IE20140241A1 - An instantaneous electric water heater and a heat recovery shower system - Google Patents

Abstract

An instantaneous electric water heater (10) comprising a) a mixer valve (15) in fluid communication with a water inlet (12), via a first flow path; b) a heater can (14) in fluid communication with a water inlet (12), via a second flow path; c) the first and second flow paths being separate so that water may flow from said inlet (12) to the heater can (14) without passing through the mixer valve (15) and vice versa; d) the heater can (14) having an outlet (22) in fluid communication with the mixer valve (15) to allow relatively hot water to be mixed with relatively cold water from said inlet (12); and e) a downstream flow restrictor (28) being provided in the second flow path to prevent or hinder water from flowing to the heater can (14) at above downstream maximum flow rate.

Description

Title: An Instantaneous Electric Water Heater and a Heat Recovery Shower System Field of the Invention This invention relates generally to the field of electric showers and associated water heaters, and relates, more specifically, but not exclusively, to ways in which the operating efficiency of such devices can be improved.

Background to the Invention and Overview of the Prior Art Instantaneous electric water heaters, of the type which provide heated water 15 on request, are found commonly in domestic sanitary and ablutionary environments, in showers, hand wash heaters and the like. In simple terms, the electrical power input and/or flow rate of the water can typically be adjusted, in order to regulate an outlet (showering/hand washing) temperature, with a variety of configurations being known.

Electric showers of this type are generally referred to as "instantaneous because the water is heated to a desired temperature substantially on demand, shortly after the unit has been switched on.

The performance of conventional electric showers can sometimes suffer, owing to external factors beyond a user’s control. One of these is a variation in the mains water supply. This provides the incoming "cold" feed, which is then heated/blended, as required.

It is important to understand that the term "cold" is used here - and should be interpreted - as a relative term. This is because the "cold" feed, from the mains, will always be colder than the water which has been heated by the shower - i.e. the "hot" water.

The cold feed is largely seasonal; thus, during summer months, mains cold water can be received at up to around 20°C (sometimes slightly higher) but this mains temperature can drop to less than 5°C, during the winter. In consequence, for a given power setting on an electric shower unit, a relatively low flow rate is required during the winter, to compensate for the lower temperature of the incoming water. This, of course, is so that the water can spend more time in contact with the heating element, to allow it to be heated to the required degree. On the other hand, when the mains cold water is warmer (e.g. during the summer), an increased flow is required, to balance this - whilst this can be beneficial from a "showering experience" perspective, it can also be extremely wasteful of water, somewhat lessening the shower’s environmental credentials.

In an attempt to reduce the power requirement of an electric shower, various proposals have been made to pre-heat an incoming cold supply, using heat from the shower’s waste water, which otherwise is simply lost, through the drainage system. Such "recovered" heat, in boosting the incoming temperature of the cold water feed, can result in less electrical power being required to provide a desired outlet temperature, but prior art systems still suffer from one or more drawbacks.

By way of example, W02008/068500A1 (Kohler Mira Limited) discloses a "heat recovery" shower system in which the flow rate of cold water through a heater can is adjusted in response to variations in the inlet (mains) water temperature. Whilst this allows heat from a waste water supply to be used, the resulting increase in the flow of water through the heater can, with no reduction in power to the heating elements being made, results in a static (i.e. unimproved) power demand, combined with an increased water requirement. Summary of the Invention In accordance with a first aspect of the present invention, we provide an instantaneous electric water heater comprising: a) a mixer valve in fluid communication with a water inlet, via a first flow path; b) a heater can in fluid communication with a water inlet, via a second flow path; c) the first and second flow paths being separate so that water may flow from said inlet to the heater can without passing through the mixer valve and vice versa; d) the heater can having an outlet in fluid communication with the mixer valve to allow relatively hot water to be mixed with relatively cold water from said inlet; and e) a downstream flow restrictor being provided in the second flow path to prevent or hinder water from flowing to the heater can at above a downstream maximum flow rate.

Preferably, a common inlet is in fluid communication with the mixer valve and heater can.

The downstream maximum flow rate may be unaffected or substantially unaffected by the temperature of the water flowing through the downstream flow restrictor.

The downstream flow restrictor may be passive, having a deformable element which is acted upon by the pressure of the water flowing through it.

