be provided with a protrusion 147 to smoothen the flow of water toward the chimney and reduce turbulence to help to minimise the pressure drop between an inlet and an outlet of the heater. A temperature sensor 50 and control means may also be provided for the heater.

Description

INSTANTANEOUS WATER HEATER

The present invention relates to an instantaneous water heater, in particular, an instantaneous water heater for a shower. The instantaneous water heater includes 5 a heater can which is optimised for improved overall performance.

Heater cans of instantaneous water heaters of the prior art have a heating

chamber defined between a cylindrical outer housing and a cylindrical inner chimney disposed in the housing and sharing a common longitudinal axis with the housing.

Water enters the heater can from the bottom between the outer housing and the inner 10 chimney. The water rises up between the side walls of the outer housing and the inner chimney where it is heated by coiled heater elements. Heated water rises to the top and is forced down the chimney through an outlet defined by an open end of the chimney. The top of the heater can is formed by a copper plate. At least one heater element is used to heat the water. A medial portion of that heater element is formed 15 in coils around the chimney and end portions of the coils are bent to protrude through the top copper plate of the outer housing. A temperature sensor is positioned on the copper plate which interacts with a control means to control power to the heater element. A problem with the heater can of the prior art is that sharp bends of a small

20 radius need to be made in the heater element between the medial portion and the end portions. This has several disadvantages. First, the lifetime of the heater element is reduced by having small radius bends as well as by having a large number of bends.

Second, the prior art arrangement of heater element requires a great amount of

material per number of coils.

25 The present invention provides an instantaneous water heater including a heater can, wherein said heater can has: an outer housing; an inner tubular chimney with a longitudinal axis and positioned inside said outer housing; 30 a heating chamber with an inlet and an outlet, said heating chamber being

defined between an inner surface of walls of said outer housing and an outer surface of walls of said inner tubular chimney; and at least one heater element; wherein a medial portion of said at least one heater element is disposed in 5 said heating chamber and formed in coils around said longitudinal axis of said chimney, end portions of said at least one heater element, either side of said medial portion, being substantially perpendicular to said longitudinal axis of said chimney.

In this way, the ratio of number of bends to number of coils is lower than in heater elements of the prior art.

10 Preferably the heater element is formed such that bends in said coils of said heater element have the smallest radius of all bends in said heater element. In this way, the lifetime of the heater element, for a given radius of coils, is further increased over the heater elements of the prior art.

Preferably the end portions protrude through a side wall of the walls of the 15 outer housing, the side wall being substantially parallel to the longitudinal axis of the chimney. This arrangement ensures that the minimum amount of material is used for the heater elements.

Preferably the coils and the walls of the chimney have substantially the same crQss-sectional shape in a plane perpendicular to the longitudinal axis of the chimney.

20 In this way, the most efficient use is made of the space between the chimney and the outer housing.

Preferably the coils have an elongate shape in a plane perpendicular to the longitudinal axis of the chimney. This allows the cross-sectional area of both the chimney and the heating chamber perpendicular to the longitudinal axis of the 25 chimney to be increased without increasing the depth of the heater can. An increased area results in a lower pressure drop in the heater can which is advantageous.

Furthermore, a low profile (i.e. low depth) heater can is more aesthetically pleasing and therefore also advantageous.

Preferably the coils substantially have the shape of a lozenge in a plane 30 perpendicular to the axis of the chimney. It has been found that the lozenge shape is

-3 a particularly good shape for having a large cross-sectional area in a plane perpendicular to the longitudinal axis of the chimney in the heating chamber whilst maintaining a low profile of the heater can.

A further problem with the heater can of the prior art is that it has a high

5 pressure drop because of the turbulence and various accelerations imparted to water as it flows from the inlet of the heating chamber to the outlet.

The present invention provides an instantaneous water heater including, a heater can, wherein said heater can has: a heating chamber with an inlet and an outlet, said heating chamber being 10 defined between an inner surface of walls of an outer housing and an outer surface of walls of an inner tubular chimney, said chimney being positioned in spaced apart relationship from and within said outer housing, and said outlet being defined by an open end of said tubular chimney positioned facing an end wall of said walls of said outer housing, 15 wherein said end wall of said outer housing includes a central protrusion extending into said chamber and positioned and shaped such that water flowing from between said side walls of said outer housing and said chimney towards said end wall has its direction of flow changed gradually by interaction with said protrusion towards said outlet.

