Patent Publication Number: US-2010123013-A1

Title: Thermostatic mixing valve

Description:
PRIORITY CLAIM 
     This application is a continuation application of and claims the benefit of U.S. patent application Ser. No. 11/804,631 filed on May 18, 2007, which is a continuation application of and claims the benefit of U.S. patent application Ser. No. 10/607,025, filed on Jun. 26, 2003, now U.S. Pat. No. 7,240,850, each application being incorporated herein in their entireties. 
    
    
     FIELD OF THE INVENTION 
     This invention concerns improvements in or relating to thermostatic mixing valves. The invention has particular, but not exclusive, application to thermostatic mixing valves for water supply installations such as showers. 
     BACKGROUND 
     Thermostatic mixing valves provide a source of water having a desired temperature and are operable to maintain the desired water temperature substantially constant. Typically, the desired water temperature is obtained by controlling the relative proportions or hot and cold water admitted to a mixing chamber and adjusting the relative proportions to maintain the desired water temperature substantially constant. 
     The known thermostatic mixing valves employ an actuator responsive to water temperature for adjusting the relative proportions in which the hot and cold water are mixed to maintain the desired water temperature substantially constant. Various types of actuators for providing the thermal control of the water temperature are known including thermally responsive elements positioned in the water flow for actuating the valve in response to the detected water temperature. For example, wax capsules or bimetal or memory metal type actuators. Alternatively, a motor may be provided to actuate the valve in response to the water temperature detected by temperature sensors. 
     In use, the outlet water temperature can deviate from the desired temperature if the temperature and/or pressure of one or both of the hot and cold water supplies to the mixing valve changes. A sudden increase or decrease in temperature that is sufficient to be discernible to the user may result in an uncomfortable experience. As a result, steady state temperature performance requirements are becoming increasingly more stringent with reductions in the permitted temperature deviations being introduced. For example, in mixing valves for healthcare applications such as in hospitals or care homes for the elderly or disabled, temperature deviations of only a few degrees are permitted. 
     In addition to steady state temperature performance requirements, transient temperature performance requirements dealing with temperature overshoots or undershoots when the operating conditions suddenly change are increasingly being included in valve specifications for certain applications, especially in the healthcare market. Transient temperature changes typically arise when the desired water temperature is changed, for example from cold to hot or where the valve is initially turned on. Under these conditions, the valve may initially change the relative proportions of hot and cold water more than is required before settling to produce water having the new desired temperature. The size and duration of any temperature overshoot or undershoot may only last a few seconds but is again discernible to the user if more than a few degrees and can be uncomfortable even if not presenting a safety risk. 
     A common approach to meet these tighter performance standards has been to seek to improve the thermal control system and in particular the accuracy and speed of response of the system to detected changes in the desired temperature of the water. This approach is based on the assumption that the water has been properly mixed so that the system responds equally to any changes tending to increase or decrease the desired water temperature. 
     This approach has not been completely successful, however, and in many cases valve performance is generally not as good as predicted by theoretical calculations. Often the valve responds more to changes in one water supply than the other and the thermal control system is adapted by trial and error to produce an arrangement in which the response is consistent to changes in either supply. In particular, the outlet water temperature may deviate from that selected if the inlet pressures change and skewing the response of the valve to inlet pressure changes may be required to ensure that any deviation of the outlet water temperature fits within a permitted tolerance range. Such skewing of the response is undesirable however as it may not result in optimum performance for all the circumstances that may arise in use. 
     As a result of extensive testing, we have now found that in many existing valve designs incomplete mixing of the water occurs and the water temperature detected by the thermal control system is made up of the temperature of partially mixed and unmixed streams of hot and cold water. Indeed, for some designs, as little as 25% of the water stream is made up of mixed water having the desired temperature. 
     Typically, the waterways within the valve are made up of spaces between valve components and are not particularly streamlined. This can produce variations in the flow through the valve which, together with incomplete mixing of the water, is now believed to be a reason for quite significant variations in performance occurring from one valve to another. For example, we have found that when a valve that fails to meet the required performance standards during testing is taken apart, there is often nothing wrong with it and, when re-assembled with the same components, the valve can pass the performance standards on re-testing. 
     As a result, a considerable amount of time and attention has been spent in ensuring that only valve components of the highest quality are used and that assembly is carried out very carefully. This adds considerably to production costs and the fundamental problem of variations in performance between valves assembled from the same components still persists. 
     The present invention has been made from a consideration of the aforementioned disadvantages and drawbacks of existing thermostatic mixing valves. 
     SUMMARY 
     It is a desired object of the present invention to provide an improved thermostatic mixing valve that is operable in a reliable manner. 
     It is yet another preferred object of the present invention to provide an improved thermostatic mixing valve having performance characteristics consistent with theoretical calculations. 
     It is a still further desired object of the present invention to provide an improved thermostatic mixing valve having application to installations for different requirements. 
     Other preferred objects of the present invention will be apparent from the description later herein of exemplary embodiments. 
     According to a first aspect of the present invention we provide a thermostatic mixing valve having a hot water inlet for connection to a supply of hot water, a cold water inlet for connection to a supply of cold water, an outlet for temperature controlled water, valve means for controlling the relative proportions of hot and cold water admitted to a mixing chamber, the outlet communicating with the mixing chamber to receive temperature controlled water having a desired temperature, temperature control means for adjusting the valve means in accordance with the desired temperature of the temperature controlled water, each inlet communicating with a multi-stage plenum chamber constructed and arranged to distribute flow of water to porting of the valve means for admitting the water to the mixing chamber. 
     By this invention, the incoming flows of hot and cold water are managed to produce conditions that reduce asymmetric flow patterns and promote thorough mixing of the water flows. As a result, a faster, more accurate response to change in the outlet water temperature can be achieved both for steady state operation to maintain a desired outlet water temperature substantially constant and to reduce transient temperature overshoots/undershoots when the desired outlet water temperature is adjusted by the user. 
     This approach to manage the incoming water flows is totally different to the prior art. More especially, the present invention recognises and provides a solution to the problems of incomplete mixing of the hot and cold flows on the accuracy and reliability of the thermal control systems employed to adjust the relative proportions of the hot and cold flows. In particular, the present invention enhances the mixing of the hot and cold flows by distributing the flows uniformly with respect to the porting for admitting the flows to the mixing chamber. 
     In this way, the development of asymmetric flow patterns that tend to keep the flows separate is reduced or eliminated. As a result, substantially complete mixing of the flows to provide a fully blended flow can be achieved within a mixing chamber of relatively small volume. This enables detection of the outlet water temperature to be carried out soon after the flows have been brought together and enhances the response of the valve to changes in the desired water temperature. 
     Each inlet preferably communicates with an annular outer chamber of a two stage plenum chamber having an annular inner chamber separated from the outer chamber by partition means arranged so that water flows around the outer chamber and into the inner chamber at a position axially spaced from the porting of the valve means. 
     In this way, the water is initially distributed around the outer chamber and approaches the porting in an axial direction within the inner chamber. As a result, swirling flow vectors are significantly reduced as the water approaches the porting and the distribution of water volume and velocity energy is substantially even around the porting for both flows. This produces essentially identical mixing conditions for both flows entering the mixing chamber that in turn leads to enhanced mixing that avoids the formation of separate streams of mixed and unmixed water. 
     Preferably, each plenum chamber is of similar size and shape so that the distribution of flows is substantially the same. As a result, both flows are matched so that any asymmetry is cancelled out when the flows merge within the mixing chamber. In this way, conditions in which separate streams of mixed and unmixed water may be formed are eliminated to a large extent. 
     The partition means separating the outer and inner chambers may be an annular wall provided with at least one opening for water to flow into the inner chamber. Preferably, the opening provides a substantially uniform distribution of the water flow around the inner chamber. For example, the opening may be in the form of a continuous annular slot in the wall between the outer and inner chambers. Alternatively, the opening may be in the form of a series of slots or holes of uniform size and shape formed in the wall between the outer and inner chambers with a regular spacing between the slots in the circumferential direction. 
     Preferably, the opening is offset relative to the point at which the water flow enters the outer chamber. In this way, the water is prevented from flowing directly into the inner chamber and is confined to flow around the outer chamber. This further contributes to a uniform distribution of water flowing towards the porting within the inner chamber. 
     In a preferred arrangement, the opening is axially offset relative to the point at which the water flow enters the outer chamber. In this way, the water flow is distributed around the outer chamber and approaches the opening in an axial direction before flowing into the inner chamber. This leads to a further reduction in swirling flow vectors and enhances uniform distribution of the water flow towards the porting within the inner chamber. 
     The valve means may comprise a shuttle valve mounted for axial movement relative to annular hot and cold seats to vary the relative proportions of hot and cold water admitted to the mixing chamber. Preferably, the hot and cold seats are close together so that the water flows are brought together and merge quickly. The shuttle valve may comprise a cylindrical shuttle of short axial length mounted between the hot and cold seats and having an annular sealing face at each end for co-operating with the hot and cold seats. More preferably, however, the hot and cold seats are positioned between a pair of hot and cold shuttles having annular sealing faces for co-operating with the hot and cold seats. For example, the hot and cold seats may be provided by opposite sides of an annular seating member such as a washer. In this way, the hot and cold flows may enter the mixing chamber at substantially the same axial position. In either arrangement, the hot seat at least may be resilient for enhanced sealing contact with the opposed sealing face of the shuttle to cut-off the flow of hot water. 
     Alternatively, the valve means may comprise a spool valve mounted for axial movement relative to an annular flow separator to vary the relative proportions of hot and cold water admitted to the mixing chamber. The spool valve may comprise a cylindrical shuttle axially movable relative to an O-ring to vary the area of axially extending slots in the shuttle to the flows of hot and cold water. This arrangement brings the flows of hot and cold water together quickly and promotes mixing of the flows. The slots may be inclined to the longitudinal axis of the shuttle so that the flows of hot and cold water are offset in the circumferential direction. This causes the flows to interlace and further promotes mixing of the flows. 
     Preferably, the flow of hot and cold water across the hot and cold seats is in a radial inwards direction and both flows are then turned in an axial direction to merge within the mixing chamber. For example, the flows may contact curved surfaces arranged to guide the flows in the axial direction. One or both of the hot and cold flows may be provided with a curved surface on the inboard edge of the porting such that turning the flows in the axial direction is assisted by the Coanda effect. Turning the flows in the axial direction creates an area of low pressure on the upstream side of that flow that may be used to entrain and assist the other flow. This may usefully be employed where the hot water flow is at a higher pressure to entrain the cold water flow and thereby improve the response of the thermal control system to change in the desired temperature of the water. 
     The mixing chamber is preferably sized to match the total flow through the valve. The total flow is dependent on the combined waterway cross-sectional areas at the hot and cold seats and is substantially constant for all adjusted positions of the valve means. By sizing the mixing chamber to match the permitted flow in this way, the velocity energy of the hot and cold flows admitted to the mixing chamber is largely maintained. This contributes to the creation of turbulent flow within the mixing chamber that promotes thorough mixing of the hot and cold water flows. As a result, substantially complete mixing can be achieved over a relatively short distance from the point the hot and cold flows are brought together within the mixing chamber. In this way, fast, accurate response of the thermostat to changes in the desired water temperature is achieved by ensuring the thermostat is exposed to water that has been properly mixed and by reducing transport delays. 
     We have found that the above benefits and advantages are optimised if the cross-sectional area of the mixing chamber is from 1 to 1.5 times and more preferably from 1 to 1.25 times the combined cross-sectional areas of the hot and cold flows and the axial length of the mixing chamber is at least 5 times the width of the mixing chamber and more preferably from 5 to 10 times the width of the mixing chamber. 
     The mixing chamber may be of any suitable shape and is preferably of annular ring shape between inner and outer walls. In this way, the width of the mixing chamber can be kept small thereby reducing the axial length required to achieve complete mixing of the hot and cold flows. As a result, the mixing chamber can be accommodated without having to increase the overall size of the valve compared to existing valves. 