Alternatively, the downstream flow restrictor may be active, having a flow sensor and a valve in communication with the flow sensor, to set the downstream maximum flow rate.

A temperature sensor may be provided, to monitor the temperature of the water at the heater can outlet.

Preferably, the temperature sensor is in communication with a control system 5 which is operative to maintain a constant or substantially constant water temperature at the heater can outlet.

The control system may be operative to vary the power supplied to a heating element of the can, in accordance with the output of the temperature sensor.

Preferably, the mixer valve is thermostatic.

The mixer valve may comprise a mixing chamber in which the relatively hot and relatively cold water is mixed, the valve being operative substantially only to vary the amount of relatively cold water which flows to the mixing chamber.

Preferably, an upstream flow restrictor is provided, upstream of both the mixer valve and the downstream flow restrictor, to prevent or hinder water from entering the first and second flow paths at above an upstream maximum flow rate, where the upstream maximum flow rate is greater than the downstream maximum flow rate.

Preferably, the upstream maximum flow rate is unaffected or substantially unaffected by the temperature of the water flowing through the upstream flow restrictor.

The upstream flow restrictor may be passive, having a deformable element which is acted upon by the pressure of the water flowing through it.

Alternatively, the upstream flow restrictor may be active, having a flow sensor and a valve in communication with the flow sensor, to set the upstream maximum flow rate.

The instantaneous electric water heater may further comprise a heat recovery system for pre-heating a cold water supply which flows to said inlet, by heat exchange with waste water from the water heater’s outlet.

Preferably, the control system is operative to reduce the power supplied to the heating element in response to an increase in the inlet water temperature, with no or substantially no change being made to the flow rate of water through the heater can.

In accordance with a second aspect of the present invention, we provide a heat recovery shower system comprising an electric shower unit and a heat recovery system operatively associated therewith, the electric shower unit comprising: a) a mixer valve in fluid communication with a water inlet, via a first flow path; b) a heater can having an electric heating element in fluid communication with the inlet, via a second flow path; c) the first and second flow paths being separate so that water may flow from said inlet to the heater can without passing through the mixer valve and vice versa; d) the heater can having an outlet in fluid communication with the mixer valve to allow relatively hot water to be mixed with relatively cold water from said inlet; e) a downstream flow restrictor in the second flow path to prevent or hinder water from flowing to the heater can at above a downstream maximum flow rate; f) an upstream flow restrictor, upstream of both the mixer valve and heater can, having an upstream maximum flow rate which is greater than the downstream maximum flow rate and which limits the rate of water flow into the mixer valve; and g) a temperature control system to monitor and maintain substantially constant the temperature of the water at the heater can outlet; the heat recovery system comprising: h) a heat exchanger for pre-heating a cold water supply which flows to said inlet by heat exchange with waste water from the shower’s outlet; i) the temperature control system being operative to reduce the power supplied to the heating element in response to an increase in the inlet water temperature caused by the recovery of heat from the waste water, and with no or substantially no change being made to the flow rate of water through the heater can.

The invention, in its second aspect, may comprise one or more of the features as described in relation to the first aspect.

Detailed Description of the Invention and Overview of the Drawings Specific but non-limiting embodiments of the present invention will now be described in further detail, but strictly by way of example only, by reference to the accompanying drawings, of which: FIGURE 1 is a schematic plan view of an instantaneous electric water heater in accordance with the invention; FIGURE 2 is a cut-way, schematic sectional view of a flow switch and flow restrictor; FIGURE 3 is a cut-way, schematic sectional view of an additional flow restrictor; and FIGURES 4, 5, 6 and 7 are graphical illustrations of inlet water pressures, inlet water temperatures, ambient water temperatures, shower unit outlet temperatures, flow rates and power consumptions associated with use of the invention.

Referring first to Figure 1, there is shown, schematically and generally at 10, an instantaneous electric water heater in the form of an electric shower.

The shower has an external casing 11, within which are housed the various operating components. The shower thus comprises a primary, mains cold water inlet 12, fed from a mains coid supply 13, a heater can assembly 14, a thermostatic mixer valve 15 and an outlet chamber 16 leading to an outlet orifice 17 which, in use, is connected to a shower head (not shown) by way of a flexible coupling, hose or the like.

