Patent Publication Number: US-2023160604-A1

Title: Pumpless top-up water heating tank

Description:
The present invention relates to a water heating tank and a method for operating it. In particular, a water heating tank with an electrical heating element that can heat relatively small quantities of water quickly and supply them without the use of a pump. 
     It is desirable to heat only the amount of water that is needed for use and it is desirable to be able to heat that water quickly, once a user has decided that they need it. In conventional water heating tanks the heating element is located at the base of the tank and when hot water is demanded by the user the cold water at the base of the tank is heated and rises to the top of the tank under the action of a convection current. There is inevitably mixing of the heated water as it rises through the tank therefore reducing the efficiency and speed with which a volume at hot water can be provided at the top of the tank from where is drawn off for use. 
     There is therefore a need for a water heating tank that can quickly and efficiently provide hot water to a user on demand. 
     Accordingly the present invention provides a water heating tank comprising a reservoir with a cold water inlet into the reservoir and a hot water outlet from the reservoir, a heater enclosure located within the reservoir, the heater enclosure enclosing at least part of a heater, the heater enclosure having an inlet, a vent and an outlet, a duct connected to the outlet and the duct having an exit located in an upper portion of the reservoir. This arrangement is advantageous because the heater enclosure encloses a relatively small volume of water. This enables sufficient heat to be imparted to that water by the heating element to cause the temperature of the water to be raised quickly to a temperature suitable for use (e.g. above 50 degrees Celsius). The duct facilitates the transfer of the heated water from the heater enclosure to the top of the tank, from where it will be drawn off for use, with the minimum of loss of temperature, because the heated water is shielded from the rest of the water in the tank by the wall of the duct. 
     Preferably, the vent of the heater enclosure is an enclosure vent that passes through a wall of the heater enclosure and that is covered by an openable enclosure vent cover and wherein there is also provided an actuator to open the enclosure vent. 
     The enclosure vent can provide a relief path once initial heating is achieved, to permit better heating in lower portions of the reservoir and to prevent excessive heating in the top of the reservoir and in the heater enclosure. The actuator is preferably configured to open the enclosure vent at least partially once a predetermined threshold temperature is reached at a specific location in the reservoir. Various combinations of suitable threshold temperature and specific location in the reservoir may be appropriate. For example, suitable specific locations may include: at a top of the reservoir, in the top half of the reservoir, in the top third of the reservoir, in the heater enclosure, in the duct, or in the reservoir at a particular height between the heater and the top of the duct. The predetermined threshold temperature may for example be one or more of: above 45° C., above 50° C., around 55° C., below 70° C., and below 60° C. The predetermined threshold temperature may be selected in dependence on the specific location in the reservoir. 
     The vent may be an enclosure vent. The enclosure vent may pass through a wall of the heater enclosure. The enclosure vent may be closed by an enclosure vent closure. An actuator may be provided to move the enclosure vent closure. The heater enclosure may comprise an enclosure vent closure suitable for closing the enclosure vent. The heater enclosure may comprise an actuator arranged to move the enclosure vent closure. 
     Preferably the actuator is a thermal actuator, such as a bimetallic actuator or a wax motor. Preferably, the actuator is a wax motor. For effective thermal response the bulb of the wax motor is preferably located inside the heater enclosure. For effective venting the bulb of the wax motor is preferably located within the top half of the heater enclosure. Preferably the actuator is configured to open the enclosure vent at least partially above a threshold temperature. The threshold temperature may be above 50° C., further preferably around 55° C. This can prevent water at the top of the reservoir from becoming too hot and water at the bottom of the reservoir from failing to become heated. Preferably the threshold temperature is below 70° C., further preferably below 60° C. 