     Advantageously, the mixing chamber has smooth walls and is shaped to provide substantially unobstructed flow with a gradual increase in cross-sectional area in the direction of flow. In this way, mixing of the hot and cold flows is further enhanced and some of the velocity energy required for turbulent flow may be recovered as pressure energy for discharge of the mixed water from the valve. 
     Preferably, the temperature control means is linked to the valve means for user selection of a desired water temperature and is operable to maintain the selected temperature substantially constant. In this way, user selection of a range of water temperatures, for example from cold to 60° C. may be permitted by any suitable means, for example a rotatable control knob or push button or other means of temperature selection. 
     The temperature control means may be of any suitable type commonly employed in thermostatic mixing valves to respond to a detected deviation of the mixed water temperature from the desired temperature to adjust the valve means to return the mixed water temperature to the desired temperature. For example, the temperature control means may comprise a thermostat containing a filler such as wax arranged to sense the temperature of the mixed water and an actuator responsive to expansion/contraction of the filler to adjust the valve means. Alternatively, the temperature control means may comprise at least one temperature sensor such as a thermistor arranged to sense the temperature of the mixed water and an actuator such as an electric motor operable under the control of a controller such as a microprocessor to adjust the valve means. 
     The valve may include means for controlling the flow of hot and/or cold water. The flow control may be separate from the temperature control or may be linked to the temperature control. 
     In one arrangement, the flow control is separate from the temperature control and comprises flow control valves between the inlets and each plenum chamber for controlling the flows of hot and cold water. Preferably the flow control valves are linked for operation simultaneously by a common control member such as a rotatable flow control knob or any other suitable means. For example, each flow control valve may comprise a sliding plate valve with at least one fixed valve plate and one movable valve plate for controlling flow. Preferably, the movable plate is adjustable between a closed position in which openings in the plates are out of alignment to shut-off the flow and a range of open positions in which the openings overlap by varying amounts to adjust the flow. The plates may be ceramic plates. 
     In another arrangement, the flow control is linked to the temperature control and is operable to control the flows in sequence whereby the cold water flow is turned on first during start-up and the hot water flow is turned off first during close-down. In this way, the water temperature increases from full cold when the valve is initially turned on and reduces to full cold again when the valve is finally turned off. 
     Preferably, the valve comprises a main body having the inlets for connection to the hot and cold supplies and the outlet for connection to an ablutionary appliance and an opening for reception of a cartridge unit housing the valve means. The outer chamber of each plenum chamber may be defined between the valve body and the cartridge unit with the inner chamber being formed inside in the cartridge unit and communicating with the outer chamber via at least one opening in the wall of the cartridge unit. 
     Preferably the cartridge unit carries seals, for example O-rings for sealing the cartridge unit in the valve body and separating the outer chambers. The O-rings may be of decreasing diameter from the outer end to the inner end of the cartridge unit to provide clearance for insertion of the cartridge unit. In this way, fitment and removal of the cartridge unit is facilitated and sealing engagement is obtained when the cartridge unit is fully inserted in the valve body. 
     According to a second aspect of the invention we provide a thermostatic mixing valve for hot and cold water having two-stage inlet chambers for the hot and cold water flows respectively, the inlet chambers being arranged to distribute the flows uniformly with respect to porting for admitting the flows to a mixing chamber to reduce asymmetric flow patterns and promote thorough mixing of the flows within the mixing chamber. 
     According to a third aspect of the present invention we provide a method of reducing asymmetric flow patterns and promoting thorough mixing of flows of hot and cold water within a mixing chamber of a thermostatic mixing valve comprising providing multi-stage inlet chambers for the hot and cold water flows respectively, and arranging the inlet chambers to distribute the flows uniformly with respect to porting for admitting the flows to a mixing chamber. 
     According to a fourth aspect of the present invention we provide a thermostatic mixing valve having a hot water inlet for connection to a supply of hot water, a cold water inlet for connection to a supply of cold water, an outlet for temperature controlled water, valve means for controlling the relative proportions of hot and cold water admitted to a mixing chamber, the outlet communicating with the mixing chamber to receive temperature controlled water having a desired temperature, temperature control means for adjusting the valve means in accordance with the desired temperature of the temperature controlled water, wherein the mixing chamber is sized to match the total flow through the valve. 
     By sizing the mixing chamber to match the total flow through the valve, the velocity energy of the hot and cold flows admitted to the mixing chamber is largely maintained. This contributes to the creation of turbulent flow within the mixing chamber that promotes thorough mixing of the hot and cold water flows. As a result, substantially complete mixing can be achieved over a relatively short distance from the point the hot and cold flows are brought together within the mixing chamber. In this way, fast, accurate response to changes in the desired water temperature is achieved by ensuring the control means responds to water that has been properly mixed and by reducing transport delays. As a result, steady state operation to maintain a desired outlet water temperature substantially constant is more reliable and transient temperature overshoots/undershoots when the desired outlet water temperature is adjusted by the user may be reduced. 
     This approach is totally different to the prior art. More especially, the present invention recognises and provides a solution to the problems of incomplete mixing of the hot and cold flows on the accuracy and reliability of the thermal control systems employed to adjust the relative proportions of the hot and cold flows. In particular, the present invention enhances the mixing of the hot and cold flows by sizing the mixing chamber to create turbulent flow conditions. In this way, the development of asymmetric flow patterns that tend to keep the flows separate is reduced or eliminated. As a result, substantially complete mixing of the flows to provide a fully blended flow can be achieved within a mixing chamber of relatively small volume. This enables detection of the outlet water temperature to be carried out soon after the flows have been brought together and enhances the response of the valve to changes in the desired water temperature. 
     The total flow through the valve is dependent on the combined waterway cross-sectional areas of the hot and cold flows through the proportional valve means and is substantially constant for all adjusted positions of the valve means. 
     We have found that the above benefits and advantages are optimised if the cross-sectional area of the mixing chamber is from 1 to 1.5 times and more preferably from 1 to 1.25 times the combined cross-sectional areas of the hot and cold flows. In this way, turbulent flow conditions are optimised and substantially complete mixing of the hot and cold flows can be achieved if the axial length of the mixing chamber is at least 5 times the width of the mixing chamber and more preferably from 5 to 10 times the width of the mixing chamber. 
     The mixing chamber may be of any suitable shape and is preferably of annular ring shape between inner and outer walls. In this way, the width of the mixing chamber can be kept small thereby reducing the axial length required to achieve complete mixing of the hot and cold flows. As a result, the mixing chamber can be accommodated without having to increase the overall size of the valve compared to existing valves. 
     Advantageously, the mixing chamber has smooth walls and is shaped to provide substantially unobstructed flow with a gradual increase in cross-sectional area in the direction of flow. In this way, mixing of the hot and cold flows is further enhanced and some of the velocity energy required for turbulent flow may be recovered as pressure energy for discharge of the mixed water from the valve. 
     Each inlet preferably communicates with an annular outer chamber of a two stage plenum chamber having an annular inner chamber separated from the outer chamber by partition means arranged so that water flows around the outer chamber and into the inner chamber at a position axially spaced from the porting of the valve means. 
     In this way, the water is initially distributed around the outer chamber and approaches the porting in an axial direction within the inner chamber. As a result, swirling flow vectors are significantly reduced as the water approaches the porting and the distribution of water volume and velocity energy is substantially even around the porting for both flows. This produces essentially identical mixing conditions for both flows entering the mixing chamber that in turn leads to enhanced mixing that avoids the formation of separate streams of mixed and unmixed water. 
     Preferably, each plenum chamber is of similar size and shape so that the distribution of flows is substantially the same. As a result, both flows are matched so that any asymmetry is cancelled out when the flows merge within the mixing chamber. In this way, conditions in which separate streams of mixed and unmixed water may be formed are eliminated to a large extent. 
     The partition means separating the outer and inner chambers may be an annular wall provided with at least one opening for water to flow into the inner chamber. Preferably, the opening provides a substantially uniform distribution of the water flow around the inner chamber. For example, the opening may be in the form of a continuous annular slot in the wall between the outer and inner chambers. Alternatively, the opening may be in the form of a series of slots or holes of uniform size and shape formed in the wall between the outer and inner chambers with a regular spacing between the slots in the circumferential direction. 
     Preferably, the opening is offset relative to the point at which the water flow enters the outer chamber. In this way, the water is prevented from flowing directly into the inner chamber and is confined to flow around the outer chamber. This further contributes to a uniform distribution of water flowing towards the porting within the inner chamber. 
     In a preferred arrangement, the opening is axially offset relative to the point at which the water flow enters the outer chamber. In this way, the water flow is distributed around the outer chamber and approaches the opening in an axial direction before flowing into the inner chamber. This leads to a further reduction in swirling flow vectors and enhances uniform distribution of the water flow towards the porting within the inner chamber. By this use of two-stage plenum chambers, the incoming flows of hot and cold water are managed to produce conditions that help to reduce asymmetric flow patterns and further promote thorough mixing of the water flows within the mixing chamber. More especially, the plenum chambers distribute the flows uniformly with respect to the porting for admitting the flows to the mixing chamber. In this way, the development of asymmetric flow patterns that tend to keep the flows separate is reduced or eliminated. 
     The valve means may comprise a shuttle valve mounted for axial movement relative to annular hot and cold seats to vary the relative proportions of hot and cold water admitted to the mixing chamber. Preferably, the hot and cold seats are close together so that the water flows are brought together and merge quickly. The shuttle valve may comprise a cylindrical shuttle of short axial length mounted between the hot and cold seats and having an annular sealing face at each end for co-operating with the hot and cold seats. More preferably, however, the hot and cold seats are positioned between a pair of hot and cold shuttles having annular sealing faces for co-operating with the hot and cold seats. For example, the hot and cold seats may be provided by opposite sides of an annular seating member such as a washer. In this way, the hot and cold flows may enter the mixing chamber at substantially the same axial position. In either arrangement, the hot seat at least may be resilient for enhanced sealing contact with the opposed sealing face of the shuttle to cut-off the flow of hot water. 
     Alternatively, the valve means may comprise a spool valve mounted for axial movement relative to an annular flow separator to vary the relative proportions of hot and cold water admitted to the mixing chamber. The spool valve may comprise a cylindrical shuttle axially movable relative to an O-ring to vary the area of axially extending slots in the shuttle to the flows of hot and cold water. This arrangement brings the flows of hot and cold water together quickly and promotes mixing of the flows. The slots may be inclined to the longitudinal axis of the shuttle so that the flows of hot and cold water are offset in the circumferential direction. This causes the flows to interlace and further promotes mixing of the flows. 
     Preferably, the flow of hot and cold water across the hot and cold seats is in a radial inwards direction and both flows are then turned in an axial direction to merge within the mixing chamber. For example, the flows may contact curved surfaces arranged to guide the flows in the axial direction. One or both of the hot and cold flows may be provided with a curved surface on the inboard edge of the porting such that turning the flows in the axial direction is assisted by the Coanda effect. Turning the flows in the axial direction creates an area of low pressure on the upstream side of that flow that may be used to entrain and assist the other flow. This may usefully be employed where the hot water flow is at a higher pressure to entrain the cold water flow and thereby improve the response of the thermal control system to change in the desired temperature of the water. 
     Preferably, the temperature control means is linked to the valve means for user selection of a desired water temperature and is operable to maintain the selected temperature substantially constant. In this way, user selection of a range of water temperatures, for example from cold to 60° C. may be permitted by any suitable means, for example a rotatable control knob or push button or other means of temperature selection. 
     The temperature control means may be of any suitable type commonly employed in thermostatic mixing valves to respond to a detected deviation of the mixed water temperature from the desired temperature to adjust the valve means to return the mixed water temperature to the desired temperature. For example, the temperature control means may comprise a thermostat containing a filler such as wax arranged to sense the temperature of the mixed water and an actuator responsive to expansion/contraction of the filler to adjust the valve means. Alternatively, the temperature control means may comprise at least one temperature sensor such as a thermistor arranged to sense the temperature of the mixed water and an actuator such as an electric motor operable under the control of a controller such as a microprocessor to adjust the valve means. 