20 In this way, the introduction of turbulence in water as it transfers from

flowing between the side walls of the outer housing and the chimney through the outlet into the chimney is minimised. This is achieved as water from one side of the chimney in a plane perpendicular to the longitudinal axis will not collide with water coming in the opposite direction from the other side of the chimney meeting at a 25 central portion adjacent the end wall thereby inducing turbulence.

Preferably the end wall is shaped to minimise a pressure drop between the inlet and the outlet of the chimney. This is advantageous because a high flow rate of water through the instantaneous water heater is desired and having, for example, a shower with a high flow rate of water is a more pleasurable experience than having a 30 shower with a low flow rate of water in which it can be difficult to remove soap from

-4 the skin or shampoo from the hair.

Preferably an outer portion of the end wall adjacent the side walls of the outer housing is curved towards the side walls of the outer housing to form a smooth transition from the side walls to the end wall. Also, preferably a medial portion of 5 the end wall next to the central protrusion is curved towards the central protrusion to form a smooth transition from the medial portion to the central portion. These features are advantageous as they help still further to decrease the pressure drop between the inlet and the outlet.

Preferably inner sides of the walls of the chimney define a water exit passage 10 for water from the outlet, the exit passage having a crosssectional area perpendicular to the axis of the chimney, and the heating chamber having a cross-sectional area perpendicular to the axis of the chimney, the cross-sectional areas being substantially the same. A heater can with this arrangement is particularly advantageous because it means that there are no constrictions in the flow path of water through the heater can 15 thereby serving to minimise the pressure drop through the heater can.

Another disadvantage with the heater can of the prior art is that there is an

abrupt transition from the smooth side walls of the outer housing to the end wall of the outer housing. This abrupt transition results in turbulence being set up in the heating chamber. Also, a large amount of heater element material is required for the 20 end portions to be long enough to exit through the end wall.

The present invention provides an instantaneous water heater comprising a heater can, wherein said heater can has: a heating chamber with an inlet and an outlet, said heating chamber being defined between an inner surface of walls of an outer housing and an outer surface of 25 walls of an inner tubular chimney with a longitudinal axis and which is positioned inside of said outer housing; and at least one heater element; wherein end portions of said heater element protrude through a metal member of said housing which at least partly forms a side wall of said walls of said housing 30 and is shaped on its inner side to conform to the overall shape of said inner side of

-5 said walls of said housing.

In this way abrupt transitions in the heating chamber are avoided and the conformity of the overall shape of the outer housing ensures uniform heating of the water by the heater element. Also, the amount of heater element material required is 5 reduced as the end portions may be shorter than in prior art heater cans. The heating

chamber has at least two planes of symmetry parallel to said longitudinal axis.

Preferably the chimney has a cross-sectional shape in a plane perpendicular to its longitudinal axis and the outer housing has a crosssectional shape in a plane perpendicular to the longitudinal axis, the cross-sectional shapes being substantially 10 the same and wherein part of the cross-sectional shape of the housing is defined by the metal member. This arrangement ensures that the amount of water between the chimney and the housing is the same all around the chimney such that uniform heating of the water results.

Preferably the overall shape of the housing in a plane perpendicular to the 15 longitudinal axis is that of a lozenge and a curved part of the lozenge is formed by the metal member. The lozenge shape is particularly preferable as it results in a heater can with a low profile which is ergonomically pleasing to the consumer.

Preferably the metal member is a plate or sheet and is preferably made of copper. Such a metal plate or sheet is particularly easy to form to the desired shape 20 and may be attached by edges of the metal plate or sheet being bent over an edge of the housing.

A further disadvantage with the prior art heater can is that the temperature

sensor is positioned far away from the coils such that when no water is in the heater can and current is flowing in the coils it can take quite some time for the temperature 25 of the temperature sensor to reach a cut- out temperature. This can have serious consequences if there is only a small amount of water in the heater can as that water can quickly reach boiling point and can be dangerous to the user of the instantaneous water heater.