     For effective venting the enclosure vent may be located above the heater. For effective venting the enclosure vent may be located near the heater, preferably near a top half of the heater or at or near a top of the heater. This can permit venting of water heated by the heater. For effective venting the enclosure vent may be located within 30 cm height or 20 cm height or 10 cm height above or below the top of the heater. For effective venting the enclosure vent is preferably located within the top half of the heater enclosure, more preferably at or near the outlet of the heater enclosure. For effective venting the enclosure vent may be located within 30 cm distance or 20 cm distance or 10 cm distance of the outlet of the heater enclosure. For effective fluid flow the outlet of the heater enclosure is preferably located at or near the top of the heater enclosure. For effective fluid flow the outlet of the heater enclosure is preferably located above a top of the heater. 
     Preferably, the water heating tank further comprises a cut-out thermostat for the heater, wherein the temperature sensor of the thermostat is located within the heater enclosure and above the heater. The temperature sensor is located above the heating element in order that it is located within the hottest zone of the heater enclosure. This is done to reduce the risk of water at a scalding temperature being drawn from the tank. 
     Alternatively, the temperature sensor of the thermostat could be located in an upper portion of the reservoir, above the exit from the duct. 
     Preferably, the water heating tank further comprises a temperature and pressure relief valve fluidly connected to the reservoir by a pipe, wherein the inlet to the pipe is located above the exit from the duct. 
     Preferably, the water heating tank further comprises a chimney valve that, in use, can prevent fluid flow from the heater enclosure to the duct or prevent fluid flow from the duct. 
     Preferably, the chimney valve is actuated by a wax motor. The wax motor may be located within upper region of the heater enclosure. The wax motor may also actuate the enclosure vent closure. The chimney valve may be actuated by the actuator of the heater enclosure. 
     The enclosure vent closure and the chimney valve may be provided by a combined valve. The wax motor may actuate the combined valve. The actuator of the heater enclosure may be arranged to actuate the combined valve. 
     In an alternative embodiment, the duct comprises at least one duct vent that passes through a wall of the duct and that is covered by an openable duct vent cover. There may also be provided a duct actuator to open the duct vent. The openable duct vent cover may be actuated by the actuator of the heater enclosure. The actuator of the heater enclosure may be arranged to actuate the openable duct vent cover. 
     Optionally the heater enclosure includes a plurality of enclosure vents. 
     Preferably, the enclosure vent is a hole. Preferably, the openable enclosure vent cover is an enclosure sleeve. Preferably the enclosure sleeve is located adjacent to the heater enclosure. Preferably the enclosure sleeve has an enclosure sleeve vent. Preferably the enclosure sleeve vent is a hole that passes through the wall of the enclosure sleeve. 
     Preferably, the at least one duct vent is a hole that passes through a wall of the duct. Preferably, the openable duct vent cover is a duct sleeve. Preferably, the duct sleeve is located adjacent to the duct. Preferably, the duct sleeve has a duct sleeve vent. Preferably, the duct sleeve vent is a hole that passes through the wall of the duct sleeve. 
     Preferably, the enclosure sleeve is moveable relative to the heater enclosure. The actuator of the heater enclosure may be arranged to move the enclosure sleeve relative to the heater enclosure. Preferably in one position there is no overlap of the enclosure vent and the enclosure sleeve vent and in another position there is at least some overlap of the enclosure vent and the enclosure vent sleeve. 
     Preferably the duct sleeve is moveable relative to the duct. The actuator of the heater enclosure may be arranged to move the duct sleeve relative to the duct. Preferably in one position there is no overlap of the duct vent and the duct sleeve vent. Preferably in another position there is at least some overlap of the duct vent and the duct sleeve vent. The duct sleeve may be fixed to the enclosure sleeve. 