     The valve may include means for controlling the flow of hot and/or cold water. The flow control may be separate from the temperature control or may be linked to the temperature control. 
     In one arrangement, the flow control is separate from the temperature control and comprises flow control valves between the inlets and each plenum chamber for controlling the flows of hot and cold water. Preferably the flow control valves are linked for operation simultaneously by a common control member such as a rotatable flow control knob or any other suitable means. For example, each flow control valve may comprise a sliding plate valve with at least one fixed valve plate and one movable valve plate for controlling flow. Preferably, the movable plate is adjustable between a closed position in which openings in the plates are out of alignment to shut-off the flow and a range of open positions in which the openings overlap by varying amounts to adjust the flow. The plates may be ceramic plates. 
     In another arrangement, the flow control is linked to the temperature control and is operable to control the flows in sequence whereby the cold water flow is turned on first during start-up and the hot water flow is turned off first during close-down. In this way, the water temperature increases from full cold when the valve is initially turned on and reduces to full cold again when the valve is finally turned off. 
     Preferably, the valve comprises a main body having the inlets for connection to the hot and cold supplies and the outlet for connection to an ablutionary appliance and an opening for reception of a cartridge unit housing the valve means. The outer chamber of each plenum chamber may be defined between the valve body and the cartridge unit with the inner chamber being formed inside in the cartridge unit and communicating with the outer chamber via at least one opening in the wall of the cartridge unit. 
     Preferably the cartridge unit carries seals, for example O-rings for sealing the cartridge unit in the valve body and separating the outer chambers. The O-rings may be of decreasing diameter from the outer end to the inner end of the cartridge unit to provide clearance for insertion of the cartridge unit. In this way, fitment and removal of the cartridge unit is facilitated and sealing engagement is obtained when the cartridge unit is fully inserted in the valve body. 
     According to a fifth aspect of the present invention we provide a thermostatic mixing valve having a hot water inlet for connection to a supply of hot water, a cold water inlet for connection to a supply of cold water, an outlet for temperature controlled water, valve means for controlling the relative proportions of hot and cold water admitted to a mixing chamber, the outlet communicating with the mixing chamber to receive temperature controlled water having a desired temperature, temperature control means for adjusting the valve means in accordance with the desired temperature of the temperature controlled water, wherein the mixing chamber has a cross-sectional area 1 to 1.5 times the combined cross-sectional areas of the hot and cold flows through the proportioning valve means. 
     According to a sixth aspect of the present invention we provide a thermostatic mixing valve having a hot water inlet for connection to a supply of hot water, a cold water inlet for connection to a supply of cold water, an outlet for temperature controlled water, valve means for controlling the relative proportions of hot and cold water admitted to a mixing chamber, the outlet communicating with the mixing chamber to receive temperature controlled water having a desired temperature, temperature control means for adjusting the valve means in accordance with the desired temperature of the temperature controlled water, wherein the mixing chamber has an axial length at least 5 times the width of the mixing chamber. 
     According to a seventh aspect of the present invention we provide a thermostatic mixing valve having a hot water inlet for connection to a supply of hot water, a cold water inlet for connection to a supply of cold water, an outlet for temperature controlled water, valve means for controlling the relative proportions of hot and cold water admitted to a mixing chamber, the outlet communicating with the mixing chamber to receive temperature controlled water having a desired temperature, temperature control means for adjusting the valve means in accordance with the desired temperature of the temperature controlled water, wherein the mixing chamber has a cross-sectional area relative to the combined cross-sectional areas of the hot and cold flows such that the velocity energy of the hot and cold flows is sufficient to create turbulent flow conditions within the mixing chamber. 
     Preferably, the cross-sectional area of the mixing chamber is at least equal to the combined cross-sectional areas of the hot and cold flows. 
     According to an eighth aspect of the present invention we provide a thermostatic mixing valve having a hot water inlet for connection to a supply of hot water, a cold water inlet for connection to a supply of cold water, an outlet for temperature controlled water, valve means for controlling the relative proportions of hot and cold water admitted to a mixing chamber, the outlet communicating with the mixing chamber to receive temperature controlled water having a desired temperature, temperature control means for adjusting the valve means in accordance with the desired temperature of the temperature controlled water, wherein the mixing chamber is arranged so that incoming streams of hot and cold water are turned to flow in the same direction such that flow of the hot stream entrains and assists flow of the cold stream. 
     By this invention, the incoming streams of hot and cold water are managed so that the interaction between the hot and cold streams tend to aid the temperature control process. In particular, flow of the entrained cold stream is increased by an increase in pressure of the hot stream tending to maintain the initial proportions of hot and cold water and assist the response of the temperature control means to maintain the desired temperature. 
     Preferably, the hot and cold streams enter the mixing chamber in a radial direction and are turned in an axial direction to merge within the mixing chamber. For example, one or both of the streams may contact curved surfaces arranged to guide the streams in the axial direction. The curved surfaces may be provided by radially inner and outer walls of the mixing chamber. The inner wall may assist turning one of the streams and the outer wall assist turning the other stream. In this way, undesirable crossing of the streams affecting the flow through the valve may be reduced. Turning the water stream by the inner wall may be assisted by the Coanda effect. 
     Advantageously, the hot and cold streams enter the mixing chamber close together in the axial direction of flow whereby each stream entrains and assists the flow of the other stream. In this way an increase in pressure of either stream tends to increase the flow of the other stream to assist the response of the temperature control means to maintain the desired temperature. 
     The valve means may comprise a shuttle valve mounted for axial movement relative to annular hot and cold seats to vary the relative proportions of hot and cold water admitted to the mixing chamber. 
     Preferably, the hot seat at least is resilient for enhanced sealing contact with an opposed sealing face of the shuttle valve to cut-off the flow of hot water. 
     The shuttle valve may comprise a cylindrical shuttle of short axial length mounted between the hot and cold seats and having annular sealing faces at opposite ends for co-operating with the hot and cold seats. 
     More preferably, however, the hot and cold seats are positioned between a pair of hot and cold shuttles having annular sealing faces for co-operating with the hot and cold seats. In this way, the hot and cold streams enter the mixing chamber at substantially the same axial position so that the streams are brought together and merge quickly to promote mixing of the streams. 
     In one arrangement, the hot and cold seats are provided by opposite sides of an annular seating member such as a washer. The seating member may be incorporated in the valve body. 
     Alternatively, the valve means may comprise a spool valve mounted for axial movement relative to an annular flow separator to vary the relative proportions of hot and cold water admitted to the mixing chamber. 
     The spool valve may comprise a cylindrical shuttle axially movable relative to an O-ring to vary the area of axially extending slots in the shuttle to the streams of hot and cold water. This arrangement brings the streams of hot and cold water together quickly and promotes mixing of the streams. 
     The slots may be inclined to the longitudinal axis of the shuttle so that the streams of hot and cold water are offset in the circumferential direction. This causes the streams to interlace and further promotes mixing of the streams. 
     Preferably, each inlet communicates with an annular outer chamber of a two stage plenum chamber having an annular inner chamber separated from the outer chamber by partition means arranged so that water flows around the outer chamber and into the inner chamber at a position axially spaced from the porting of the valve means. 
     In this way, the water is initially distributed around the outer chamber and approaches the porting in an axial direction within the inner chamber. As a result, swirling flow vectors are significantly reduced as the water approaches the porting and the distribution of water volume and velocity energy is substantially even around the porting for both flows. This produces essentially identical mixing conditions for both flows entering the mixing chamber that in turn leads to enhanced mixing that avoids the formation of separate streams of mixed and unmixed water. 
     Preferably, each plenum chamber is of similar size and shape so that the distribution of flows is substantially the same. As a result, both flows are matched so that any asymmetry is cancelled out when the flows merge within the mixing chamber. In this way, conditions in which separate streams of mixed and unmixed water may be formed are eliminated to a large extent. 
     The partition means separating the outer and inner chambers may be an annular wall provided with at least one opening for water to flow into the inner chamber. Preferably, the opening provides a substantially uniform distribution of the water flow around the inner chamber. For example, the opening may be in the form of a continuous annular slot in the wall between the outer and inner chambers. Alternatively, the opening may be in the form of a series of slots or holes of uniform size and shape formed in the wall between the outer and inner chambers with a regular spacing between the slots in the circumferential direction. 
     Preferably, the opening is offset relative to the point at which the water flow enters the outer chamber. In this way, the water is prevented from flowing directly into the inner chamber and is confined to flow around the outer chamber. This further contributes to a uniform distribution of water flowing towards the porting within the inner chamber. 
     In a preferred arrangement, the opening is axially offset relative to the point at which the water flow enters the outer chamber. In this way, the water flow is distributed around the outer chamber and approaches the opening in an axial direction before flowing into the inner chamber. This leads to a further reduction in swirling flow vectors and enhances uniform distribution of the water flow towards the porting within the inner chamber. 
     In this way, the development of asymmetric flow patterns that tend to keep the flows separate is reduced or eliminated. As a result, substantially complete mixing of the flows to provide a fully blended flow can be achieved within a mixing chamber of relatively small volume. This enables detection of the outlet water temperature to be carried out soon after the flows have been brought together and enhances the response of the valve to changes in the desired water temperature. 
     The mixing chamber is preferably sized to match the total flow through the valve. The total flow is dependent on the combined waterway cross-sectional areas at the hot and cold seats and is substantially constant for all adjusted positions of the valve means. By sizing the mixing chamber to match the permitted flow in this way, the velocity energy of the hot and cold flows admitted to the mixing chamber is largely maintained. This contributes to the creation of turbulent flow within the mixing chamber that promotes thorough mixing of the hot and cold water flows. 
     As a result, substantially complete mixing can be achieved over a relatively short distance from the point the hot and cold flows are brought together within the mixing chamber. In this way, fast, accurate response of the thermostat to changes in the desired water temperature is achieved by ensuring the thermostat is exposed to water that has been properly mixed and by reducing transport delays. 
     We have found that the above benefits and advantages are optimised if the cross-sectional area of the mixing chamber is from 1 to 1.5 times and more preferably from 1 to 1.25 times the combined cross-sectional areas of the hot and cold flows and the axial length of the mixing chamber is at least 5 times the width of the mixing chamber and more preferably from 5 to 10 times the width of the mixing chamber. 
     The mixing chamber may be of any suitable shape and is preferably of annular ring shape between inner and outer walls. In this way, the width of the mixing chamber can be kept small thereby reducing the axial length required to achieve complete mixing of the hot and cold flows. As a result, the mixing chamber can be accommodated without having to increase the overall size of the valve compared to existing valves. 
     Advantageously, the mixing chamber has smooth walls and is shaped to provide substantially unobstructed flow with a gradual increase in cross-sectional area in the direction of flow. In this way, mixing of the hot and cold flows is further enhanced and some of the velocity energy required for turbulent flow may be recovered as pressure energy for discharge of the mixed water from the valve. 
     Preferably, the temperature control means is linked to the valve means for user selection of a desired water temperature and is operable to maintain the selected temperature substantially constant. In this way, user selection of a range of water temperatures, for example from cold to 60° C. may be permitted by any suitable means, for example a rotatable control knob or push button or other means of temperature selection. 
     The temperature control means may be of any suitable type commonly employed in thermostatic mixing valves to respond to a detected deviation of the mixed water temperature from the desired temperature to adjust the valve means to return the mixed water temperature to the desired temperature. For example, the temperature control means may comprise a thermostat containing a filler such as wax arranged to sense the temperature of the mixed water and an actuator responsive to expansion/contraction of the filler to adjust the valve means. Alternatively, the temperature control means may comprise at least one temperature sensor such as a thermistor arranged to sense the temperature of the mixed water and an actuator such as an electric motor operable under the control of a controller such as a microprocessor to adjust the valve means. 
     The valve may include means for controlling the flow of hot and/or cold water. The flow control may be separate from the temperature control or may be linked to the temperature control. 
