GB2183789A

GB2183789A - Valve

Info

Publication number: GB2183789A
Application number: GB08530351A
Authority: GB
Inventors: Nicholas John Beck; Robert Meadows; Michael Whiting
Original assignee: Kohler Mira Ltd
Current assignee: Kohler Mira Ltd
Priority date: 1985-12-10
Filing date: 1985-12-10
Publication date: 1987-06-10
Anticipated expiration: 2005-12-10
Also published as: GB2183789B; IE59262B1; IE863164L; GB8530351D0

Abstract

A fluid flow control valve 2, preferably for an instantaneous water heater to reduce risk of scalding, has a closure member 35 coupled with a flow regulator 38 positioned downstream of an inlet port 21 to control the flow rate from an inlet chamber 22 to an outlet 46a. The flow regulator 38 is constructed to provide a gradual reduction in the flow rate, from a maximum obtained on rotating in one sense a control knob mounted on the spindle 25 to open the inlet port 21, by further rotation of the control knob in the same sense. The closure member 35 and flow regulator 38 are preferably carried by a piston 30 connected to a spindle 25 through interengaging screw threads 28,29 for converting rotational movement of the spindle 25 into sliding movement of the piston 30 for moving the closure member 35 to open and close the inlet port 21 and simultaneously moving the flow regulator 38 to adjust the flow rate. <IMAGE>

Description

SPECIFICATION Valve This invention concerns valves for controlling fluid flow and water heaters incorporating such valves, in particular so called instantaneous water heaters.

Instantaneous water heaters provide a source of hot water on demand by heating a supply of cold water flowing continuously through the heater and are commonly provided in ablutionary installations, especially for showering.

Generally, the known instantaneous water heaters include a heat exchanger to heat rapidly the water and a valve to control the flow rate through the heat exchanger and hence the final water temperature.

It is important in heaters of this type that the heat exchanger should be turned off when there is less than a predetermined minimum flow rate through the heat exchanger corresponding to a safe maximum water temperature so as to prevent water temperatures higher than the safe maximum or even boiling of the water being obtained.

For this purpose it is already known in instantaneous electric water heaters to control the power supply to a heating element of considerable heating capacity, generally about 7kW, by means of a switch assembly responsive to the water pressure in the heat exchanger downstream of the valve.

A disadvantage of such known arrangement is that the water pressure in the heat exchanger is relatively low as cpmpared with the supply pressure and varies with the selected flow rate leading to unreliable operation of the switch assembly. For example, it is possible for the switch assembly to cut off the power to the heating element when selecting low flow rates to obtain high water temperatures.

To overcome this disadvantage it is known to arrange for the switch assembly to be responsive to the water pressure in the valve between a closure member operable to turn the water flow on and off and a flow regulator downstream of and operable independently of the closure member to adjust the flow rate to obtain the desired water temperature. With this arrangement, when the water flow is turned on, the water pressure controlling the switch assembly is substantially the water supply pressure. The supply pressure is considerably higher than the water pressure downstream of the valve in the heat exchanger and is substantially unaffected by variations in the water pressure downstream of the flow regulator in the heat exchanger as above-described leading to reliable operation of the switch assembly.

A disadvantage of this arrangement however, is that the closure member can be operated to turn the water flow off with the flow regulator adjusted to provide the minimum flow rate and, if the closure member is operated to turn the water flow on with the flow regulator in this position of adjustment, the maximum water temperature is obtained.

There is a significant risk of the user being scalded by such operation and this risk is further increased if the water flow is only turned off for a short period before being turned on again. In these circumstances, the residual heat in the heating element on turning the water flow off heats the water remaining in the heat exchanger and, on turning the water flow on again, this heated water is further heated so that a water temperature higher than the maximum water temperature is obtained initially.

It is an objet of the present invention to provide a valve and a water heater incorporating the valve which overcome the problems and disadvantages afore-mentioned.