The present invention provides an instantaneous water heater comprising a 30 heater can, wherein said heater can has:

- -6 a heating chamber with an inlet and an outlet, said heating chamber being defined between an inner surface of walls of an outer housing and an outer surface of walls of an inner tubular chimney with a longitudinal axis and which is positioned inside of said housing and said outlet being S deemed by an open end of said tubular chimney positioned facing an end wall of said walls of said housing; at least one heater element; a control means for controlling power to said at least one heater element; and a temperature sensor interacting with said control means for reducing power 10 to said at least one heater element when the temperature sensor senses that the; temperature of water exceeds a predetermined value; wherein a medial portion of said at least one heater element is disposed in said heating chamber and formed in coils around said longitudinal axis of said chimney and said temperature sensor is positioned on a side wall of said housing, 15 said side wall being substantially parallel to said longitudinal axis of said chimney outside of said housing such that said temperature sensor is positioned outside said heating chamber as close as possible to said coils.

Thus, the reaction time of the temperature sensor to changes in temperature of the elements is fast as it is positioned close to the heater element and the control 20 means can quickly reduce power to the heater elements when the temperature of the water in the heater can rises above a pre-determined level. Furthermore, when there is no water in the heater can and the temperature sensor senses the temperature of the heater elements through radiation, because of the short distance between the temperature sensor and the heater elements this scenario can also be quickly dealt 25 with by the present invention.

Preferably a boss protrudes from the side wall into the heating chamber thereby to ensure substantially no boundary layer in water at the side wall opposite the temperature sensor. Ensuring that there is no boundary layer at that position is advantageous because the presence of a boundary layer could cause the temperature 30 sensor to sense a temperature which is not representative of the temperature of the

-7 majority of water in the heating chamber.

The present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 illustrates the front cover of an instantaneous water heater according 5 to the present invention; Figure 2 illustrates an exploded view of the functional components of an instantaneous water heater according to the present invention; Figure 3 illustrates the assembled heater can of Figure 2; Figure 4a and 4b illustrate vertical cross-sections through the assembled 10 heater can of Figure 3; and Figure 5 illustrates a horizontal crosssection through the heater can of Figure As illustrated in Figure 1, an instantaneous water heater includes a front cover 2 including a water temperature control 4 and an on/off button 6.

15 The front cover 2 houses a number of functional components for providing hot water of a desired temperature.

Figure 2 illustrates a suitable arrangement of components embodying the present invention. i Water is provided to one of two alternative inlets 8 and 10. With the water 20 heater running, the water is then channelled by a flow rate control valve 12 to the inside of an outer housing 14 of a heater can. Within the heater can, the water flows up around the outside of a chimney 16 in contact with one or more heater element(s) 18. The water then flows down through the inside of the chimney 16 and out of an inlet 19.

25 A main valve 20 operates under the control of a button 22a, 22b to control whether or not there is any flow from the inlets 8 or 10 to the flow rate control valve 12. Dependent upon the position of the main valve 20 and/or flow of water through the water heater, a rod 24 operates an electrical switch 26 to turn on or off power to the heater element 18.

30 The flow rate control valve 12 may be operated by a control member 28 so as

to vary the flow rate through the heater can 14 and, hence, the outlet temperature.

The constructional details of various of the components of the heater can will now be described in detail with reference to Figures 2 and 3. A heating chamber is defined between an inner side of walls 142, 145 of the outer housing 14 and an outer 5 side of walls of the inner tubular chimney 16 which is positioned inside of the housing 14. The chimney I6 is positioned in spaced apart relationship from the side walls 142 of the outer housing and from an end wall 145 of the walls of the outer housing 14. The chimney 16 is attached to a bottom wall 147. Thus, the heating chamber of the heater can has the shape of an annulus in the region of the bottom 10 wall 147 and is a solid cavity in the region of the end wall 145.

The flow of water through the heater can is as follows. Water flows into the bottom of the heater can through an inlet (which is the outlet of the flow rate control valve 12) and the flow rate is controlled by the flow rate control valve 12. Water then passes between the side walls 142 of the outer housing and the chimney 16 in I 5 the region where the heating chamber has the shape of an annulus. The water flows to the end wall 145 where it changes direction and is channelled into the top end of the chimney 16 which forms an outlet of the heating chamber. Water then flows down the chimney to the outlet 19 at the bottom of the chimney.