Disposed between the inlet 12 and the outlet orifice 17 are an on/off solenoid valve 18 (electrically operable so as to allow/deny water flow into the unit), a flow switch 19 (having a flow-induced moveable element, in operative association with the electrical circuitry), a TRIAC (Triode for Alternating Current) and heat sink 20, which modulates the supply of power to the heater can 14, and a temperature sensor 21 which monitors the temperature of heated water, exiting the heater can 14 at the heater can exit 22.

Electric power is supplied to the heater can 14 through heating elements extending into the can (the tops of which are shown at 23), with the power supplied to the elements 23 being determined by an electronic control system (not shown) associated with the temperature sensor 21.

In generally conventional manner, the shower operates by blending relatively hot (i.e. electrically heated) and relatively cold water flows in a mixing chamber (not shown) of the thermostatic mixer valve 15. The hot water flows to the thermostatic mixer valve through a hot feed line 24, whereas the cold water passes to the mixer valve 15 by way of a diversion ("bleed") from the mains (cold) feed line 25, just downstream of the flow switch 19. The path taken by the cold water to the mixer valve 15 constitutes a first flow path; the path taken by the non-diverted/non-bled cold water, through to the heater can, provides a second flow path.

In more detail, the thermostatic mixer valve 15 is of the (generally known) wax capsule type, whereby a pre-set outlet temperature (i.e. the temperature of the water at the mixing chamber outlet 26) is automatically regulated, by adjustment of the amount of incoming cold water, through an opening/orifice in a side of the cold feed line 25.

In generally conventional manner, the thermostatic mixer valve is pre-set such that an acceptable range of outlet (i.e. showering) temperatures can be obtained (typically between 35°C and 47°C), by way of mixing an incoming heated flow (maintained at around 50°C - 55°C) from the heater can with the cold mains flow, in adjustable proportions.

In other words, the general mode of operation is as follows: Relatively cold (i.e. non-electrically heated) water enters the unit through the mains cold inlet 12. From there, the water flows through the solenoid valve 18 (once it has been opened, by the shower unit being turned on), and then through the flow switch 19, which enables power to be supplied to the heating elements 23. The cold water then flows through the cold feed line 25, past the aperture/orifice in the thermostatic mixer valve, with most of the cold water flowing past the thermostatic valve, unhindered. The cold water then enters the heater can 14, towards its base, through the elbow joint 27.

The cold water then flows upwardly, through the heater can (the specific configuration of heater element is not germane to the present invention), over a weir/baffle (not shown) towards the top of the inside of the heater can and then back down, through the heater can, through a central channel, exiting at the heater can exit 22, and passing, via the temperature sensor 21, back towards the thermostatic mixer valve 15. The wax capsule arrangement (not shown) contained within the thermostatic valve 15 is then operative, automatically, to adjust the amount of cold water which is "bled" from the incoming cold water feed line 25, and passed to the mixing chamber of the mixer valve 15, to provide a water blend having the desired, pre-set, outlet temperature.

Importantly, the shower unit comprises a downstream flow restrictor, towards the end of the cold water flow path, shown at 28. The downstream flow restrictor 28 is contained within the (second) flow path of the cold water and serves to restrict the flow rate of the cold water, into the heater can, to about four litres per minute. It will be appreciated that other downstream maximum flow rates will also allow the invention to operative satisfactorily, with the applicants having found that a downstream maximum flow rate range of between three and five litres per minute operates well, in conjunction with a (fairly standard and widely-used) 8.5kW heater can.

The downstream flow restrictor 28, in this example, is of the passive type, and, specifically, is in the form of a body and a dynamic O-ring, with the O-ring reacting to increases in the incoming water pressure (the pressure being dependant upon flow rate). Specifically, increases in the incoming water pressure/flow rate cause the shape of the O-ring to be changed, so as to decrease the amount of water which can flow through an array of apertures, surrounding it. Such flow restrictors, which are essentially ‘factory-set’ (i.e. their maximum permitted flow rates cannot be ‘user-adjusted’) are well known in themselves, and are commercially available under the "NEOPERL" trade mark.

It will be appreciated that, in contrast to such a passive arrangement, an active configuration is also envisaged. Although not illustrated, such an active configuration could comprise a pressure/flow rate sensor, and an electrically operated valve, effective to restrict the water flow, beyond a particular threshold.