     Preferably, the duct comprises a plurality of duct vents, the duct sleeve comprises a plurality of duct sleeve vents and wherein in one position a first group of the duct vents and a first group of the duct sleeve vents are aligned and in another position another group of the duct vents and another group of the duct sleeve vents are aligned. The configuration of overlapping vents in the duct and duct sleeve can be arranged to provide a flow path out of the side of the duct which drops progressively down the height of the duct as the thermocline within the tank is established and moves downwards through the cylinder. This approach maximises the conversion of electrical power to hot water which can be made available to a user at a useful temperature. In alternative embodiments, there may be fewer vents in the duct and/or duct sleeve. In these alternative embodiments, the first phase of heating develops hot water at a useful temperature up to the point where a maximum can temperature is reached, beyond which heated water is released into the cylinder underneath the thermocline where that heated water is mixed with cooler water located in the bottom of the tank. The approach of the alternative embodiments is not as optimal as the first, however it provides the advantage of reduced cost of manufacture of the water heating tank and the simplicity aids reliability by reducing the number of moving surfaces that are required in the more complex embodiments that are employed to achieve the most optimal dispatch of heated water into the cylinder. These embodiments assist with reducing the amount of energy needed to heat all of the water in the cylinder to a point at which it will be sterilised and in reduced the time taken to achieve the sterilisation temperature. 
     Preferably, the heater enclosure comprises at least one aperture towards its base, wherein the aperture acts as the inlet and the vent. Preferably, the heater enclosure has a heating zone within which zone there is located an active part of the heating element that is, in use, able to transfer heat to water within the heater enclosure and wherein the part of the aperture acting as the vent is located within the heating zone. 
     Preferably the water heating tank further comprises an openable aperture cover. This allows the area of the inlet region and/or the area of the vent region to be changed, thus allowing control of the temperature of the water within the heater enclosure. It is advantageous to be able to reduce the flow area in order to maximise the efficiency with which water within the heater enclosure can be heated. If the water within the heater enclosure is at a low temperature then it is advantageous to reduce the flow area so that the optimum amount of water travels up through the convective duct and loss of heat to the water at the bottom of the tank is minimised. 
     Preferably, the water heating tank further comprises a valve located at the inlet to the duct, at the outlet from the duct or within the duct. Advantageously, the valve is able to partially or fully close the duct. 
     Preferably, the degree of opening of the valve can be changed by an adjuster. The adjuster may be located at least partially outside of the reservoir. It is advantageous to have an external control because this facilitates changing the behaviour of the valve (e.g. the temperatures at which it opens and closes) after manufacture of the tank, whether in use, or immediately prior to installation in a hot water system. 
     Preferably, the water heating tank further comprises a second heater, wherein the second heater is located within the reservoir and externally to the heater enclosure. The second heater can be any suitable heater such as an electrical heating element, or it can be a coil of pipe supplied with hot water that has been heated by a gas boiler or by a heat pump. A heat pump can heat the water via a coil of pipe or via an external plate heat exchanger. 
     Preferably, the water heating tank further comprises a third heater, wherein the third heater is located within the reservoir and externally to the heater enclosure. The third heater can be any suitable heater such as an electrical heating element, or it can be a coil of pipe supplied with hot water that has been heated by a gas boiler or by a heat pump. A heat pump can heat the water via a coil of pipe or via an external plate heat exchanger. In one arrangement, the heat pump would heat the bulk of the water outside of the water heating tank to a low temperature. The arrangement of the heater enclosure and duct of the water heating tank to increase the temperature of relatively small amounts of water to a useable temperature, as and when they are demanded by a user. 
     Preferably, the water heating tank further comprises a hot water baffle located within the reservoir and positioned above the second heater. 