     In one arrangement, the flow control is separate from the temperature control and comprises flow control valves between the inlets and each plenum chamber for controlling the flows of hot and cold water. Preferably the flow control valves are linked for operation simultaneously by a common control member such as a rotatable flow control knob or any other suitable means. For example, each flow control valve may comprise a sliding plate valve with at least one fixed valve plate and one movable valve plate for controlling flow. Preferably, the movable plate is adjustable between a closed position in which openings in the plates are out of alignment to shut-off the flow and a range of open positions in which the openings overlap by varying amounts to adjust the flow. The plates may be ceramic plates. 
     In another arrangement, the flow control is linked to the temperature control and is operable to control the flows in sequence whereby the cold water flow is turned on first during start-up and the hot water flow is turned off first during close-down. In this way, the water temperature increases from full cold when the valve is initially turned on and reduces to full cold again when the valve is finally turned off. 
     Preferably, the valve comprises a main body having the inlets for connection to the hot and cold supplies and the outlet for connection to an ablutionary appliance and an opening for reception of a cartridge unit housing the valve means. The outer chamber of each plenum chamber may be defined between the valve body and the cartridge unit with the inner chamber being formed inside in the cartridge unit and communicating with the outer chamber via at least one opening in the wall of the cartridge unit. 
     Preferably the cartridge unit carries seals, for example O-rings for sealing the cartridge unit in the valve body and separating the outer chambers. The O-rings may be of decreasing diameter from the outer end to the inner end of the cartridge unit to provide clearance for insertion of the cartridge unit. In this way, fitment and removal of the cartridge unit is facilitated and sealing engagement is obtained when the cartridge unit is fully inserted in the valve body. 
     According to a ninth aspect of the present invention we provide a thermostatic mixing valve having a hot water inlet for connection to a supply of hot water, a cold water inlet for connection to a supply of cold water, an outlet for temperature controlled water, valve means for controlling the relative proportions of hot and cold water admitted to a mixing chamber, the outlet communicating with the mixing chamber to receive temperature controlled water having a desired temperature, temperature control means for adjusting the valve means in accordance with the desired temperature of the temperature controlled water, wherein the proportioning valve means comprises a shuttle valve having opposed sealing faces arranged for simultaneous movement relative to respective hot and cold valve seats positioned between the sealing faces for controlling the relative proportions of hot and cold water admitted to the mixing chamber. 
     By positioning the hot and cold valve seats between opposed sealing faces of the shuttle valve, the flows of hot and cold water are admitted to the mixing chamber close together in the axial direction and mixing of the flows is promoted. For example, the hot and cold valve seats may be provided on opposite sides of a thin, plate member such as a washer extending between the sealing faces. 
     Preferably, the sealing faces are provided by hot and cold shuttles shaped to assist turning the flows of hot and cold water in the same axial direction. For example, the shuttles may be provided with opposed curved surfaces on radially inner and outer walls of the mixing chamber. The curved surface on the inner wall may assist turning the flow due to the Coanda effect. 
     According to a tenth aspect of the present invention we provide a thermostatic mixing valve having a hot water inlet for connection to a supply of hot water, a cold water inlet for connection to a supply of cold water, an outlet for temperature controlled water, valve means for controlling the relative proportions of hot and cold water admitted to a mixing chamber, the outlet communicating with the mixing chamber to receive temperature controlled water having a desired temperature, temperature control means for adjusting the valve means in accordance with the desired temperature of the temperature controlled water, wherein the proportioning valve means comprises a shuttle valve having a proportioning shuttle axially slidable in a valve body relative to hot and cold seats for controlling the proportions of hot and cold water admitted to the mixing chamber, wherein the hot and cold seats are provided by opposite sides of an annular seating member integral with the valve body. 
     The seating member may comprise a thin, flat plate element such as a washer such that the flows of hot and cold water are admitted to the mixing chamber close together in the axial direction. 
     The valve body and seating member may be united in a single component by arranging the seating member as an insert in a plastics moulding die for the valve body. 
     According to an eleventh aspect of the present invention we provide a thermostatic mixing valve having a hot water inlet for connection to a supply of hot water, a cold water inlet for connection to a supply of cold water, an outlet for temperature controlled water, valve means for controlling the relative proportions of hot and cold water admitted to a mixing chamber, the outlet communicating with the mixing chamber to receive temperature controlled water having a desired temperature, temperature control means for adjusting the valve means in accordance with the desired temperature of the temperature controlled water, wherein the proportioning valve means comprises a shuttle valve having a valve body and a shuttle axially slidable in the valve body relative to hot and cold valve seats for controlling the relative proportions of hot and cold water admitted to the mixing chamber, the hot and cold seats being provided between opposed sealing faces of the shuttle, and guide means for maintaining the sealing faces square with respect to the valve seats. 
     According to a twelfth aspect of the present invention we provide a thermostatic mixing valve having a hot water inlet for connection to a supply of hot water, a cold water inlet for connection to a supply of cold water, an outlet for temperature controlled water, valve means for controlling the relative proportions of hot and cold water admitted to a mixing chamber, the outlet communicating with the mixing chamber to receive temperature controlled water having a desired temperature, temperature control means for adjusting the valve means in accordance with the desired temperature of the temperature controlled water, wherein each of the flows of hot and cold water is admitted to the mixing chamber at a plurality of openings. 
     Preferably, the hot flow openings alternate with the cold flow openings such that the flows interlace as they enter the mixing chamber thereby promoting mixing of the hot and cold water flows admitted to the mixing chamber. 
     Advantageously, the mixing chamber is of annular ring shape and the openings are arranged so that the flows of hot and cold water are offset in the circumferential direction causing the flows to interlace and promote mixing of the flows within the mixing chamber. 
     In one arrangement, the openings are formed in a cylindrical shuttle of a spool valve, the shuttle being mounted for axial movement relative to an annular flow separator to vary the relative proportions of hot and cold water admitted to the mixing chamber. For example, the shuttle may be axially movable relative to an O-ring separator to vary the area of axially extending slots in the shuttle to the flows of hot and cold water, the slots being inclined to the longitudinal axis of the shuttle so that the flows of hot and cold water are offset in the circumferential direction causing the flows to interlace and promote mixing of the flows within the mixing chamber. 
     In another arrangement, the valve means controls the relative proportions of hot and cold water admitted to separate hot and cold water chambers and the openings are provided between the hot and cold water chambers and the mixing chamber. For example, the hot and cold water chambers may be arranged concentrically at one of the mixing chamber. The valve means may be a proportioning mechanism to adjust the hot and cold flows inversely to one another. Alternatively, the valve means may comprise two separate valves that are separately controlled. 
     According to a thirteenth aspect of the present invention we provide a thermostatic mixing valve having a hot water inlet for connection to a supply of hot water, a cold water inlet for connection to a supply of cold water, an outlet for temperature controlled water, valve means for controlling the relative proportions of hot and cold water admitted to a mixing chamber, the outlet communicating with the mixing chamber to receive temperature controlled water having a desired temperature, temperature control means for adjusting the valve means in accordance with the desired temperature of the temperature controlled water, wherein the hot and cold water streams admitted to the mixing chamber are co-entrained. 
     Co-entraining the flows is beneficial in reducing potentially instability effects caused by differences between the hot and cold water pressures. Thus, if the water pressures are very different the streams of hot and cold water will have very different levels of energy. If the streams of hot and cold water entered the mixing chamber on opposite sides of the mixing chamber, there would be a risk that the higher energy stream could suppress the flow of the lower energy stream, resulting in a sudden deviation from the intended proportions of hot and cold water. An instability of the whole valve could result. Co-entraining the flows reduces suppression of the flow of the lower energy stream by the higher energy stream. 
     Various other features, benefits and advantages of the invented thermostatic mixing valve will be apparent from the description hereinafter of exemplary embodiments. 
     This invention will now be described in more detail, by way of example only, with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a front perspective view of a thermostatic mixing valve according to a first embodiment of the invention; 
         FIG. 2  is a front view, similar to  FIG. 1 , with the control knobs and cartridge cover removed; 
         FIG. 3  is an isometric sectioned view through the valve of  FIG. 1 ; 
         FIG. 4  is a transverse section through the valve of  FIG. 1 ; 
         FIG. 5  is a perspective view of a thermostatic mixing valve according to a second embodiment of the invention; 
         FIG. 6  is a longitudinal section through the valve of  FIG. 5 ; 
         FIG. 7  is longitudinal section through the valve of  FIG. 5  in part isometric projection; 
         FIG. 8  is an isometric sectioned view of part of the valve shown in  FIG. 6  as viewed from the left hand end of  FIG. 6 ; 
         FIG. 9  is an isometric sectioned view of part of the valve shown in  FIG. 6  as viewed from the right hand end of  FIG. 6 ; 
         FIG. 10  is a perspective view of a thermostatic mixing valve according to a third embodiment of the invention; 
         FIG. 11  is a perspective view of the body of the valve shown in  FIG. 10  with the cartridge unit removed; 
         FIG. 12  is a longitudinal section through the valve shown in  FIG. 10 ; 
         FIG. 13  shows the section of  FIG. 12  in part isometric projection; 
         FIG. 14  is a section of the hot seat insert of the valve shown in  FIGS. 10 to 13  in part isometric projection; 
         FIG. 15  is a perspective view of a thermostatic mixing valve according to a fourth embodiment of the invention; 
         FIG. 16  is a perspective view of the cartridge unit of the valve shown in  FIG. 15 ; 
         FIG. 17  is a longitudinal section through the inlets of the valve shown in  FIG. 15 ; 
         FIG. 18  is a longitudinal section through the outlet of the valve shown in  FIG. 17 ; 
         FIG. 19  is an isometric view, partly sectioned, showing a modification to the valve of  FIGS. 15 to 18 ; and 
         FIG. 20  is a longitudinal section showing part of a thermostatic mixing valve according to a fifth embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring first to  FIGS. 1 to 4  of the accompanying drawings, a thermostatic mixing valve  1  according to a first embodiment of the invention is shown. The mixing valve  1  has inlets  2  and  3  for connection to respective supplies of cold and hot water (not shown) and an outlet  4  for discharging temperature controlled water to an ablutionary appliance (not shown) such as a spray fitting for a shower or other washing equipment. In this embodiment, each inlet  2 , 3  has two ports  2   a , 2   b  and  3   a , 3   b  at right angles to each other for connecting the valve  1  to supply pipes from above or behind the valve  1 . A blanking plug (not shown) is provided for closing each port that is not connected to a supply pipe. 
     The valve  1  has a rotatable temperature control knob  5  detachably mounted on a drive spindle  6  of a temperature control mechanism described in more detail later herein. The knob  5  is rotatable relative to a fixed indicator ring  7  for user selection of a range of outlet-water temperatures, for example from cold to 60° C. 
     A stop (not shown) is provided to limit rotation of the knob  5  for user selection of outlet water temperatures up to a pre-set temperature, for example 40° C. for safe washing/showering. The knob  5  includes an over-ride button  8  operable by the user to release the stop and allow selection of outlet water temperatures higher then the pre-set temperature up to the maximum. 
     In this way, accidental or inadvertent selection of an outlet water temperature above the pre-set temperature is prevented but the user can purposively select higher temperatures if desired. The stop is automatically re-set when knob  5  is rotated to select a temperature below the pre-set temperature. 
     The valve  1  also has an annular flow control ring  9  mounted on a drive spindle  10  of a flow control mechanism described in more detail later herein. The control ring  9  is concentric with the control knob  5  and has a lever  11  for manual rotation of the control ring  9  for user control of a range of flows, for example from off to fully open. 
     As best shown in  FIG. 2 , the valve  1  has a body  12  with an oval opening  13  in the front for reception of a cartridge unit  14  incorporating the temperature control mechanism and flow control mechanism. The cartridge unit  14  is releasably secured in the body  12  by four screws  15  and is provided with cut-outs  16  in the marginal edge for releasably attaching a front cover  17  ( FIG. 1 ) for the cartridge unit. The valve body  1  is symmetrical and the cartridge unit  14  can be fitted to allow connection of the hot and cold supplies either way round to suit the installation lay-out. 