The heater element(s) 18 is coiled around the chimney 16 and the coils are 20 generally co-axial with a longitudinal axis of the chimney 16 and also with a longitudinal axis of the outer housing 14. Thus, as water passes from the inlet of the heating chamber through the annular part of the heating chamber defined between the chimney 16 and the side walls of the outer housing 14, it is heated by the heating elements. 25 Several features of the heater can result in a heater can which is efficient, has a low pressure drop between its inlet and outlet, has a slim profile, is cheap and is also safer.

As can be seen from Figures 2 and 3 the heater can has a very low depth.

This is advantageous because it enables the instantaneous water heater to maintain a 30 low profile which is aesthetically phasing. However, the heater can still allows a

-9- large flow of water through the heater can. This is achieved by providing both the outer housing 14 and the chimney 16 to have a crosssectional shape of a lozenge in a direction perpendicular to their common longitudinal axes. This is most clearly shown in Figure 5.

5 If both the chimney and the outer housing have the same shape, this is an advantage because then the water passing through the annular part of the heating chamber will be evenly heated by the coils of the heater element(s) 18 which are present in that annular part of the heating chamber. The lozenge shape enables the heating chamber to have a large cross-sectional area in a plane perpendicular to the 10 longitudinal axis of the housing 14 and chimney 16 whilst maintaining a low depth.

Preferably the cross-sectional area of the annular part of the heating chamber in a plane perpendicular to the axis of the housing 14 is substantially the same as the cross-sectional area of the inside of the chimney 16 which provides an exit passage for water which has passed through the heating chamber outlet formed by the top of 15 the chimney 16 and is on its way to the outlet l 9 of the instantaneous water heater.

This arrangement has the advantage that there are no particular constrictions for the flow of water through the heater can so that the pressure drop between the inlet and the outlet of the heater can can be minimised. There are several other features of the heater can of the present invention which have the same aim and which are described 20 below.

In order to ensure that water passing through the heating chamber is evenly heated, there may be baffles or other means to ensure that water which enters the heating chamber is evenly distributed and flows evenly past the coils of the heater elements 18.

25 The heater can as illustrated has two heater elements 18. This is not necessarily the case and there may be only a single heater element or more than two heater elements.

The medial portions of the heater elements are formed as coils and, when in the heater chamber, are co-axial with the chimney 16 and the outer housing 14. End 30 portions of the heater elements 18, either side of the medial portion, exit through a

-10 side wall 142 of the outer housing 14.

As can be seen from Figure 2, the end portions are substantially perpendicular to the axis of the coils, chimney and housing such that there are no extra bends between the coils and the end portions. This is advantageous because bends in heater 5 elements can reduce their lifetime. This is especially so for small radius bends which are required if the end portions of the heater elements are bent such that the end portions protrude from the housing through the top wall 145 as in prior art heater can

arrangements. Also, the arrangement of the end portions leaving through the side wal] 142 of the outer housing 14 requires less material to be used thereby reducing 10 the cost of the heater element(s) 18. Thus, it can be said that the bends in the heater elements 18 which form the coils of the medial portion have the smallest radius of all bends of the heater element(s) 18.

The coils of the medial portion of the heater element(s) 18 have the same cross-sectional shape in a plane perpendicular to their longitudinal axis as the 15 chimney 16 and the housing 14. As can be seen from Figure 5 this ensures an even distribution of heater elements in the annular portion of the heating chamber where the water is heated such that uniform heating of the water can be expected.

Typically, the heater element is comprised of a central resistance wire coated in a layer of MgO which is coated with a thin layer of copper. The diameter of a 20 heater element might be perhaps 6mm. The cross-sectional shape of the heater elements 18 also has the advantage of allowing the lozenge cross-sectional shape of the chimney 16 described above, thereby indirectly also helping in minimising the pressure drop between the inlet and the outlet of the heater can.

Power is supplied to the heater elements 18 at the end of the end portions by a 25 control means which controls power to the heater elements. The amount of power supplied to the heater elements 18 affects the temperature of the heater elements 18 and thereby the amount that the water passing through the heating chamber is heated.

The side wall 40 through which the end portions of the heater elements 1 X pass is comprised of a metal. Preferably the metal is copper and is in the form of a 30 sheet, though a plate of copper may also be used. A weld is formed around the

- 1 1 circumference of the end portions of the heater elements and the copper plate 40 where they pass through the plate to form a watertight seal. As can be seen from Figures 2 and 3, if the copper side wall 40 is of a sheet of copper, it can be attached to the rest of the outer housing by bending edges over a lip of the outer housing 14.