     Preferably, the water heating tank further comprises a thermostatic valve on the outlet. The provision of a thermostatic valve facilitates a reduction in the amount of time that it takes to raise the temperature of all of the water in the tank to, for example, a temperature needed to sterilise the water. In a conventional water heating tank, the temperature of the water should not exceed an upper limit, because if it does the risk of a user being scalded by the hot water is increased. In the water heating tanks of the present invention, if the maximum temperature of the water in the tank is limited to, for example 70 degrees Celsius, then it will take an undesirably long time to heat all of the water in the tank to a temperature that will cause it to be sterilised, e.g. when all of the water in the tank is above 60 degrees Celsius. This is because of the way that the tanks operate. The heating element is located within the can and the heated water rises to the top of the tank as a result of a convection current set up within the chimney. If the water in the top of the tank is not to exceed 70 degrees Celsius, then the heating element cannot be operated at full power continuously (if it is heated continuously the temperature of the water at the top of the tank will exceed 70 degrees Celsius). Instead, the heating element must be periodically switched off, or operated at a lower power. In the periods when the heating element is off, or operating at low power, a greater proportion of the heat within the water enclosed by the can and chimney passes into the water surrounding the can and chimney (rather than the water at the top of the tank). This may be via conduction through the walls of the can or the chimney or via water leaving the can through the flow ports. This enables the temperature of the water in the lower portions of the tank to be raised, but the time taken to raise the temperature is reduced because the heating element is not being operated continuously. If a thermostatic valve is provided then the water at the top of the tank can be allowed to exceed 70 degrees Celsius because the thermostatic valve will mix that water with water at a lower temperature thus ensuring that the water provided to the user is not at a temperature that is likely to scold them. If the water at the top of the tank can be allowed to exceed 70 degrees Celsius then the heating element can be operated at full power continuously, or at least for a greater period of time, thereby enabling the temperature within the tank to be raised to a sterilisation temperature more quickly. 
     Preferably, the reservoir comprises a base and the water heater is located adjacent to the base and extends into the reservoir away from the base. 
     Preferably, the duct has a lower cross-sectional area than the heater enclosure. 
     Preferably, the actuator is a wax motor. Preferably the wax reservoir of the wax motor is located within the heater enclosure, further preferably in its upper portion. 
     Preferably, the heater is an electrical water heater. 
     Preferably, the water heating tank further comprises a control system with a means for varying heat provided by the heater. The heater may be an electrical water heater and the control system may comprise means for varying a supply of electrical power to the heater. The control system is preferably adapted to cause the heater to provide heat until a first temperature threshold is crossed in the reservoir, preferably at the top of the reservoir. The first temperature threshold may be in the range of 65-75° C., preferably around 70° C. The control system may be adapted to control the heater to maintain a temperature in the reservoir (preferably a temperature at the top of the reservoir) in the region of the first temperature threshold. The control system is preferably adapted to cause the heater to stop providing heat until a second temperature threshold is crossed in the reservoir (preferably a temperature at the top of the reservoir). The second temperature threshold may be 1 to 10° C. below the first temperature threshold, preferably around 5° C. below the first temperature threshold. The second temperature threshold may be in the range of 60-70° C., preferably around 65° C. 
    
    
     
       Preferably, the water heating tank further comprises a diffuser at the duct exit in the upper portion of the reservoir. The diffuser may be co-axial with duct. The diffuser is preferably adapted to inhibit mixing of water exiting the duct with water in the top of the reservoir. The diffuser may be a plate diffuser or any other suitable diffuser. 
         FIG.  1    is a schematic cross-sectional view of a water heating tank according to a first embodiment of the present invention; 
         FIG.  2    is a view of the heating element enclosure of the water heating tank of  FIG.  1    showing the flow ports; 
         FIG.  3    is a schematic cross-sectional view of a water heating tank according to a second embodiment of the present invention; 
         FIG.  4    is a view of the heating element enclosure of the water heating tank of  FIG.  3    showing the flow ports; 
         FIG.  5    is a schematic cross-sectional view of a water heating tank according to a third embodiment of the present invention; 
         FIG.  6    is a schematic cross-sectional view of a water heating tank according to a fourth embodiment of the present invention; 