The downstream flow restrictor 28 is shown in more detail in Figure 3, from which can be seen the O-ring 29, disposed in the path of the incoming (cold) water flow. The (approximate) direction of flow is shown by the arrow "F".

The effect of the downstream flow restrictor 28 is to prevent (or substantially hinder) any flow of water, into the heater can, at above the maximum level permitted by the restrictor. This, of course, has the effect, in turn, of influencing the amount of cold water which is forced to flow into the mixing chamber of the mixer valve 15, where the overall flow of wafer into the unit exceeds the rate permitted by the downstream restrictor 28.

Notably, the downstream flow restrictor 28 is operative to restrict the flow of water through it (up to its maximum flow rate), irrespective of the temperature of the cold water itself. In other words, the downstream flow restrictor 28 will operate to allow water to flow through it, until the flow rate reaches its maximum, with the maximum not being influenced in any way by the temperature of the incident water.

The effect of this is that as the temperature of the mains cold water increases, the flow restrictor is unable to compensate for it by permitting any greater flow into the heater can, with the control system thus being forced to reduce the power supplied to the heating elements, to compensate for the increased inlet water temperature. This, of course, has significant power saving advantages, in addition to water saving credentials, on the basis that the maximum flow rate cannot be exceeded.

It will be understood, of course, that in order for the downstream flow restrictor to operate in this way (i.e. to provide the savings and benefits described), the level of its maximum flow rate needs to be appropriate. At one extreme, a downstream maximum flow rate which is too high could have the effect of permitting unfettered flow into the heater can, meaning that the heater can would continue to ‘run’ at full power. At another extreme, a downstream maximum flow rate which is too low would (by simple physics) restrict the amount of hot (i.e. electrically-heated) water from the heater can, and thus which is made available to the mixer valve - which would then not be able to function properly.

Recognising the (generally domestic) arena is which the current system is designed and intended to operate within, the applicants, through detailed experimentation, have found that it is necessary for the downstream flow restrictor to have a maximum flow rate which is lower than that required for an 8.5 kW heater can to run at full power, over a range of inlet temperatures from 5 to around 35°C, whilst providing a heater can outlet temperature of about 5CTC.

By way of example, an 8.5 kW heater can, operating at full power, requires the following ‘incoming’ flow rates to provide a heated flow at about 50°C: An inlet temperature of 28°C needs 5.5 l/min; An inlet temperature of 30’C needs 6.0 l/min; An inlet temperature of 32°C needs 6.7 l/min; and An inlet temperature of 34°C needs 7.6 l/min.

Plainly, and unsurprisingly, the higher the inlet temperature, the greater the flow rate into the heater can which is needed, to maintain an output at about 50°C.

For this reason, the applicants envisage a maximum downstream flow rate of about 4 litres per minute as being particularly well-suited to the present system and arrangement.

Because mains pressure (and thus flow rate) figures cannot accurately be predicted, it is important to ensure that the thermostatic mixer valve 15 (which operates using a pre-set wax capsule configuration) is always able to "cope with the amount of incoming cold water.

As will be appreciated, an unfettered mains inlet configuration, in conjunction with the downstream flow restrictor, could mean that the mixer valve 15 became "deluged’’ with an incoming water flow, with the pre-set, regulated wax capsule arrangement then being ineffective to manage the increased cold water flow.

To mitigate against this, the water heater thus also comprises an upstream flow restrictor 30, contained within the housing of the flow switch 19, and just downstream of the on/off solenoid valve 18. This is shown in more detail in Figure 2.

The maximum flow rate of the upstream flow restrictor 30 is higher than that of the downstream flow restrictor 28.

In this preferred embodiment, the upstream flow restrictor (which also takes the form of a passive, O-ring configuration, much like the downstream flow restrictor) has a maximum permitted flow rate of seven litres per minute, in contrast to the four litres per minute maximum permitted flow rate of the downstream flow restrictor 28, situated just prior to the heater can 14. The applicants have found that an upstream maximum flow rate range of between six and eight litres per minute operates well, in conjunction with an 8.5kW heater can.

The upstream flow restrictor allows the hot and cold water flows (into the mixing chamber of the mixer valve 15) to be balanced, and avoids too much cold water being fed to the chamber, which could mean that the wax capsule thermostatic arrangement was unable to compensate, and provide the desired outlet temperature.