     With particular reference now to  FIGS. 3 and 4 , the flow control mechanism includes a separate flow control valve  18 , 19  for each supply. Each valve  18 , 19  is similar and comprises an assembly of three ceramic discs  20 , 21 , 22 . The outer discs  20 , 21  are fixed and the centre disc  22  is movable relative thereto to vary the overlap of openings in the discs to control the flow through the valve. For the purposes of illustration, the valve  18  is shown fully open and the valve  19  is shown fully closed. It will be understood, however, that in use both valves  18 , 19  are assembled to open and close at the same time. 
     Each valve  18 , 19  is operatively connected to the control ring  9  by a gear drive  23 , 24  for reciprocating movement of the associated centre disc  22  relative to the outer discs  20 , 21  in response to rotation of the control ring  9  in opposite directions. The gear drives  23 , 24  are linked to the control ring  9  for simultaneous adjustment to open and close both valves  18 , 19  in a synchronised manner. In this way, the selected temperature is substantially unaffected by adjustment of the valves  18 , 19  to increase/decrease the total flow of water through the valve  1 . 
     The temperature control mechanism includes a proportioning shuttle valve  25  for controlling the relative proportions of hot and cold water admitted to an annular mixing chamber  26 . The temperature control mechanism also includes a wax thermostat  27  arranged to sense the temperature of the mixed water and adjust the shuttle valve  25  to maintain a selected temperature substantially constant. 
     The shuttle valve  25  comprises a pair of shuttle valve members  28 , 29  mounted on the thermostat  27  and secured by a nut  30  screwed onto the thermostat  27 . Each valve member  28 , 29  has an annular seal face  31 , 32  arranged to co-operate with opposed annular seal faces  33 , 34  of an annular valve seat  35  positioned between the seal faces  31 , 32 . 
     The valve members  28 , 29  are fixed relative to each other and are axially movable together relative to the valve seat  35  between a first end position and a second end position. In the first end position, the valve member  28  engages the valve seat  35  to shut-off the flow of cold water. In the second end position, the valve member  29  engages the valve seat  35  to shut-off the flow of hot water. Between the end positions, the valve members  28 , 29  are spaced from the valve seat  35  to control the relative proportions of hot and cold water admitted to the mixing chamber  26  according to the axial position of the valve members  28 , 29 . 
     The valve seat  35  comprises a thin metal washer coated on both sides with a layer of rubber or similar elastomeric material. In this way, the seal faces  33 , 34  are resilient for engagement with the seal faces  31 , 32  of the shuttle valve members  28 , 29 . As a result, fluid tight engagement of the valve members  28 , 29  with the valve seat  35  is assured in each of the end positions. 
     The thermostat  27  contains a wax filler and has an actuator rod  36  that is axially movable in response to expansion/contraction of the wax filler in response to temperature of the mixed water sensed by the thermostat  27 . The free end of the rod  36  engages a cap  37  biased towards the rod  36  by an overload spring  38  acting between the cap  37  and the inner end of a sleeve member  39  screwed into a threaded bore  40  at the inner end of the drive spindle  6 . The cap  37  and overload spring  38  are retained in the sleeve member  39  by an end stop in the form of a U-shaped wire clip  37   a  inserted through holes (not shown) in the sleeve member  39  to locate the cap  37  in a pre-loaded state. 
     The sleeve member  39  is axially slidable in the shuttle valve member  29  and is located against rotation so as to be axially movable in response to rotation of the drive spindle  6  by user actuation of the control knob  5 . In this way, axial movement of the sleeve member  39  in response to user selection of the desired water temperature is transmitted to the shuttle valve members  28 , 29  via the thermostat  25 . As a result, the position of the valve members  28 , 29  relative to the valve seat  35  is adjusted to vary the relative proportions of hot and cold water admitted to the mixing chamber  26  to produce mixed water having the selected temperature. 
     If the temperature of the mixed water increases, the wax filler expands to increase the projecting length of the actuator rod  36 . This causes the thermostat  27  to be displaced axially away from the cap  37  against the biasing of a return spring  41  which is weaker than the overload spring  38 . 
     The thermostat  27  carries with it the shuttle valve members  28 , 29  causing the seal face  32  to move towards the seal face  34  of the valve seat  35  to reduce the flow of hot water and simultaneously increase the flow of cold water by moving the seal face  31  away from the seal face  33  of the valve seat  35 . In this way the relative proportions of hot and cold water admitted to the mixing chamber  26  are adjusted to return the temperature of the mixed water to the selected temperature. 
     If the temperature of the mixed water exceeds the maximum permitted, for example if cold water supply fails, expansion of the wax filler causes the valve member  29  to engage the valve seat  35  to shut-off the flow of hot water. Further elongation of the actuator rod  36  is accommodated by compression of the overload spring  38  to prevent damage to the internal components of the cartridge unit  14 . 
     If the temperature of the mixed water decreases, the wax filler contracts and the thermostat  27  is displaced axially towards the cap  37  reducing the projecting length of the rod  36  under the biasing of the return spring  41 . 
     As a result of this movement, seal face  32  of shuttle member  29  moves away from seal face  34  of the valve seat  35  to increase the flow of hot water and simultaneously the flow of cold water is reduced by seal face  31  of shuttle member  28  moving towards the seal face  33  of the valve seat  35 . In this way the relative proportions of hot and cold water admitted to the mixing chamber  26  are adjusted to return the temperature of the mixed water to the selected temperature. 
     As best shown in  FIGS. 3 and 4 , each flow control inlet valve  18 , 19  leads to a two-stage plenum chamber  42 , 43  respectively. Each plenum chamber  42 , 43  is divided internally into concentric annular outer and inner chambers  42   a , 43   a  and  42   b , 43   b  by an axially extending partition wall  44 . In this embodiment, the partition wall  44  and valve seat  35  are integrated in a single component by using the valve seat  35  as an insert in a plastic moulding die for the partition wall  44 . In this way, the valve seat  35  is an integral part of the cartridge body. 
     One end of the partition wall  44  forms a weir  45  separating the chambers  42   a , 42   b  and the other end forms a weir  46  separating the chambers  43   a , 43   b . The weir  45  is axially spaced from the point of entry of cold water to the outer chamber  42   a  and the point of exit of cold water from the inner chamber  42   b . Likewise, the weir  46  is axially spaced from the point of entry of hot water to the outer chamber  43   a  and the point of exit of hot water from the inner chamber  43   b.    
     In this way, the incoming water to each plenum chamber  42 , 43  is distributed around the outer chamber  42   a , 43   a  and is confined to flow in an axial direction towards the associated weir  45 , 46  where it flows over the weir  45 , 46  into the inner chamber  42   b , 43   b  and is again confined to flow in an axial direction towards the valve seat  35  where it flows into the mixing chamber  26 . 
     As a result, the water flows are uniformly distributed around the shuttle valve members  28 , 29  and swirling flow vectors are reduced to substantially insignificant values as the water approaches the valve seat  35 . The velocity vectors as the water approaches the valve seat  35  are substantially axial and the flow velocities across the valve seat  35  are radially inwards and even all around the valve seat  35 . 
     In this way, the distribution of water volume and velocity energy is even around the valve seat  35  for both flows. As a result, the plenum chambers  42 , 43  provide substantially identical mixing conditions around the porting of the shuttle valve members  28 , 29  that prevents asymmetric flow patterns developing to any significant extent as the hot and cold water flows enter the mixing chamber  26 . 
     Each shuttle valve member  28 , 29  is sealed relative to the cartridge body by an O-ring  47 , 48  respectively to close-off the inner chambers  42   b , 43   b . The diameter of the O-rings  47 , 48  is matched to that of the valve seat  35  so that the inlet water pressure exerts no resultant force on the shuttle valve  25 . The O-rings  47 , 48  also act to provide a guidance system for axial movement of the shuttle valve  25  that maintains the alignment of the shuttle valve  25  relative to the valve seat  35 . 
     In this way, the seal faces  31 , 32  of the shuttle valve members  28 , 29  are maintained square to the seal faces  33 , 34  of the valve seat  35 . This further contributes to producing substantially identical mixing conditions around the porting of the shuttle valve members  28 , 29  that prevents asymmetric flow patterns developing to any significant extent as the water flows enter the mixing chamber  26 . 
     In addition to providing a uniform distribution of water volume and velocity energy of the water flows, the arrangement of the valve seat  35  between the seal faces  31 , 32  of the shuttle valve members  28 , 29  provides porting that enables the water flows to enter the mixing chamber  26  close together. As a result, the flows meet radially inwardly of the valve seat  35  and are swept into an axial direction by curved surfaces  28   a , 29   a  of the shuttle valve members  28 , 29 . In this way, interaction between the flows is enhanced to promote thorough mixing of the water around the valve seat  35  and the formation of separate streams of water having different temperatures is substantially eliminated. 
     Moreover, velocity energy generated is maintained by matching the cross-sectional area of the mixing chamber  26  perpendicular to the direction of flow to the combined cross-sectional area of the hot and cold flows across the valve seat  35 . As a result, turbulent flow conditions are created within the mixing chamber  26  over the normal flow rates. Turbulent flow has numerous random eddies which give rise to random lateral flows throughout the water stream that cause the hot and cold flows to merge producing a fully blended flow within a relatively short axial distance. More particularly, we have found that if the cross-sectional area of the mixing chamber  26  is 1 to 1½ times the combined cross-sectional area of the hot and cold flows across the valve seat  35 , substantially complete mixing of the hot and cold flows is achieved if the length of the mixing chamber  26  is approximately 5 times the width. 
     In this embodiment, the diameter of the shuttle valve  25  is large relative to the operating stroke (or valve lift) and the mixing chamber  26  has a relatively small width. This results in a compact size of mixing chamber  26  that further promotes mixing of the water flows. 
     In addition, the mixing chamber  26  provides substantially unobstructed flow of water and can be slightly tapered to increase gradually the cross-sectional area in the direction of flow. As a result, some of the velocity energy may be recovered and converted into pressure energy for the mixed water discharged from the outlet  4 . 
     Furthermore, in the circumstances where one of the flows has a higher pressure, the higher pressure flow creates a low pressure region immediately inside the mixing chamber causing the lower pressure flow to increase and so assist the response of the thermostat  27  to maintain the selected temperature. 
     Thus, in the event the hot water pressure is higher, the flow of hot water into the mixing chamber  25  is turned from a radial inward direction to an axial direction by curved surface  29   a . This creates a low pressure region causing the cold water flow to become entrained in the hot water flow, mixing with it and tending to maintain the initial proportions of hot and cold water. 
     Alternatively, if the cold water pressure is higher, the flow of cold water into the mixing chamber  25  clings to the curved surface  28   a  due to the Coanda effect and is turned into an axial direction. This creates a low pressure region causing the hot water flow to become entrained in the cold water flow, mixing with it and tending to maintain the initial proportions of hot and cold water. 
     The mixed water leaving the mixing chamber  25  flows over the temperature responsive part  27   a  of the thermostat  27 . The thermostat  27  is exposed to water that is fully blended thereby improving the accuracy of response to change in temperature of the blended water. More particularly, we have found that temperature deviations from the desired temperature are reduced by approximately 50% to 70% compared to existing mixer valves in which the hot and cold water flows are not fully blended. Also the speed of response to sudden temperature changes is similarly improved. 
     An opening  49  in the inner end of the cartridge unit  14  allows the mixed water to flow into an outlet chamber  50  defined between the cartridge unit  14  and the valve body  12 . The outlet chamber  50  communicates with the outlet  4  for discharge of temperature controlled water to an ablutionary appliance such as a shower handset (not shown) connected to the outlet via a flexible hose (not shown). 
     Referring now to  FIGS. 5 to 9  of the drawings, there is shown an electronically controlled thermostatic mixing valve  101  according to a second embodiment of the present invention. The mixing valve  101  has spaced parallel inlets  102 , 103  for connection to supplies of hot and cold water (not shown) and two outlets  104 , 105  for discharging temperature controlled blended water. The outlets  104 , 105  are spaced apart 180 degrees for selective connection to an ablutionary appliance such as a shower spray fitting. 