According to a first aspect of the present invention, there is provided a valve having an inlet for connection to a supply of fluid, the inlet being in fluid communication with an inlet chamber through an inlet port, a closure member operatively connected to a control knob and movable on rotation of the control knob in opposed senses to open and close the inlet port to turn the fluid flow on and off respectively and, a flow regulator coupled to the closure member for movement therewith on rotation of the control knob to control the flow rate from the inlet chamber to an outlet, the arrangement being such that, in use, the flow rate is gradually reduced from a maximum flow rate obtained on rotation of the control knob in one sense to open the inlet port by further rotation of the control knob in said one sense.

By using a common control knob to operate both the closure member and the flow regulator it is ensured that, when the inlet port is closed, the flow regulator is automatically set to provide the desired maximum flow rate on opening the inlet port.

Preferably the valve has a hollow tubular body and the closure member and flow regulator are arranged for movement longitudinally thereof by means of a hollow piston mounted for sliding movement longitudinally of the body within the inlet chamber.

Advantageously the piston has a screw thread portion engageable with a complementary screw thread portion of a spindle rotatable by the control knob to effect sliding movement of the piston and the confronting surfaces of the piston and body are shaped to prevent rotation of the piston. For example the confronting surfaces may have complementary axial splines or be of complementary non-circular crosssection.

Conveniently the closure member and flow regulator are mounted at opposed ends of an elongate rod supported intermediate its ends by a transverse web of the piston. The web is preferably formed with at least one aperture for fluid flow from the inlet chamber to the interior of the piston.

Preferably the inlet port is defined by an annular valve seat and the closure member comprises a resilient annular washer movable towards and away from the valve seat on rotation of the control knob in opposed senses to engage and disengage the valve seat to close and open the inlet port.

The flow regulator preferably comprises a substantially cylindrical element formed with a plurality of circumferentially spaced apart longitudinally extending grooves in the peripheral surface which define with an O-ring a plurality of circumferentially spaced apart openings for flow of fluid from the inlet chamber to the outlet. The grooves are of increasing crosssection in the direction away from the inlet port so that, on rotation of the control knob in said one sense, the flow regulator is moved relative to the O-ring to reduce the size of the openings and hence the flow rate. In this way the desired maximum flow rate is obtained on opening the inlet port.

Preferably a stop is provided to limit movement of the flow regulator to set the minimum flow rate that can be obtained. Conveniently the stop is adjustable to allow the minimum flow rate to be selected to suit any given application.

Advantageously, the O-ring is fixed in a bore in the inner end of the spindle in which the flow regulator is received and the stop comprises an adjustable set screw mounted in the outer end of the spindle to project into the bore for engagement with the end of the rod on which the flow regulator is mounted.

Downstream of the flow regulator the spindle bore is conveniently formed with a plurality of radial ports communicating with radial ports in a closure cap leading to the outlet.

Preferably the radial cap ports open into to an outlet chamber in the body in fluid communication with the outlet.

According to a second aspect of the present invention there is provided a water heater comprising a valve according to the first aspect of the invention and a heat exchanger connected to the outlet from the valve and adapted for heating water flowing through the heat exchanger.

By the use of the valve according to the first aspect of the invention it is ensured that the maximum flow rate and hence the coolest water temperature is always obtained on initial rotation of the control knob in said one sense to open the inlet port and higher water temperatures are obtained by further rotation of the control knob in the same sense to reduce the flow rate. In this way the risk of the user being scalded is reduced.

Furthermore by automatically setting the flow regulator to provide maximum flow rate on opening the inlet port it is also ensured that the heater is flushed with the coolest water temperature on rotation of the control knob in the other sense to close the inlet port. This coupled with the maximum flow rate on opening the inlet port reduces the risk of the user being scalded by water remaining in the heat exchanger being heated to an elevated temperature if only a short period of time elapses between the heater being turned off and turned on.

Preferably the heat exchanger is controlled by a switch assembly operable to prevent the heat exchanger being switched on to heat the water when the flow rate through the heat exchanger is less than a pre-determined minimum set by the stop which corresponds to the maximium water temperature that can be selected. In this way safe operation of the heater is ensured by preventing excessive water temperatures or even boiling of the water being reached.