The ability of the water heater to compensate for changes in the inlet, mains cold feed in this way has clear power-saving advantages, in that as the inlet feed temperature increases, the amount of electrical power required to maintain the fixed heater can outlet temperature reduces, by necessity. This is because the arrangement is such that the flow, into the heater can, cannot be increased beyond the maximum threshold governed by the downstream flow restrictor 28.

As with the considerations outlined above (in relation to the downstream flow restrictor), care is needed in choosing an appropriate level for the upstream maximum flow rate.

It will again be understood, that in order for the upstream flow restrictor to operate satisfactorily, (i.e. to work properly in association with the downstream restrictor and the substantially constant heater can output, at about 50Ό), the level of its maximum flow rate is important.

At one extreme, an upstream maximum flow rate which is too high could have the effect of permitting unfettered (cold) flow into the mixer valve, meaning that the heated outlet (from the heater can) would be Overpowered’ by the cold flow, irrespective of the flow rate from the heater can. At another extreme, an upstream maximum flow rate which is too low would (by simple physics) restrict the overall amount of water in the system as a whole, with obvious adverse consequences.

Again recognising the (generally domestic) arena is which the current system is designed and intended to operate within, the applicants, through detailed experimentation, have found that it is necessary for the upstream flow restrictor to have a maximum flow rate which is lower than that required for an 8.5 kW heater can to run at full power in conjunction with a wax capsule mixer valve (set to operate with a hot feed at 50°C), over a range of mains inlet temperatures from 5 to around 35°C.

By way of example, a system using an 8.5 kW heater can, operating at full power, and a wax capsule mixer valve with an outlet (i.e. end user) temperature of about 41'C, requires the following overall ‘incoming’ flow rates : An inlet temperature of 28°C needs 9.3 l/min; An inlet temperature of 30°G needs 11.0 l/min; An inlet temperature of 32°C needs 13.5 l/min; and An inlet temperature of 34°C needs 17.4 l/min.

Plainly, and unsurprisingly, the higher the inlet temperature, the greater the overall flow rate into the system which is needed, to maintain a blended output of about 41°C.

For this reason, the applicants envisage a maximum upstream flow rate of about seven litres per minute as being particularly well-suited to the present system and arrangement.

This ability offers clear benefits in "normal" use (recognising that inlet mains temperatures can fluctuate significantly, either throughout the day or on a seasonal basis), but the invention is particularly well suited to use in conjunction with so-called "heat recovery" systems, where waste water heat is used to warm the incoming cold feed.

A number of systems of this type are known, and they fall into two basic categories: those having a heat exchanger associated with a generally vertical mains feed pipe, and those where the heat exchanger is associated with the shower tray. it will be appreciated, in this instance, that the type of "heat recovery" system is irrelevant to the advantages which can be realised by way of the present invention, on the basis that any system which is able to increase the heat of the inlet cold water feed will inevitably allow an electrical power consumption reduction, using the system described above.

To illustrate the environmental/power-saving advantages, however, the applicants have conducted a number of tests, using the "Recoh-Vert" heat recovery system, which uses (warm) waste water to heat incoming mains (cold) water with a heat exchanger in close proximity to the incoming feed pipe.

Figures 4, 5, 6 and 7 illustrate, graphically, the effect of the present invention, on electrical power consumption, using four different initial (i.e. "out of the ground") water inlet temperatures: 5°C, 10°C, 15°C and 20"C, respectively.

In each of Figures 4, 5, 6 and 7, the ambient water temperature (i.e. the temperature of the cold mains feed) is shown at A.

Again, in each of Figures 4, 5, 6 and 7, the unit outlet temperature (i.e. the "showering" temperature, experienced by a user) is shown at B.

It will be noted, that in view of the wax capsule/thermostatic valve arrangement employed, the unit outlet temperature is substantially unaffected by the ambient inlet temperature, with an outlet temperature of about 41°C being maintained.

The electrical power demands, placed on the system (i.e. the power drawn, and supplied to the heating elements) is shown at C.

The X-axis, running along the base of each graph, is representative of the passage of time. As can be seen, power is first applied to the system (in these tests) after about 27 seconds. Up until that point the incoming water flow reads 0, the ambient water temperature (naturally) remains unchanged, the water pressure remains unchanged and the power supplied to the heating elements is also 0.