     The mixing valve  101  may be provided in a range of sizes for different applications. For example, smaller valves may supply a single shower or a group of showers. Larger valves may be connected in a water circulation system to provide a hot water circuit around a building in which the water is maintained at a constant temperature and can be supplied to a large number of appliances at different locations. 
     The valve  101  has a cylindrical body  106  with the outlets  104 , 105  at one end and an opening  107  at the other end for reception of a cartridge unit  108 . 
     The cartridge unit  108  is a push fit in the body  106  and has an external flange  109  that locates against the end of the body  106 . The cartridge unit  108  is releasably secured by an end plate  110  bolted to the body  106  by a plurality of bolts  111  extending through aligned holes in the flange  109  to engage tapped holes (not shown) in the body  106 . 
     The flange  109  carries a stepper motor  112  having a rotatable drive shaft  113  for actuating a spool valve  114  to vary the relative proportions of hot and cold water admitted to an annular mixing chamber  115  within the cartridge unit  108  as described later herein. 
     The cartridge unit  108  is sealed relative to the body  106  by three axially spaced O-rings  116 , 117 , 118  located in annular grooves  119 , 120 , 121  respectively. 
     The O-ring  116  engages the inner surface of the body  106  adjacent the opening  107 . The O-ring  117  is of smaller diameter and engages an internal rib  122  axially spaced from the opening  107 . The O-ring  118  is of smaller diameter still and engages an internal rib  123  axially spaced from the rib  122 . The arrangement of the O-rings to be of progressively smaller diameter from the outer end of the cartridge unit  108  to the inner end facilitates insertion of the cartridge unit  108  in the body  106 . Thus, the O-ring  118  is a clearance fit in the body  106  until it engages rib  123  and O-ring  117  is a clearance fit until it engages rib  122 . 
     The cartridge unit  108  defines with the body  106  two annular outer chambers  124 , 125  separated by the O-ring  117 . The inlet  102  opens to the chamber  124  and the inlet  103  opens to the chamber  125 . Each chamber  124 , 125  is of similar size and shape and the inlets  102 , 103  are arranged in parallel on the same side of the body  106 . In this way, water flowing into the chambers  124 , 125  is distributed around the chambers  124 , 125  and any asymmetry in the distribution will be the same in each chamber  124 , 125 . 
     The spool valve  114  comprises a cylindrical spool sleeve  126  received in a cylindrical body  127  of the cartridge unit  108 . The spool sleeve  126  is sealed relative to the body  127  by three axially spaced O-rings  128 , 129 , 130  located in annular grooves  131 , 132 , 133  formed in internal ribs  134 , 135 , 135   a  on the inside of the body  127 . 
     The spool sleeve  126  defines with the body  127  two annular inner chambers  136 , 137  concentric with the outer chambers  124 , 125  and separated by O-ring  129 . Each chamber  136 , 137  is of similar size and shape. 
     The outer chamber  124  communicates with the inner chamber  136  via a series of holes  138  formed in the body  127 . The holes  138  are circumferentially spaced apart and offset relative to the inlet  102  so that water flowing into the outer chamber  124  is prevented from flowing directly into the inner chamber  136  and is confined to flow around the outer chamber  124 . 
     The outer chamber  125  similarly communicates with the inner chamber  137  via a series of holes  139  formed in the body  127 . The holes  139  are circumferentially spaced apart and offset relative to the inlet  103  so that water flowing into the outer chamber  125  is prevented from flowing directly into the inner chamber  137  and is confined to flow around the outer chamber  125 . 
     The spool sleeve  126  is provided with an internal hub  140  seated against an internal rib  141  and secured by adhesive, welding or other suitable means. Attached to the hub  140  by a screw thread is a rearwardly extending tubular portion  142  to locate axially a drive nut  143  threadably engaging a lead screw portion  144  of drive shaft  113  between a pair of stops  145 , 146 . 
     The tubular portion  142  is located against rotation and guided for axial sliding movement by engagement of external axial splines  147  with internal axial splines  148  of a tubular portion  149  secured to the end plate  110 . In this way, the spool sleeve  126  is axially movable in response to rotation of the drive shaft  113  between end positions defined by engagement of the drive nut  143  with the stops  145 , 146 . 
     The spool sleeve  126  is also provided with a tubular spout  150  seated against an internal rib  151  and secured by adhesive, welding or the like. The spout  150  extends away from the hub  140  and terminates in an outlet chamber  152  communicating with the outlets  104 , 105 . 
     The spool sleeve  126  is formed with a series of slots  153  uniformly spaced apart in a circumferential direction and extending between the internal ribs  141 , 151  at an angle relative to the longitudinal axis of the spool sleeve  126 . 
     The O-ring  129  engages the spool sleeve  126  in the region of the slots  153  whereby water can flow from each inner chamber  136 , 137  into the mixing chamber  115  via the exposed portion of the slots  153 . In this way, axial movement of the spool sleeve  126  in response to rotation of the drive shaft  113  under the control of the motor  112  alters the area of the slots  153  communicating with the chambers  136 , 137  to adjust the relative proportions of water admitted to the mixing chamber  115  from each chamber  136 , 137 . The shape of the slots  153  may be adapted to profile the rate of proportioning of hot and cold water according to the axial position of the spool sleeve  126 . 
     The outer end of the spout  150  is spaced from end wall  154  of the body  106  opposite a pair of temperature sensors  155 , 156  mounted on a plug  157  secured in an opening  158  in the end wall  154  by a plurality of bolts  159  and sealed by an O-ring  159   a.    
     Water flowing from the mixing chamber  115  into the outlet chamber  152  passes over the temperature sensors  155 , 156  and is swirled around in the outlet chamber  152  by a pair of guide vanes  160 , 161  mounted on the plug  157  to force the water to impart a rotation to the water stream entering the outlet chamber  152 . 
     The temperature sensors  155 , 156  provide signals representative of the temperature of the water leaving the mixing chamber  115  to a microprocessor or other suitable control system (not shown) which in turn generates a control signal for actuating the stepper motor  112  to adjust the spool valve  114  to control the relative proportions of water admitted to the mixing chamber  115  in accordance with the desired outlet water temperature. 
     An axial control hole  162  through the body  127  of the cartridge unit  113  connects each end of the spool valve  114  to the water pressure in the outlet chamber  152  so that pressure forces acting on the spool sleeve  126  are balanced and there is no net force tending to displace the spool sleeve  126  in an axial direction. 
     In use, the inlets  102 , 103  are connected to supplies of hot and cold water via on/off valves (not shown) that may also be adjustable to vary the flow similar to the ceramic plate valves of the first embodiment. Alternatively the on/off and flow control functions may be provided by separate components. 
     The waterways are of similar size and shape whereby the connections to the inlets  102 , 103  may be reversed without altering the operation and performance of the valve  101 . 
     The incoming water flows enter the outer chambers  124 , 125  where they are forced to flow around the chambers  124 , 125  by the offset arrangement of the inlets  102 , 103  relative to the holes  138 , 139  connecting the outer chambers  124 , 125  to the inner chambers  136 , 137 . 
     The holes  138 , 139  open into the inner chambers  124 , 125  at the end spaced from the slots  153 . As a result, the water entering the inner chambers  136 , 137  is turned from a radial direction into an axial direction to flow towards the slots  153 . In this way, the water flows are distributed uniformly and evenly around the spool sleeve  126  before arriving at the slots  153 . If, however, any asymmetry remains in the flows, it will be similar for each flow and produce matching ratios of hot and cold around the spool sleeve  126  even if the total flow distribution is asymmetric. 
     The water arriving at the slots  153  flows into the mixing chamber  115  in a radial inwards direction and is swept into an axial direction between curved surfaces  140   a  and  150   a  of the hub  140  and spout  150  respectively. As in the previous embodiment, both flows enter the mixing chamber  115  close together and the curved surfaces  140   a , 150   a  guide the flows to entrain one another. As shown, the centre of the mixing chamber  115  is tapered away leading into the spout  150  and the total flow path through the mixing chamber  115  and spout  150  is designed to create turbulent flow over a distance sufficient to ensure substantially complete mixing of the flows occurs before the water stream reaches the temperature sensors  155 , 156 . 
     More particularly, the mixing chamber  115  has a cross-sectional area perpendicular to the direction of flow approximately 1 to 1½ times the combined cross-sectional area of the hot and cold flows into the mixing chamber  115 . As a result, velocity energy of the flows is maintained creating turbulent flow conditions within the mixing chamber  115  and substantially complete mixing of the hot and cold flows can be achieved if the length of the mixing chamber  115  is approximately 5 to 10 times the width. 
     By angling the slots  153  relative to the longitudinal axis of the spool sleeve  126 , the jets of hot and cold water entering the mixing chamber  115  are offset around the circumference of the spool sleeve  126 . As a result, the two flows interlace as they enter the mixing chamber  115  which further promotes mixing of the hot and cold water streams. 
     The spout  150  gradually expands towards the outer end so that the water velocity reduces thereby recovering some of the velocity energy and converting it to pressure energy. This can be beneficial in obtaining good flow rates from small sized mechanisms and may not be required for larger valves. 
     The temperature sensors  155 , 156  monitor the temperature of the water stream exiting the spout  150  and send a signal representative of the temperature to the control system e.g. a microprocessor, which in turn actuates the stepper motor  112  to adjust the axial position of the spool sleeve  126  to vary the relative proportions of hot and cold water admitted to the mixing chamber  115  in accordance with the desired water temperature. In this embodiment, the stepper motor  112  provides 1500 steps between end positions of adjustment corresponding to full hot and full cold. In this way, high temperature resolution is obtained for precise control of the desired outlet water temperature. 
     The spout  150  is made of plastic or other suitable material having a low thermal mass and conductivity. In this way, the temperature of the water stream exiting the spout  150  is substantially unaffected by contact with the spout  150  thereby improving the accuracy of the outlet water temperature detected by the sensors  155 , 156 . 
     The water flows entering the mixing chamber  115  are thoroughly mixed before reaching the temperature sensors  155 , 156 . As a result, the sensors  155 , 156  can be small, e.g. thermistors, and sampled temperatures are consistent with the average mixed water stream. In this way a large number of sensors and averaging of the detected temperatures is not required. In this embodiment, two sensors  155 , 156  are employed as a back-up to enable a sensor that has failed or is not working correctly to be detected by comparing the signals from each sensor  155 , 156 . 
     As will now be appreciated, each outer chamber  124 , 125  and associated inner chamber  136 , 137  forms a two-stage plenum chamber for distributing the water flows around the porting of the spool valve  114  to provide a substantially uniform distribution of the flows entering the mixing chamber  115  similar to the first embodiment. 
     In addition, the hot and cold water flows enter the mixing chamber  115  close together and are swept in an axial direction that promotes thorough mixing to produce a fully blended stream directed over the temperature sensors  155 , 156 . The temperature sensors  155 , 156  can therefore be quick acting to provide rapid response of the control system to change in the desired water temperature. 
     In a modification (not shown), the plug  157  is provided with a position sensor to provide a signal to the control system representative of the position of the spool sleeve  126 . Position feedback may be employed if the incoming water supply pressures are unequal to compensate for increased gain of the valve and provide accurate temperature control near to the limiting position of the spool sleeve  126  at which the gain is most noticeable. 
     Referring now to  FIGS. 10 to 14  of the accompanying drawings, a thermostatic mixing valve  201  according to a third embodiment of the present invention is shown. The valve  201  has inlets  202 , 203  for connection to supplies of hot and cold water (not shown) and an outlet  204  for discharging temperature controlled water to an ablutionary appliance such as a shower (not shown). 
     The valve  201  has a drive spindle  205  for mounting a rotatable temperature control knob (not shown) for user selection of a range of outlet water temperatures, for example from cold to 60° C. 
     As best shown in  FIG. 11 , the valve  201  has a body  206  with a cylindrical bore  207  for reception of a detachable cartridge unit  208  shown in  FIGS. 12 and 13 . 