5 This arrangement is advantageous in that it is particularly easy to connect the copper side wall 40 to the remainder of the outer housing 14 during manufacture.

Furthermore, the heater element(s) 18 can be pre-attached to the copper plate 40 and the copper plate 40 can then be attached to the housing 14 with a sealing gasket between the copper plate 40 and the housing 14. Thereafter the chimney 16 is 10 inserted into the bottom of the housing 14.

A temperature sensor 50 is attached to the copper plate 40. The temperature sensor 50 senses the temperature of water on the opposite side of the copper plate to it and provides a signal indicative of the water temperature to the control means which controls power to the heater elements 18. This is designed as a safety feature.

15 For instance, if the water temperature exceeds, for example 55 C, which is a temperature at which scalding can occur, power to the elements 18 can be reduced or entirely stopped. A further safety feature is possible in that if the temperature of the water exceeds 75 C power to the elements can be permanently stopped. If such a high temperature is achieved in the heating chamber it is likely that there is a fault 20 with the entire instantaneous water heater and that it should be checked by a qualified technician before being reused. Thus, ensuring that power to the elements is stopped entirely, means that the user will need to get such a technician to inspect the system before reusing it.

The temperature sensor 50 is positioned near the outlet of the heating 25 chamber and also close to the coils of the heating element 18. Preferably, the temperature sensor 50 is placed as close as possible to the top end of the coils of the heating elements 18 which are the last coils that the water passes before exiting the heating chamber. This ensures that the temperature sensor 50 measures the maximum temperature of water in the heating chamber and that no defects with the 30 heater elements can occur downstream of the temperature sensor.

-12 The position of the temperature sensor 50 close to the coils i.e. the side of the coils of the heater elements 18 are directly adjacent the opposite side of the copper plate 40 on which the temperature senor 50 is attached. In this way, for instance if there is no water in the heating chamber for one reason or another, the temperature 5 sensor SO can detect the heat of the elements 18 through radiation and thereby prevent burning out of the heater elements 18 because of over heating (as the temperature is not decreased by water flowing over them). Thus, the positioning of the temperature sensor 50 is advantageous because it reacts quickly to variations in the temperature of the coils of the heater elements 18.

10 Sometimes a boundary layer of water can be present on the inner surface of the copper plate 40. This can affect the temperature which the temperature sensor 50 reads such that it is not representative of the temperature of water in the heating chamber. In order to avoid this boundary layer attaching, a baffle 45, illustrated best in Figure 4, is formed on the copper plate 40 just upstream of the temperature sensor 15 50. This has the effect of detaching any boundary layer which may have formed on the copper plate 40 such that the water on the other side of the copper plate to the temperature sensor 50 is well mixed and is representative of the temperature of water at the level of the most downstream coil of the heater elements 18.

The heater can is also shaped in several ways to minimise the pressure drop 20 between the inlet and outlet 17.

The most noticeable shaping of the heater can takes place at the end wall 145 opposite the outlet 17 which is formed by an open end of the chimney 16. This is most clearly illustrated in Figure 4.

As water passes through the annular part of the heating chamber between the 25 chimney 16 and the side walls 142 of the outer housing the direction of flow must be changed at the end wall 145 such that the water flows into the outlet 17 formed by the open end of the chimney 16. The acceleration in this region can be large and can result in a large pressure drop.

In order to avoid a large amount of turbulence in the end wall 145 region' a 30 protrusion 147 is formed on the end wall 145. The protrusion 147 extends into the

-13 heating chamber opposite the outlet 17. The protrusion 147 may share a corurnon axis of symmetry with the chimney 16 as well as the outer housing 14 and also the coils of the heating elements 18.

The protrusion 147 is shaped so that the change in direction of flow of water S between the annular part of the heating chamber and that part where the water flows out of the outlet 17 is smooth or gradual by guiding the flow of water towards the outlet 17. This arrangement reduces the amount of turbulence in the water in the heating chamber adjacent the end wall 145 of the outer housing 14.

Preferably the protrusion 147 is flat at its maximum protrusion into the 10 heating chamber though it may be pointed.