Once the unit is switched on, however, using an initial inlet water pressure of 3 bar, power is applied to the heating elements (shown by a high initial power consumption at C), of approximately 8 to 8.5 kilowatts, causing an immediate increase in the unit outlet temperature, shown at line B.

In view of the upstream flow regulator which is effective to limit the overall incoming flow (into the unit) to seven litres per minute, the overall flow rate (shown at D) cannot exceed seven litres per minute, irrespective of the inlet water pressure.

During the initial phase (the first 3 bar section on each graph) the rapid increase in the unit outlet temperature, gradually enables the heat recovery process to take place. In other words, as hot waste water starts to flow around (or in close proximity to) the cold inlet feed, the temperature of the inlet feed starts to rise - as shown at E. There is inevitably a time lag, of some degree, between the inlet temperature rise and the outlet temperature rise, as it takes time for the heated waste water to come into contact with the incoming feed, and for the heat exchange to take place.

However, as the unit inlet temperature (i.e. the temperature of the water entering the shower unit) starts to rise, caused by the heat recovery process, the relatively cold water entering the heater can is also at an elevated temperature. In view of the control system, and the pre-set, maintained heater can outlet temperature (of about 50°C), the control system is forced to reduce the amount of electrical power (i.e. heat) applied to the heater can, to compensate for the elevated incoming water feed temperature. This is shown by the significant drop in the power bands shown at C, as the unit inlet temperature starts to rise. The ‘banding’ shown at C is the result of the TRIAC 20 effecting some power switching, but it will be understood that the overall amount of power supplied to the heating elements is significantly reduced, as a result of the heat being recovered from the waste water.

The overall flow is unaffected (as shown by the generally flat lines at D), in view of the two (downstream and upstream) flow restrictors, meaning that the increased inlet water temperatures do not create a greater flow demand, in contrast to the prior art discussed above.

The power-saving benefit can be realised at a range of incoming (mains) pressures, as shown by the varying pressure bands, in the figures.

Specifically, referring to Figure 4 as an example, a 5 bar mains water pressure (increased from 3 bar, towards the left of the graph) gives rise to a slightly higher overall flow rate (albeit never exceeding the 7 l/min imposed by the upstream flow restrictor) and a very slight increase in the upper power limit (shown by the top line of the power banding) being applied. However, the recovered heat still enables a significant overall power reduction to be achieved, as shown by the lower power limit, on the bottom line of the power banding.

Dropping the inlet pressure back to 3 bar (seen slightly towards the right of the 5 bar ‘section’) does not have appreciable effect on the ’heat-recovered’ inlet supply temperature, but as the system has ‘settled’ by this point, an even greater power saving (i.e. reduced demand) has been observed, by the applicants. This is shown by the drop of both the top and bottom lines of the power banding.

As the inlet pressure is reduced still further (to 2 bar, 1 bar and 0.5 bar) the inlet flow rate also drops. This means that the flow rate into the heater can is also reduced, so that, to maintain a heater can outlet temperature of about 50°C, the power needs to be reduced, quite considerably.

Figures 5, 6 and 7 illustrate the unit’s operation with different mains (cold) temperatures, at 10, 15 and 20"C.

The ‘trend’ is of course similar, in that a significant power reduction can be obtained as the heat recovery process starts to operate. However, what is also clear is that the actual power savings are notably increased, with warmer mains temperatures - as shown by the bottom lines of the power bandings at C.

When used in this specification and claims, the terms comprises and comprising and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims (21)

1. An instantaneous electric water heater comprising: a) a mixer valve in fluid communication with a water inlet, via a first flow path; b) a heater can in fluid communication with a water inlet, via a second flow path; c) the first and second flow paths being separate so that water may flow from said inlet to the heater can without passing through the mixer valve and vice versa; d) the heater can having an outlet in fluid communication with the mixer valve to allow relatively hot water to be mixed with relatively cold water from said inlet; and e) a downstream flow restrictor being provided in the second flow path to prevent or hinder water from flowing to the heater can at above a downstream maximum flow rate.

2. An instantaneous electric water heater according to claim 1 wherein a common inlet is in fluid communication with the mixer valve and heater can.