     The body  206  has an external screw thread  209  at the open end of the bore  207  for engagement of a retainer ring  210  to secure the cartridge unit  208  in the body  206 . 
     The cartridge unit  208  is located in the correct orientation and prevented from rotating in the bore  207  by engagement of lugs (not shown) on the cartridge body  211  with a pair of diametrically opposed notches  212  in the body  206  of the valve at the open end of the bore  207 . 
     The cartridge unit  208  is sealed relative to the bore  207  of the valve body  206  by three axially spaced O-rings  213 , 214 , 215  located in annular grooves  216 , 217 , 218  in the outer surface of the cartridge body  211  to form two annular outer chambers  219 , 220  separated by the O-ring  214 . 
     The inlets  202 , 203  lead to respective spiral ducts  202   a , 203   a  formed in the wall of the valve body  206  that provide tangential entry of the water flow to the outer chambers  219 , 220 . Each outer chamber  219 , 220  communicates with a respective annular inner chamber  221 , 222  via a series of circumferentially spaced slots  223 , 224  formed in the cartridge body  211 . 
     The inner chambers  221 , 222  are separated by a shuttle valve  225  arranged within the cartridge unit  208  for controlling the relative proportions of hot and cold water admitted from the inner chambers  221 , 222  to an annular mixing chamber  226 . 
     The shuttle valve  225  includes a shuttle  227  axially movable between annular valve seats  228 , 229 . The cold water valve seat  228  is fixed and the hot water valve seat  229  is provided by an insert  230  screwed into the end of the cartridge body  211 . In this way, the axial position of the hot seat  229  can be adjusted to vary the travel of the shuttle  227  for different operating requirements. 
     As best shown in  FIG. 14 , the hot seat  229  is formed by an annular rubber ring  247  that is a stretch fit around an external rebate  248  at the inner end of the insert  230  and has an annular groove  249  in the inner marginal surface for reception of a flange  250  to retain the ring  247  in position. The ring  247  is stretched about 5% which holds it in place under all operating conditions and is made of a fairly hard rubber compound to provide a resilient seat for fluid tight engagement with the shuttle  227  to shut-off flow of the hot water under certain operating conditions. 
     The shuttle  227  is connected by webs  231  to a mounting ring  232  located on a thermostat  233  and is biased by an overload spring  234  acting between the ring  232  and a retainer sleeve  235  screwed onto the thermostat  233 . In this way, the shuttle  227  follows movement of the thermostat under normal operating conditions. 
     The thermostat  233  contains a wax filler and has an actuator rod  236  that is axially movable in response to expansion/contraction of the wax filler in response to the temperature of the mixed water sensed by the thermostat  233 . 
     The free end of the rod  236  engages an axial plug  237  provided on the underside of a drive nut  238 . The plug  237  is slidably received in the end of the retainer sleeve  235  and provides an axial guide for one end of the thermostat assembly. The other end of the thermostat assembly is provided with an axial guide in the form of internal webs  239  of a mixed water guide  240  slidably received in the insert  230 . 
     The guide  240  is biased by a return spring  241  towards the thermostat  233  and engages the thermostat  233  via a spacer ring  245  having openings (not shown) for water to flow from the mixing chamber  226  through the guide  240  and into an outlet chamber  246  defined between the cartridge body  211  and the valve body  206 . The outlet chamber  246  communicates with the outlet  204  for discharging temperature controller water. 
     The drive nut  238  is located against rotation in the cartridge body  211  and is screwed into the inner end of the drive spindle  205  such that the drive nut  238  is axially movable in response to user rotation of the control spindle  205 . 
     This axial movement is transmitted via the plug  237  to the thermostat  233  for adjusting the position of the shuttle valve  227  between the hot and cold seats  228 , 229  to vary the relative proportions of hot and cold water admitted to the mixing chamber  226  in accordance with user selection of the desired outlet water temperature. 
     If the temperature of the mixed water increases, the wax filler expands to increase the projecting length of the actuator rod  236 . This causes the thermostat  233  to be displaced away from the plug  237  against the biasing of the return spring  241  which is weaker than the overload spring  234 . The thermostat  233  carries with it the shuttle  227  causing a taper seal face  242  at one end to move towards the hot seat  229  to reduce the flow of hot water and a tapered seal face  243  at the other end to move away from the cold seat  228  to increase the flow of cold water. In this way, the relative proportions of hot and cold water admitted to the mixing chamber  226  are adjusted to return the temperature of the mixed water to the selected temperature. 
     If the temperature of the mixed water exceeds the maximum permitted, for example if the cold water supply fails, expansion of the wax filler causes the shuttle valve  227  to image the hot seat  228  to shut-off the flow of hot water. Further elongation of the actuator rod  236  is accommodated by compression of the overload spring  234  to prevent damage to the internal components of the cartridge unit  208 . 
     If the temperature of the mixed water decreases, the wax filler contracts and the thermostat  233  is displaced axially towards the plug  237  reducing the projecting length of the rod  236  under the biasing of the return spring  241 . 
     As a result, seal face  243  of the shuttle valve  227  moves towards the cold seat  228  to reduce the flow of cold water and seal face  242  moves away from the hot seat  229  to increase the flow of hot water. In this way, the relative proportions of hot and cold water admitted to the mixing chamber  226  are adjusted to retain the temperature of the mixed water to the selected temperature. 
     As best shown in  FIGS. 12 and 13 , incoming water is distributed around the outer chambers  219 , 220  and flows into the inner chamber  221 , 222  via the slots  223 , 224 . The slots  223 , 224  are provided at the ends of the inner chambers  221 , 222  remote from the hot and cold seats  228 , 229 . As a result, the flow of hot and cold water is confined to flow in an axial direction towards the hot and cold seats  228 , 229 . In this way, the hot and cold water flows are evenly distributed around the inner chambers  221 , 222  and pass across the hot and cold seats  228 , 229  in a radial direction. 
     The cold water flow enters a chamber  244  surrounding the overload spring  234  and is turned in an axial direction towards the mixing chamber  226 . The hot water flow to the mixing chamber  226  is turned in an axial direction by the curved inner surface  229   a  of the hot seat  229 . The hot water clings to the surface  229   a  due to the Coanda effect. As a result, both the hot and cold water flows are turned in the same direction towards the mixing chamber  226  and are uniformly distributed around the thermostat  233  as they enter the mixing chamber  226 . 
     The mixing chamber  226  has a small radial width compatible with the required flow rates so that the two flows are thoroughly mixed in a short axial distance. More particularly, the mixing chamber  226  has a cross-sectional area perpendicular to the direction of flow approximately 1 to 1½ times the combined cross-sectional area of the hot and cold flows into the mixing chamber  226 . As a result, velocity energy of the water flows is maintained creating turbulent flow conditions within the mixing chamber  226  and substantially complete mixing of the hot and cold flows can be achieved if the length of the mixing chamber  226  is approximately 5 times the width. 
     The mixed water stream is then directed over the temperature responsive part  233   a  of the thermostat  233  by the guide  240 . In this way, the thermostat  233  provides a fast, accurate response to change in the desired mixed water temperature. Furthermore, if the hot water pressure is higher than the cold water pressure, a pressure drop is created by the hot water entering the mixing chamber  226  that effectively entrains and assists flow of cold water to assist response of the thermostat  233  to maintain the desired water temperature. 
     As will be appreciated, each outer chamber  219 , 220  and associated inner chamber  221 , 222  forms a two stage plenum chamber for distributing water flows around the shuttle  227 . In this way, substantially identical mixing conditions are created around the porting of the shuttle valve  225  that prevent asymmetric flow patterns developing to any appreciable extent as the hot and cold water flows enter the mixing chamber  226 . 
     In addition, the seal faces  242 , 243  of the shuttle  227  are maintained square to the hot and cold seats  228 , 229  by the guide system for the thermostat  223 . This further contributes to producing substantially identical mixing conditions around the porting of the shuttle valve  225  to reduce development of asymmetric flow patterns in the water admitted to the mixing chamber  226 . 
     Referring now to  FIGS. 15 to 18  of the accompanying drawings, there is shown a thermostatic mixing valve  301  according to a fourth embodiment of the present invention. 
     The mixing valve  301  has separate inlets  302 , 303  for connection to supplies of hot and cold water (not shown) and an outlet  304  for discharging temperature controlled water to an ablutionary appliance such as a shower (not shown). 
     The valve  301  has a hollow body  305  in which a detachable cartridge unit  306  is received for controlling the flow and temperature of the water discharged from the outlet  304 . 
     The cartridge unit  306  has a rotatable control spindle  307  with axial splines  308  for detachably mounting a control knob (not shown) for a combined flow and temperature control mechanism described in more detail later. 
     The cartridge unit  306  has an external screw thread  309  for engagement with a mating internal screw thread of an opening at one end of the body  305  to secure releasably the cartridge unit  306  in the body  306 . The cartridge unit  306  carries axially spaced O-rings  310 , 311  co-operable with an internal wall  312  of the body  305  to define annular outer chambers  313 , 314 . 
     The inlets  302 , 303  communicate with the outer chambers  313 , 314  respectively. The outlet  304  communicates with an annular outlet chamber  315  formed between the cartridge unit  306  of the valve body  305  and is sealed by an O-ring  316 . 
     Each outer chamber  313 , 314  communicates with an annular inner chamber  317 , 318  within the cartridge unit  306  via a series of circumferentially spaced slots  319 , 320  formed in the body of the cartridge unit  306 . 
     The slots  319 , 320  are axially offset relative to the inlets  302 , 303  so that water entering the outer chambers  313 , 314  is distributed around the chambers  313 , 314  before entering the inner chambers  317 , 318 . 
     The cartridge unit  306  has a shuttle valve  321  for controlling the relative proportions of hot and cold water admitted to an annular mixing chamber  322  in accordance with user selection of the desired outlet water temperature. 
     The shuttle valve  321  has a shuttle  323  axially movable between a cold water valve seat  324  and a hot water valve seat  325  to control the relative proportions of hot or cold water admitted to the mixing chamber  322 . 
     The shuttle  323  has an O-ring  326  separating the inner chambers  317 , 318  and is connected via webs  327  to a temperature overload housing  328  mounted on a wax filled thermostat  329 . 
     The thermostat  329  has an actuator rod  330  that is axially movable in response to expansion/contraction of the wax filler in response to the temperature of the mixed water sensed by a temperature responsive part  329   a  of the thermostat  329 . 
     The free end of the rod  330  engages a cap  331  received within the housing  328  and biased towards the rod  330  by an overload spring  332  acting between the cap  331  and an abutment collar  333  at the outer end of the housing  328 . A return spring  334  acts between an insert  335  screwed into the end of the cartridge body and the cap  331 . 
     The insert  335  carries the hot seat  325  and is axially adjustable to set the axial spacing between the hot seat  325  and cold seat  324  according to the operating requirements. The hot seat  325  is provided by a rubber ring and is resilient as described for the previous embodiment to provide a fluid-tight seal with the shuttle  323  to shut-off the flow of hot water under certain operating conditions. 
     The insert  335  is provided with internal axial ribs  345  providing an axial guide for the lower end of the overload housing  328 . The thermostat  329  provides an axial guide for the other end of the overload housing  328  and is in turn axially aligned by engagement with the piston  336 . In this way, the shuttle  323  is maintained square to the hot and cold seats  324 , 325  for axial adjustment of the position of the shuttle  323  to vary the relative proportions of hot and cold water admitted to the mixing chamber  322 . 
     The other end of the thermostat  329  remote from the rod  330  is coupled to a piston  336  received in a bore  337  of a drive nut  338 . The drive nut  338  threadably engages the control spindle  307  and is located against rotation so that rotation of the control spindle  307  causes axial movement of the drive nut  338 . 
     The lower end of the drive nut  338  is provided with an annular washer  339  co-operable with an annular valve seat  340  provided by an annular flow guide  341  surrounding the temperature responsive part  329   a  of the thermostat  329 . 