Advantageously the switch assembly is responsive to the water pressure in the inlet chamber. This water pressure is zero when the closure member engages the valve seat to close the inlet port and substantially the water supply pressure when the closure member is clear of the valve seat to open the inlet port.

This water pressure is substantially constant for all operating conditions and the switch assembly is conveniently arranged to operate at an intermediate water pressure. This intermediate water pressure is selected to be as low as possible and corresponds to the minimum water pressure required to provide the predetermined minimum flow rate for safe operation of the heater. A large pressure difference therefore exists between the minimum water pressure for operating the switch assembly and the actual water pressure in the inlet chamber when the inlet port is open. As a result, minor fluctuations in the water supply pressure do not affect operation of the switch assembly and the heat exchanger is only switched off if there is a large reduction in the water pressure in the inlet chamber, for example if the water supply pressure drops below the minimum or on closing the inlet port. In this way reliable operation of the switch assembly is ensured under all conditions of use.

Preferably, the switch assembly is responsive to the water pressure acting on a flexible diaphragm, one side of which is exposed to the water pressure in a chamber in fluid communication with the inlet chamber. In this way fast response of the switch assembly to change in the water pressure acting on the diaphragm is ensured.

Preferably the switch assembly includes an actuator engageable with the other side of the diaphragm and movable therewith in response to change in the water pressure acting on the diaphragm to control operation of the heat ex changer.

Conveniently the actuator is resiliently biassed towards the diaphragm, for example by a return spring, and the resilient biassing is selected to be overcome by the minimum water pressure for operating the switch assembly. By this arrangement the diaphragm deflects to move the actuator to turn the heat exchanger on when the water pressure in the chamber is equal to or greater than the minimum water pressure and, the actuator and diaphragm are automatically returned to their original positions to turn the heat exchanger off under the biassing of the return spring when the water pressure in the chamber is less than the minimum water pressure.In this way accurate response of the switch assembly to change in the water pressure in the chamber is obtained and the heat exchanger is only turned on to heat the water when the pre-determined minimum flow rate for safe operation of the heater is present.

Preferably the heat exchanger includes at least one electric heating element and the switch assembly includes a switch to control the power supply to the heating element.

Conveniently the switch is operable by a lever pivotal under the control of the actuator and the switch assembly includes a cam to block selectively the pivotal movement of the lever. By this arrangement the heater may be operated with or without the power input to the heating element as desired.

Advantageously, the heat exchanger has a pair of electric heating elements and the switch assembly has a pair of levers each pivotal under the control of the actuator to operate a respective switch for each heating element and a pair of cams, one for each lever, to block selectively the pivotal movement of the associated lever. By this arrangement, both, one or neither switch may be operated to alter the power input to the heating elements as desired.

Conveniently, the actuator comprises a push rod resiliently biassed towards the diaphragm by the return spring and a beam extending transversely through an elongate diametric slot intermediate the ends of the push rod. Each lever engages a respective one end of the beam and the beam is resiliently biassed, for example by a relief spring, to the outer end of the slot. In this way the relief spring permits deflection of the diaphragm to move the push rod when pivotal movement of one or both levers is blocked.

Preferably the cam or each cam is mounted on a spindle for rotation therewith by a control knob to select the required power input.

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings wherein: FIGURE 1 is a front view of a water heater incorporating a valve according to the present invention; FIGURE 2 is a side view of the water heater shown in Figure 1; FIGURE 3 is a front view of the water heater shown in Figures 1 and 2 with the front cover of the casing removed to show the internal parts; FIGURE 4 is a section through the valve shown in Figure 3; FIGURE 5 is a section on the line X-X of Figure 3; FIGURE 6 is a front view, to an enlarged scale, of the switch assembly shown in Figure 3; FIGURE 7 is a rear view of the switch assembly shown in Figure 6; and FIGURE 8 is a side view, partly in section, of the switch assembly shown in Figures 6 and 7.

Referring first to Figures 1 to 3 of the accompanying drawings, there is shown an instantaneous electric water heater having a generally rectangular casing 1 housing a valve 2, a heat exchanger 3 and a switch assembly 4.