3. An instantaneous electric water heater according to claim 1 or claim 2 wherein the downstream maximum flow rate is unaffected or substantially unaffected by the temperature of the water flowing through the downstream flow restrictor.

4. An instantaneous electric water heater according to any one of claims 1 to 3 wherein the downstream flow restrictor is passive, having a deformable element which is acted upon by the pressure of the water flowing though it.

5. An instantaneous electric water heater according to any one of claims 1 to 3 wherein the downstream flow restrictor is active, having a flow sensor and a valve in communication with the flow sensor, to set the downstream maximum flow rate.

6. An instantaneous electric water heater according to any preceding claim wherein a temperature sensor is provided to monitor the temperature of the water at the heater can outlet.

7. An instantaneous electric water heater according to claim 6 wherein the temperature sensor is in communication with a control system which is operative to maintain a constant or substantially constant water temperature at the heater can outlet.

8. An instantaneous electric water heater according to claim 7 wherein the control system is operative to vary the power supplied to a heating element of the can, in accordance with the output of the temperature sensor.

9. An instantaneous electric water heater according to any preceding claim wherein the mixer valve is thermostatic.

10. An instantaneous electric water heater according to any preceding claim wherein the mixer valve comprises a mixing chamber in which the relatively hot and relatively cold water is mixed, the valve being operative substantially only to vary the amount of relatively cold water which flows to the mixing chamber.

11. An instantaneous electric water heater according to any preceding claim wherein an upstream flow restrictor is provided, upstream of both the mixer valve and downstream flow restrictor, to prevent or hinder water from entering the first and second flow paths at above an upstream maximum flow rate, where the upstream maximum flow rate is greater than the downstream maximum flow rate.

12. An instantaneous electric water heater according to claim 11 wherein the upstream maximum flow rate is unaffected or substantially unaffected by the temperature of the water flowing through the upstream flow restrictor.

13. An instantaneous electric water heater according to claim 11 or claim 12 wherein the upstream flow restrictor is passive, having a deformable element which is acted upon by the presence of the water flowing through it.

14. An instantaneous electric water heater according to claim 11 or claim 12 wherein the upstream flow restrictor is active, having a flow sensor and a valve in communication with the flow sensor, to set the upstream maximum flow rate.

15. An instantaneous electric water heater according to any preceding claim further comprising a heat recovery system for pre-heating a cold water supply which flows to said inlet, by heat exchange with waste water from the water heater’s outlet

16. An instantaneous electric water heater according to any one of claims 8 to 15 wherein the control system is operative to reduce the power supplied to the heating element in response to an increase in the inlet water temperature, with no or substantially no change being made to the flow rate of water through the heater can.

17. A heat recovery shower system comprising an electric shower unit and a heat recovery system operatively associated therewith, the electric shower unit comprising: a) a mixer valve in fluid communication with a water inlet, via a first flow path; b) a heater can having an electric heating element in fluid communication with the inlet, via a second flow path; c) the first and second flow paths being separate so that water may flow from said inlet to the heater can without passing through the mixer valve and vice versa; d) the heater can having an outlet in fluid communication with the mixer valve to allow relatively hot water to be mixed with relatively cold water from said inlet; e) a downstream flow restrictor in the second flow path to prevent or hinder water from flowing to the heater can at above a downstream maximum flow rate; f) an upstream flow restrictor, upstream of both the mixer valve and heater can, having an upstream maximum flow rate which is greater than the downstream maximum flow rate and which limits the rate of water flow into the mixer valve; and g) a temperature control system to monitor and maintain substantially constant the temperature of the water at the heater can outlet; the heat recovery system comprising; h) a heat exchanger for pre-heating a cold water supply which flows to said inlet by heat exchange with waste water from the shower’s outlet; i) the temperature control system being operative to reduce the power supplied to the heating element in response to an increase in the inlet water temperature caused by the recovery of heat from the waste water, and with no or substantially no change being made to the flow rate of water through the heater can.

18. A heat recovery shower system further comprising one or more of the features of any of claims 1 to 16.

19. An instantaneous electric water heater substantially as hereinbefore described and/or as shown in the accompanying drawings.

20. A heat recovery shower system substantially as hereinbefore described and/or as shown in the accompanying drawings.

21. Any novel feature or novel combination of features described herein 5 and/or in the accompanying drawings.