     Rotation of the control spindle  307  in one direction lowers the drive nut  338  to cause the shuttle  323  to engage the hot seat  325  to shut-off the flow of hot water. Further rotation of the control spindle  307  in the same direction causes the thermostat  329  to move relative to the housing  328  to compress the overload spring  332  and return spring  334  until the washer  339  engages the valve seat  340  to cut-off the flow of cold water. 
     Rotation of the control spindle  307  in the opposite direction moves the washer  339  away from the valve seat  340  to allow flow of cold water with the flow of hot water cut-off until the compression of the over-load spring  332  is taken up. The housing  328  is then coupled for movement with the thermostat  329  via the return spring  334  and further rotation of the control spindle in the same direction moves the shuttle  323  away from the valve seat  325  to allow flow of hot water. 
     The amount of travel before the shuttle  323  is coupled for movement with the thermostat  329  is pre-set by adjusting the position of the piston  336  via an adjusting screw  342  threadably mounted in a tapped bore  343  of the drive nut  338 . The bore  343  extends axially to the outer end of the drive nut  338  for insertion of a tool such as a screwdriver or alien key to adjust the position of the piston  336 . 
     In use, hot and cold water flowing into the annular outer chambers  313 , 314  is confined to flow around the chambers  313 , 314  and passes through the slots  319 , 320  into the inner chambers  317 , 318 . The slots  319 , 320  are axially offset relative to the valve seats  324 , 325  so the water flows are evenly distributed around the chambers  317 , 318  and approach the valve seats  324 , 325  in an axial direction. 
     The hot water flows across the valve seat  325  in a radial direction and is turned in an axial direction by a curved surface  328   a  of the housing  328 . The hot water flow also clings to a curved surface  323   a  of the shuttle  323  due to the Coanda effect that assists flow of the hot water in the axial direction. 
     The cold water flows across the valve seat  324  in a radial direction and is turned in an axial direction by a curved surface  323   b  of the shuttle  323 . The cold water flow also clings to a curved surface  324   a  of the seat  324  due to the Coanda effect that assists flow of the cold water in the axial direction. 
     The shuttle  323  is of short axial length so that both flows meet quickly and are thoroughly mixed in the mixing chamber  322 . More particularly, the mixing chamber  115  has a cross-sectional area perpendicular to the direction of flow approximately 1 to 1½ times the combined cross-sectional area of the hot and cold flows into the mixing chamber  115 . As a result, turbulent flow conditions are created within the mixing chamber  322  and substantially complete mixing of the hot and cold flows can be achieved if the length of the mixing chamber  322  is approximately 5 times the width. The mixed water stream flowing over the temperature responsive part  329   a  of the thermostat  329  within the flow guide  341  is substantially free from any streams of unmixed or partially mixed water. As a result, the thermostat response is enhanced for accurate adjustment of the shuttle  323  to control the relative proportions of hot and cold water admitted to the mixing chamber  322  according to the desired outlet water temperature. 
     The mixed water exiting from the flow guide  341  flows into the outlet chamber  315  through a series of circumferentially spaced holes  344  in the cartridge unit  306 . The water flows from the outlet chamber  315  to the outlet  304  for discharge to the ablutionary appliance. 
     If the desired temperature of the mixed water increases, the wax filler expands to increase the projecting length of the actuator rod  330 . The causes the housing  328  to be displaced axially relative to the thermostat  329  against the biasing of the return spring  334 . The housing  328  carries with it the shuttle  323  causing the flow of hot water to be reduced and the flow of cold water to be increased. In this way the relative proportions of hot and cold water admitted to the mixing chamber  322  are adjusted to return the temperature of the mixed water to the selected temperature. 
     If the temperature of the mixed water exceeds the maximum permitted, for example if the cold water supply fails, expansion of the wax filler causes the shuttle  323  to engage the hot seat  325  to shut-off the flow of hot water. Further elongation of the actuator rod  330  is accommodated by compression of the overload spring  332  to prevent damage to the internal components of the cartridge unit  306 . 
     If the desired temperature of the mixed water decreases, the wax filler contracts and the housing  328  is displaced axially towards the thermostat  329  reducing the projecting length of the rod  330  under the biasing of the return spring  334 . As a result, the axial position of the shuttle  323  is adjusted to increase the flow of hot water and reduce the flow of cold water entering the mixing chamber  322  to return the temperature of the mixed water to the selected temperature. 
     As will be appreciated, each outer chamber  313 , 314  and associated inner chamber  317 , 318  forms a two stage plenum chamber for distributing water flows around the shuttle  323 . In this way, substantially identical mixing conditions are created around the porting of the shuttle valve  321  that prevent asymmetric flow patterns developing to any appreciable extent as the hot and cold water flows enter the mixing chamber  322 . 
     In addition, the seal faces of the shuttle  323  are maintained square to the hot and cold seats  324 , 325  by the guide system for the overload housing  328 . This further contributes to producing substantially identical mixing conditions around the porting of the shuttle valve  321  to reduce development of asymmetric flow patterns in the water admitted to the mixing chamber  322 . 
     Furthermore, the hot and cold flows are turned in an axial direction towards the mixing chamber  322  which has a small radial width to create turbulent flow that ensures thorough mixing of the flows within a short axial distance. Moreover, the flows are introduced close together and turned in an axial direction so that, if either flow is at a higher pressure, it creates a pressure drop that entrains and assists the other flow to enhance response of the thermostat to a change in the desired temperature of the outlet water. 
     Referring now to  FIG. 19  of the accompanying drawings, there is shown a modification to the valve shown in  FIGS. 14 to 18 . For convenience, like reference numerals are used to indicate corresponding parts. 
     In the modification shown in  FIG. 19 , the inner chambers  317 , 318  are provided with a series of axially extending flow guide vanes  346  uniformly spaced apart in a circumferential direction. The guide vanes  346  further assist in confining the water to flow in an axial direction towards the porting of the shuttle valve  321  so that flow across the valve seats  324 , 325  is radial. In this way swirl flow vectors in the water admitted to the mixing chamber  322  are reduced. It will be understood that flow guide vanes  346  may be provided in the inner chambers of any of the previous embodiments. 
     Referring now to  FIG. 20 , there is shown part of an electronically controlled thermostatic mixing valve  401  according to a fifth embodiment of the invention. This embodiment provides interlacing of the hot and cold streams to promote mixing in similar manner to the embodiment of  FIGS. 5 to 9  but with some advantages compared to the arrangement shown in  FIGS. 5 to 9 . 
     As shown the valve  401  has an inner water chamber  402  and a concentric outer water chamber  403 . Valve means (not shown) controls the flow of hot water to one of the chambers  402 , 403  and the flow of cold water to the other chamber  402 ,  403 . The valve means may be of any suitable type to adjust the relative proportions of hot and cold water to control the outlet water temperature in accordance with user selection and to maintain the selected outlet water temperature substantially constant. 
     For example, the valve device may be a proportioning mechanism such as a shuttle valve or spool valve to adjust the hot and cold flows inversely to each other. Alternatively, the valve device may comprise separate flow control valves for each flow. If separate valves are used, the total flow rate can be controlled by simultaneous adjustment of the valves to increase or reduce both flows while keeping the relative proportions the same to maintain the required outlet water temperature. 
     Each chamber  402 ,  403  is provided with a plurality of transfer ports or nozzles  402   a ,  403   a  respectively that open into a mixing chamber  404 . The mixing chamber  404  has an annular ring shaped inlet portion  404   a  that leads to a tubular exit portion  404   b  that opens to an outlet chamber (not shown) for discharge of temperature controlled output water from the valve  401 . The exit portion  404   b  acts in the manner of a diffuser to recover some of the velocity energy in the water. 
     The chambers  402 ,  403  allow the flows of hot and cold water to be distributed evenly before entering the mixing chamber  404  via the nozzles  402   a ,  403   a . In this embodiment each chamber  402 ,  403  is provided with twelve nozzles  402   a ,  403   a  uniformly spaced apart in a circumferential direction at one end of the mixing chamber  404  with the nozzles  402   a  alternating with the nozzles  403   a . It will be understood, however that more or fewer nozzles  402   a ,  403   a  may be provided. 
     Arranging the nozzles  402   a ,  403   a  alternately causes the incoming flows of hot and cold water to interlace and promote mixing within the mixing chamber  404  assisted by construction of the mixing chamber  404  to keep the flows moving fast so that they are fully turbulent as described previously. 
     The temperature of the mixed water stream is sensed by means of a temperature sensor  405  and the valve device is operable via an electronic control system (not shown) such as a programmable microprocessor responsive to input of a desired output water temperature and the temperature sensed by the sensor  405  to control the valve device, for example by means of an electric motor, to provide and maintain the selected outlet water temperature. 
     The temperature sensor  405  has to be sited sufficiently downstream of the junction between the hot and cold streams in order to allow the streams to merge enough for an accurate temperature to be measured. This gives rise to some transport delay due to the time it takes for the water to travel from the junction to the sensor  405 . The valve transient response to any change in the input parameters (pressures or temperatures of the inlet water or the set temperature) is significantly affected by the transport delay. Therefore it is desirable that the streams are mixed effectively and quickly. 
     We believe that the mixing may be substantially complete about 25% along the mixing chamber  404  from the junction of the hot and cold water streams with the rest of the mixing chamber  404  serving as a diffuser to recover some of the velocity energy in the water. 
     We have found interlacing of the streams of hot and cold water is particularly effective in getting the streams to merge very quickly and enables the temperature sensor  405  to be positioned close to the junction of the hot and cold streams. As a result, the transport delay may be very short allowing any suitable valve device to be used to control the hot and cold water streams. 
     In this embodiment, the entry nozzles  402   a ,  403   a  to the annular mixing chamber  404  can be sited close to one another Consequently the annular inlet portion  404   a  of the mixing chamber  404  can be of small volume. Also the nozzles  402   a ,  403   a  are directed in substantially the same direction so that the streams will entrain one another very effectively. 
     As a result, we have found that the temperature sensor  405  can be positioned within the mixing chamber  404  closer to the junction between the streams of hot and cold water compared to the embodiment of  FIGS. 5 to 9  where the incoming hot and cold streams are arranged on opposite sides of the O-ring separator. 
     In this embodiment, the nozzles  402   a ,  403   a  are arranged around an annular mixing chamber  404 . This is a convenient way to provide the arrangement of interlaced nozzles  402   a ,  403   a  and keep the volume of the mixing annulus close to the size required to maintain turbulent flow. For example, in this embodiment we may provide nozzles  402   a ,  403   a  approximately 3.28 mm in diameter arranged in a circle of mean diameter about 30 mm with the spout  404   b  about 11.5 mm diameter at the smallest point. It will be understood, however that other configurations could be used that allow the interlacing of hot and cold water streams and that keep the mixing chamber volume small. 
     In a modification (not shown) to the arrangement of  FIG. 20 , the chambers  402 ,  403  could be provided with annular slots that communicate with the mixing chamber  404 . Such arrangement also reduces transport delays by bringing the hot and cold flows together quickly within the mixing chamber and allows co-entrainment of the flows to reduce suppression of the flow of the lower energy stream by the higher energy stream. 
     As will now be appreciated, in each of the above-described embodiments, the hot and cold water flows are effectively managed to produce a fully blended stream of water flowing over the temperature responsive part of the thermal control system that results in improved thermal control of the outlet water temperature. More particularly, the effects of temperature and/or pressure changes of one or both supplies on the desired outlet water temperature are reduced to a level at which they are substantially unnoticed by the user. Furthermore, the size and duration of transient temperature overshoots or undershoots produced by a change in the desired outlet water temperature are also reduced to a level that is not uncomfortable and/or a risk to the user. 
     While the invention has been described with reference to the best means known to the applicant, it will be understood that the invention is not limited to the exemplary embodiments above-described and is intended to include equivalents to any feature described herein. Moreover, the invention is not intended to be limited to the combination of features described in the exemplary embodiments and that the invention includes any novel feature described herein separately or in combination with any other feature of any of the embodiments. Furthermore, it will be understood that other variations and modifications falling within the spirit and scope of the following claims are also included.