The casing 1 has a base plate 5 and a front cover 6 on which are arranged two rotatable control knobs 7,8. The front cover 6 is mounted on the base plate 5 and held in position by a screw 9 to permit removal of the front cover 6 from the base plate 5. The base plate 5 is formed with a number of holes 10 by means of which the casing 1 is surface mounted, for example on a wall, by any suitable fasteners, such as screws 11. The base plate 5 also has a number of ducts (not shown) to permit top, bottom, side or rear entry of a cold water supply for connection to an inlet 12 to the valve 2 and an electrical supply for connection to one side of a terminal block 13 within the casing.

The control knob 7 selects the power input to the heat exchanger 3 and, as shown in Figure 1, has three angularly spaced apart settings A,B and C corresponding to a power input of OkW,4kW and 7.2kW respectively.

The control knob 8 operates the valve 2 to turn the water flow on and off and to adjust the flow rate through the heat exchanger 3 and hence the final temperature of the water for the selected power input.

The heat exchanger 3 comprises a hollow body 14 having an inlet 15 connected to the valve 2 and an outlet 16 extending through the bottom of the cover 6 for connection to an ablutionary appliance, for example a shower (not shown). Arranged within the body 14 are two sheathed heating elements (not shown) coiled concentric with one another and connected through the switch assembly 4 to the other side of the terminal block 13.

The switch assembly 4 controls the power to the heating elements and is set by the control knob 7 to provide automatically the selected power input in response to the water pressure in the valve 2. The heat exchanger 3 further includes a thermal cut-out 17 to prevent overheating of the heating elements and a safety valve 18 to release automatically an excessive pressure build-up in the body 14.

The safety valve 18 is the subject of our copending U.K. Patent Application No.84.30520 to which the reader is directed for further details of the construction and operation.

Referring now to Figure 4, the valve 2 comprises a tubular body 19 having a lateral passage 20 leading from the inlet 12 to an inlet port 21 opening to an inlet chamber 22. The inlet port 21 is defined by an annular valve seat 23 arranged centrally on the longitudinal axis at one end of the body 19. The other end of the body 19 is closed by a cap 24 in which a spindle 25 is mounted and retained by a circlip 26 permitting rotation of the spindle 25 about the longitudinal axis of the body 19 but preventing movement of the spindle 25 longitudinally of the body 19.

The outer end of the spindle 25 is formed with axial splines 27 for engagement with complementary axial splines (not shown) on the control knob 8 to lock the spindle 25 for rotation with the knob 8 whilst permitting the knob 8 to be slid off the spindle 25, for example when removing the front cover 6.

The inner end of the spindle 25 is formed with a screw thread 28 on the outer surface engageable with a complementary screw thread 29 formed on the inner surface of a hollow piston 30. The outer surface of the piston 30 is of hexagonal shape in cross-section and the confronting inner surface of the body 19 is of complementary hexagonal shape in cross-section to prevent rotation of the piston 30 in the body 19 whilst permitting sliding movement of the piston 30 longitudinally of the body 19 on rotation of the spindle 25.

The end of the piston 30 remote from the spindle 25 is formed with a web 31 having a central through bore 32 and a plurality of circumferentially spaced apart holes 33. An elongate rod 34 extends through the bore 32 and is secured in fluid-tight manner by any suitable means, for example ultrasonic welding.

A closure member comprising a resilient annular washer 35 is mounted on one end of the rod 34 facing the valve seat 23. The washer 35 is seated in a recessed portion of the web 31 and is releasably secured by a nut 36 engaging a threaded end portion of the rod 34. The washer 35 engages the valve seat 23 to close the inlet port 21 in one extreme position of the piston 30 as shown in Figure 4 and disengages the valve seat 23 to open the inlet port 21 on clockwise rotation of the control knob 8 to slide the piston 30 to the left, as viewed in Figure 4, away from the valve seat 23 enabling water to enter the inlet chamber 22 and flow through the holes 33 to the interior of the piston 30.

The inner end of the spindle 25 is formed with a central bore 37 in which is received a flow regulator 38 mounted on the other end of the rod 34.