There are further refinements to the end wall 145 which also help in minimising the pressure drop between the inlet and outlet of the heater can. These are to provide a smooth transition between the side walls 142 and the end wall 145 of the heater can. This can be seen as the curved outer sections of Figures 4a and 4b at 15 the periphery of the end wall 145. Also, a medial portion of the end wall 145 between the periphery of the end wall 145 and the protrusion 147 is also curved such that the transition to the protrusion 147 from the medial portion is smooth. Thus, the end wall 145 is shaped to minimise the pressure drop between the inlet and the outlet.

- In a similar way, as is illustrated in Figure 5, the copper sheet 40 is also 20 shaped to conform to the overall shape of the inner surface of the side walls 142 of the outer housing 14. If the side walls of the outer housing 142 have a cross- sectional shape of a lozenge, the copper plate 40 is curved to ensure that

the side wall which it defines conforms to that shape. This is an advantage of using a sheet of copper rather than a plate as the sheet is easier to form and bend to the appropriate 25 shape. Shaping the copper sheet 40 in this way has the advantage that water in the annular part of the heating chamber is evenly distributed and cannot run in right angled corners between a side wall 142 and the copper plate 40 where it might not be heated as much as elsewhere. Thus, shaping the copper sheet 40 in this way ensures even heating of water in the heater can.

30 The instantaneous water heater described above can be used in a shower.

Claims

-14 CLAIMS

1. An instantaneous water heater including a heater can, wherein said heater can has: 5 an outer housing; an inner tubular chimney with a longitudinal axis and positioned inside said outer housing; a heating chamber with an inlet and an outlet, said heating chamber being defined between an inner surface of walls of said outer housing and an outer surface 10 of walls of said inner tubular chimney; and at least one heater element; wherein a medial portion of said at least one heater element is disposed in said heating chamber and formed in coils around said longitudinal axis of said chimney, end portions of said at least one heater element, either side of said medial 15 portion, being substantially perpendicular to said longitudinal axis of said chimney.

2. An instantaneous water heater according to claim l, wherein said heater element is formed such that bends forming said coils of said heater element have the smallest radius of all bends in said heater element.

3. An instantaneous water heater according to claim 1 or 2, wherein said end portions are situated partly within and partly outside of said heating chamber.

4. An instantaneous water heater according to claim 1, 2 or 3, wherein said end 25 portions protrude through a side wall of said walls of said outer housing, said side wall being substantially parallel to said longitudinal axis of said chimney.

5. An instantaneous water heater according to claim 4, wherein said side wall is comprised of metal, particularly copper.

-15 6. An instantaneous water heater according to claim 5, wherein welds between said end portions and said side wall form water tight seals.

7. An instantaneous water heater according to any one of the preceding claims, 5 wherein said coils and said walls of said chimney have substantially the same cross-

sectional shape in a plane perpendicular to said longitudinal axis of said chimney.

8. An instantaneous water heater according to any one of the preceding claims, wherein said coils have an elongate shape in a plane perpendicular to said 10 longitudinal axis of said chimney.

9. An instantaneous water heater according to claim 8, wherein said coils substantially have the shape of a lozenge in a plane perpendicular to said axis of said chimney. 10. An instantaneous water heater including, a heater can, wherein said heater can has: a heating chamber with an inlet and an outlet, said heating chamber being defined between an inner surface of walls of an outer housing and an outer surface of 20 walls of an inner tubular chimney, said chimney being positioned in spaced apart relationship from and within said outer housing, and said outlet being defined by an open end of said tubular chimney positioned facing an end wall of said walls of said outer housing, wherein said end wall of said outer housing includes a central protrusion 25 extending into said chamber and positioned and shaped such that water flowing from between said side walls of said outer housing and said chimney towards said end wall has its direction of flow changed gradually by interaction with said protrusion towards said outlet.

30 11. An instantaneous water heater according to claim 10, wherein said end wall is

-16 shaped to minimise pressure drop between said inlet and outlet.

12. An instantaneous water heater according to claim 10 or 11, wherein an outer portion of said end wall adjacent said side walls of said outer housing is curved 5 towards said side walls of said outer housing to form a smooth transition from said side walls to said end wall.

13. An instantaneous water heater according to claim 10, 1 l or 12, wherein a medial portion of said end wall next to said central protrusion is curved towards said 10 central protrusion to form a smooth transition from said medial portion to said central portion. 1 4. An instantaneous water heater according to any one of claims 10 to 13, wherein said central protrusion has a cross-sectional shape in a plane perpendicular to 15 a longitudinal axis of said tubular chimney and said tubular chimney has a cross-

sectional shape in a plane perpendicular to its longitudinal axis, said cross-sectional shapes being substantially the same.

15. An instantaneous water heater according to any one of claims 1 0 to 1 4, 20 wherein said central protrusion has an axis of syrornetry in common with said tubular chimney. 16. An instantaneous water heater according to any one of claims 10 to 15, wherein said central protrusion is flat at a maximum protrusion into said chamber 25 from said end wall.

17. An instantaneous water heater according to any one of claims 10 to 15, wherein said central protrusion curves to a point at maximum protrusion into said chamber from said end wall.

-17 18. An instantaneous water heater according to any one of the preceding claims, wherein inner sides of said walls of said chimney define a water exit passage for water from said outlet, said exit passage having a cross-sectional area perpendicular to said axis of said chimney and said heating chamber having a cross-sectional area S perpendicular to said axis of said chimney, said cross-sectional areas being substantially the same.

19. An instantaneous water heater comprising a heater can, wherein said heater can has: 10 a heating chamber with an inlet and an outlet, said heating chamber being defined between an inner surface of walls of an outer housing and an outer surface of walls of an inner tubular chimney with a longitudinal axis and which is positioned inside of said outer housing; and at least one heater element; 15 wherein end portions of said heater element protrude through a metal member of said housing which at least partly forms a side wall of said walls of said housing and is shaped on its inner side to conform to the overall shape of said inner side of said walls of said housing.

20 20. An instantaneous water heater according to claim 19, wherein said chimney has a cross-sectional shape in a plane perpendicular to said axis and said outer housing has a cross-sectional shape in a plane perpendicular to said axis, said cross-

sectional shapes being substantially the same and wherein part of said cross-sectional shape of said housing is deemed by said metal member.

21. An instantaneous water heater according to claim 19 or 20, wherein said chimney and said housing share a cozmnon longitudinal axis of symmetry.

22. An instantaneous water heater according to claim 19, 20 or 21, wherein said 30 overall shape of said housing in a plane perpendicular to said longitudinal axis is that

- 1 8 of a lozenge and a curved part of said lozenge is formed by said metal member.

23. An instantaneous water heater according to any one of claims 19 to 22, wherein said metal member is of a sheet.

24. An instantaneous water heater according to any one of claims 19 to 23, wherein edges of said metal member are bent over an edge of said outer housing thereby to attach said metal member to said housing.

10 25. An instantaneous water heater according to any one of claims 19 to 24, wherein said metal member is copper.

26. An instantaneous water heater according to any one of claims 19 to 25, wherein said outlet is defined by an open end of said tubular chimney positioned 15 facing an end wall of said walls of said outer housing.

27. An instantaneous water heater comprising a heater can, wherein said heater can has: a heating chamber with an inlet and an outlet, said heating chamber being 20 defined between an inner surface of walls of an outer housing and an outer surface of walls of an inner tubular chimney with a longitudinal axis and which is positioned inside of said housing and said outlet being defined by an open end of said tubular chimney positioned facing an end wall of said walls of said housing; 25 at least one heater element; a control means for controlling power to said at least one heater element; and a temperature sensor interacting with said control means for reducing power to said at least one heater element when the temperature sensor senses that the temperature of water exceeds a predetermined value; 30 wherein a medial portion of said at least one heater element is disposed in

-l9- said heating chamber and formed in coils around said longitudinal axis of said chimney and said temperature sensor is positioned on a side wall of said housing, said side wall being substantially parallel to said longitudinal axis of said chimney outside of said housing such that said temperature sensor is positioned outside said 5 heating chamber as close as possible to said coils.

28. An instantaneous water heater according to claim 27, wherein said temperature sensor is positioned on said side wall substantially in the same plane perpendicular to said longitudinal axis as said outlet.

29. An instantaneous water heater according to claim 27 or 2S, further comprising a boss protruding from said side wall into said heating chamber thereby to ensure substantially no boundary layer at said side wall opposite said temperature sensor. 30. An instantaneous water heater according to claim 27, wherein said one of said side walls is comprised of copper.

20 31. A shower comprising an instantaneous water heater according to any one of the preceding claims.

32. An instantaneous water heater substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.