The flow regulator 38 comprises a substantially cylindrical element extending longitudinally of the body 19 and having a plurality of longitudinally extending circumferentially spaced apart grooves (not shown) formed in the outer surface. An O-ring 39 located and retained against a shoulder 40 in the spindle bore 37 by a ring 41 engages the outer surface of the flow regulator 38 to define with the grooves a plurality of circumferentially spaced apart openings (not shown) enabling water to flow from the interior of the piston 30 to the spindle bore 37 downstream of the flow regulator 38.

The grooves are tapered to be of increasing depth and/or width in a direction away from the valve seat 23 so that the size of the openings decreases on clockwise rotation of the control knob 8 to slide the piston 30 to the left, as viewed in Figure 4, away from the valve seat 23. In this way, the maximum flow rate is obtained on initial clockwise rotation of the control knob 8 to open the inlet port 21 and thereafter further clockwise rotation of the control knob 8 gradually reduces the flow rate. The minimum flow rate is set by a stop comprising an adjustable set screw 42 mounted in the outer end of the spindle 25 and extending into the bore 37 for engagement with the end of the rod 34 to limit sliding movement of the piston 30 to the left, as viewed in Figure 4.The set screw 42 is adjusted on installation to set the minimum flow rate to obtain a safe maximum water temperature when using the highest power input available.

The water pressure in the inlet chamber 22 is substantially the same as the water supply pressure when the washer 35 is moved clear of the valve seat 23 by clockwise rotation of the control knob 8 to open the inlet port 21.

Variations in the supply pressure are automatically accommodated by movement of the 0ring 39 into or out of the grooves thereby adjusting the size of the openings to maintain the flow rate substantially constant for any given position of the flow regulator 38.

Downstream of the flow regulator 38, the spindle 25 is formed with a plurality of radial ports 43 leading from the bore 37. The cap 24 is also formed with a plurality of radial ports 44 and defines with the spindle 25 an annular channel 45 providing a flow path between the spindle ports 43 and cap ports 44 for all rotational positions of the spindle 25.

The cap ports 44 open into a part circumferential outlet chamber (not shown) formed in the body 19 which opens into an axial passage 46 leading to an outlet 46a from the valve 2 for connection to the heat exchanger inlet 15.

To render the valve 2 fluid-tight, the cap 24 is sealed in the body by a pair of longitudinally spaced apart O-rings 47,48 positioned between confronting surfaces of the cap 24 and body 19 on opposite sides of the cap ports 44. Likewise the spindle 25 is sealed in the cap 24 by a pair of longitudinally spaced apart O-rings 49,50 positioned between confronting surfaces of the cap 24 and spindle 25 on opposite sides of the spindle ports 43.

Referring now to Figures to 8, the switch assembly 4 includes a pair of bellcrank levers 51,52 pivotal under the control of an actuator 53 responsive to the water pressure in the inlet chamber 22 to operate respective microswitches 54,55 for each heating element and a pair of cams 56,57 mounted on a spindle 58 for rotation by the control knob 7 to block selectively the pivotal movement of both, one or neither lever 51,52 to provide the selected power input to the heating elements.

The spindle 58 is formed with axial splines 59 for engagement with complementary axial splines (not shown) on the control knob 7 to lock the spindle 58 for rotation with the knob 7 whilst permitting the knob 7 to be slid off the spindle 58, for example when removing the front cover 6. The cam 56 is formed with a plurality of recesses 60 in the marginal edge for selective engagement by a resilient abutment 61 to hold the spindle 58 and cams 56,57 at any selected one of positions A,B and C of the control knob 7.

The marginal edge of a flexible diaphragm 62 is clamped between co-operating flanges 63,64 of the valve body 19 and a mounting plate 65 for the actuator 53 to define a fluid chamber 66 on the outside of the valve body 19. The chamber 66 communicates with the inlet chamber 22 through an orifice 67 in the wall of the valve body 19 and a longitudinally extending groove 68 formed in the inner surface of the valve body 1 9 so that the water pressure in the chamber 66 is substantially the same as the water pressure in the inlet chamber 22.

The actuator 53 comprises a push rod 69 arranged for sliding movement in an opening in the mounting plate 65 and a beam 70 positioned on the outside of the mounting plate 65 and extending transversely through an elongate diametric slot 7 1 intermediate the ends of the push rod 69.

The inner end of the push rod 69 has a diaphragm plate 72 seated on the diaphragm 62 and a return spring 73 encircling the push rod 69 acts between the mounting plate 65 and the diaphragm plate 72 to bias the actuator 53 towards the diaphragm 62 for movea ment therewith in response to the water pressure in the chamber 66.

Each bellcrank lever 51,52 engages a respective one end of the beam 70 and a relief spring 74 received within the push rod 69 acts between the beam 70 and diaphragm plate 72 to bias the beam 70 to the outer end of the slot 71.

When the bias of return spring 73 exceeds the water pressure in the chamber 66, the actuator 53 and diaphragm 62 adopt the position shown in Figures 6,7 and 8. In this position both microswitches 54,55 are open and there is no power input to the heating elements.

When the water pressure in the chamber 66 overcomes the resilient biassing of return spring 73, the diaphragm 62 deflects and moves the push rod 69 outwards of the mounting plate 65 to provide the power input to the heating elements selected by the control knob 7 as now explained.

In position A, the cams 56,57 block pivotal movement of both levers 51,52. The beam 70 is thus held stationary by the levers 51,52 compressing the relief spring 74 on outward movement of the push rod 69 so that neither microswitch 54,55 is closed and the power input is OkW.

In position B, the cam 57 blocks pivotal movement of lever 52 while cam 56 permits pivotal movement of lever 51. One end of the beam is thus held stationary by the lever 52 causing the beam 70 to tilt in the slot 71 under the bias of relief spring 74 on outward movement of the push rod 69 and pivot lever 51 to close the associated microswitch 54 and the power input is 4.0kW.

In position C, the cams 56,57 permit pivotal movement of both levers 51,52. The beam 70 is thus free to move with the push rod 69 under the bias of relief spring 74 on outward movement of the push rod 69 and pivots each lever 51,52 to close the associated microswitch 54,55 and the power input is 7.2kW.

The bias of return spring 73 is selected to be overcome by a water pressure corresponding to the minimum water pressure required to provide the predetermined minimum flow rate for safe operation of the heater. This water pressure is low as compared with the water supply pressure and in this way it is ensured that the selected power input to the heating elements is only provided when there is sufficient water flow through the heater and is substantially unaffected by minor variations in the water supply pressure for all conditions of use.

As will be appreciated from the foregoing description, by utilising a valve in which the closure member and flow regulator are coupled for operation by a common control knob to provide the maximum flow rate on rotation of the control knob to open the inlet port and thereafter a gradual reduction in flow rate on further rotation of the control knob in the same direction, it is ensured that the coolest water temperature is always obtained on initial rotation of the control knob for any selected power input. Additionally, on rotation of the control knob to close the inlet port, the flow rate increases to the maximum before the inlet port is closed so that the heater is flushed with the coolest water temperature. In this way, the risk of the user being scalded is considerably reduced, especially when the heater is only turned off for a short period of time before re-use.

It will be understood that the invention is not limited to the embodiment above-described and that the valve has application to all types and constructions of instantaneous electric water heaters as well as instantaneous water heaters in which the heat exchanger is adapted to heat the water by fuel-fired heating devices. For example, the electric heating elements of the heat exchanger above-described may be replaced by at least one gas burner and the switch assembly modified to operate a respective gas valve for controlling the supply of gas to the or each burner in response to the water pressure in the inlet chamber.

Finally the valve may be used in appliances other than instantaneous water heaters to control the flow rate of a fluid for any desired purpose.

Claims

1. A valve having an inlet for connection to a supply of fluid, the inlet being in fluid communication with an inlet chamber through an inlet port, a closure member operatively connected to a control knob and movable on rotation of the control knob in opposed senses to open and close the inlet port to turn the fluid flow on and off respectively and, a flow regulator coupled to the closure member for movement therewith on rotation of the control knob to control the flow rate from the inlet chamber to an outlet, the arrangement being such that, in use, the flow rate is gradually reduced from a maximum flow rate obtained on rotation of the control knob in one sense to open the inlet port by further rotation of the control knob in said one sense.

2. A valve according to claim 1 wherein the flow regulator comprises an elongate element formed with a plurality of circumferentially spaced apart longitudinally extending grooves in the peripheral surface which co-operate with sealing means to define a plurality of circumferentially spaced apart openings for controlling flow of fluid from the inlet chamber to the outlet.

3. A valve according to claim 2 wherein the sealing means comprises an O-ring.

4. A valve according to claim 2 or claim 3 wherein the grooves are of increasing crosssection in the longitudinal direction leading away from the inlet port and the flow regulator is movable longitudinally relative to the sealing means to vary the size of the openings.

5. A valve according to any one of the preceding claims wherein a stop is provided to limit movement of the flow regulator to set the minimum flow rate.

6. A valve according to claim 5 wherein the stop is adjustable.

7. A valve according to any one of the preceding claims wherein the flow regulator and closure member are carried by a hollow piston mounted in the inlet chamber for sliding movement towards and away from the inlet port on rotation of the control knob in opposed senses.

8. A valve according to claim 7 wherein piston and inlet chamber are provided with complementary formations to prevent rotation of the piston.

9. A valve according to claim 7 or claim 8 wherein the flow regulator and closure member are mounted at opposed ends of an elongate rod supported intermediate its ends by a transverse web of the piston.

10. A valve according to any one of claims 7 to 9 wherein the control knob is mounted on a spindle, and the spindle and piston are connected by means for converting rotation of the spindle into sliding movement of the piston.

11. A valve according to claim 10 wherein the spindle has a bore in communication with the outlet and the flow regulator is received in the bore.

12. A valve according to claim 11 wherein the bore is in communication with the outlet through radial ports formed in the spindle downstream of the flow regulator.

13. A valve according to any one of the preceding claims wherein the closure member comprises a resilient annular washer for engagement and disengagement with an annular valve seat to close and open the inlet port.

14. A valve substantially as hereinbefore described with reference to Figure 4 of the accompanying drawings.

15. A water heater comprising a valve according to any one of the preceding claims and a heat exchanger connected to the outlet from the valve and adapted for heating water flowing through the heat exchanger.

16. A water heater according to claim 15 wherein the heat exchanger includes a heating element controlled by a switch assembly responsive to the water pressure in the inlet chamber for controlling the power supply to the heating element.

17. A water heater according to claim 16 wherein the switch assembly comprises a lever pivotal under the control of an actuator responsive to the water pressure in the inlet chamber to operate a switch for controlling the power supply to the heating element.

18. A water heater according to claim 17 wherein the actuator comprises a push rod of which one end is operatively connected to the lever and the other end engages one side of a flexible diaphragm, the other side of the diaphragm being subject to the water pressure in the inlet chamber.

19. A water heater according to claim 18 wherein the push rod is resiliently biassed towards the diaphragm.

20. A water heater according to claim 18 or claim 19 wherein the switch assembly includes a cam operable to block selectively the pivotal movement of the lever.

21. A water heater according to claim 20 wherein the heat exchanger has a pair of heating elements and the switch assembly has respective levers pivotal under the control of the actuator to operate respective switches for controlling the power supply to each heating element, and respective cams operable to block selectively the pivotal movement of each lever.

22. A water heater according to claim 21 wherein each lever engages a respective one end of a beam extending transversely through the push rod and resiliently biassed to said one end.

23. A water heater according to claim 21 or claim 22 wherein the cams are mounted on a common spindle rotatable by a second control knob for selectively blocking neither, one or both levers.

24. A water heater according to any one of claims 16 to 23 wherein the heat exchanger includes a thermal cut-out to prevent overheating of the or each heating element.

25. A water heater according to any one of claims 16 to 24 wherein the heat exchanger includes a pressure relief valve to release automatically an excessive pressure build-up in the heat exchanger.

26. A water heater according to any one of claims 16 to 25 wherein the valve, heat exchanger and switch assembly are housed in a casing.

27. A water heater substantially as hereinbefore described with reference to Figures 1 to 8 of the accompanying drawings.