         FIG.  7    is a schematic cross-sectional view of a water heating tank according to a sixth embodiment of the present invention; 
         FIG.  8    is a schematic cross-sectional view of a water heating tank according to a seventh embodiment of the present invention with a mechanism for varying the flow area of flow ports in the heating element enclosure using a sliding sleeve; 
         FIG.  9    is a schematic cross-sectional view of a heating element enclosure with a mechanism for varying the flow area of the flow ports using bimetallic elements; 
         FIG.  10    is a schematic cross-sectional view of a heating element enclosure with a mechanism for varying the flow area of the flow ports using wax motors and sliding shutters; 
         FIG.  11    is a schematic cross-sectional view of a water heating tank according to an eighth embodiment of the present invention, having a thermostatic valve; 
         FIG.  12    is a schematic cross-sectional view of a water heating tank with a control system; 
         FIG.  13    is a schematic cross-sectional view of a water heating tank with an active hot water vent operated by a wax motor; 
         FIG.  14    is a schematic cross-sectional view of a water heating tank with a secondary heating element, hot and cold water baffles and a hot water outlet at the bottom; 
         FIG.  15    is a schematic cross-sectional view of a water heating tank with a rotary hot water vent sleeve; 
         FIG.  16    is a partially cut-away close up view of the rotary hot water vent sleeve of  FIG.  15   ; 
         FIG.  17    is a schematic cross-sectional view of a water heating tank with a linearly displaceable hot water vent sleeve; 
         FIG.  18    is a schematic view of the can, chimney and linear hot water vent sleeve of  FIG.  17   ; 
         FIG.  19    is a diagram of flow through the can and chimney of the water heating tank of  FIG.  17    when the can sleeve is in its bottom position, in a middle position and in its top position; 
         FIG.  20    is a diagram of a water heating tank with a temperature and pressure relief valve and a heating element cut-out thermostat; and 
         FIG.  21    is a diagram of a water heating tank with a temperature and pressure relief valve and a heating element cut-out thermostat and a chimney valve. 
     
    
    
     A water heating tank  1 , for the provision of hot water, and according to a first embodiment of the present invention is illustrated in  FIG.  1   . The water heating tank  1  comprises a hollow cylinder  3  with a domed top wall  4  and a domed bottom wall  6 . An electrical water heater  5  is located at the bottom of the cylinder  3 , it is fixed to the bottom wall  6  and it is aligned with the longitudinal axis of symmetry of the cylinder  3 . A tubular heating element enclosure, or can,  7  is located over the heater  5  and forms a heating zone  9 . The can  7  has passive flow ports  11  arranged around the periphery of its lower portion. A tubular convective duct, or chimney,  13 , extends vertically upwards from the top of the can  7  and co-axially with the can  7  and the cylinder  3 . The can  7  and the chimney  13  each have a circular cross-sectional profile of constant diameter. The chimney  13  terminates close to the top surface of the cylinder  3 . An annular plate, or diffuser,  15  is located at the top of the chimney  13  and co-axially with it. 
     The water heater  5  has a base  17  and a heating element  19 . The base  17  is fixed to the bottom of the cylinder  3  and the heating element  19  extends vertically upwards from the base  17 . The vertical height of the heating element  19  is less than the height of the can  7  and the top of the heating element  19  is located lower than the bottom of the chimney  13 . The can  7  can be divided into an active zone  21 , in which the heating element is located, and an inactive zone  23 . 
     The passive flow ports  11  are arched and extend parallel to the axis X-X from the bottom of the cylinder  3  into the active zone  21 . 
     The can  7 , chimney  13  and diffuser  15  are made from a material that is approved for use with potable water, such as  316  stainless steel, or duplex or cross-linked polyethylene. 
     The cylinder  3  has a cold water inlet  25  and a hot water outlet  27 . 
     In use, the cylinder  3  is filled with cold water via the cold water inlet  25  and power is provided to the electrical water heater  5 . The electrical water heater  5  starts to heat the cold water within the can  7 , within the heating zone  9 . The temperature of the water increases, its density decreases and it rises upwards, through the can  7  and into the chimney  13 . Cold water is drawn into the can  7  through the flow ports  11  to replace the water that is rising upwards. 
     When the heated water has passed up through the height of the chimney  13  it exits into the top of the cylinder  3 . The diffuser  15  inhibits mixing of the hot water exiting the chimney  13  with the cold water in the top of the cylinder  3 . A reduction in mixing facilitates the production of a volume of hot water at the top of the cylinder  3  that is sufficiently hot to be used. 
     Once a sufficient volume of hot water has been obtained the electrical water heater  5  is switched off. 
     Typically, the tank  1  only needs to produce a relatively small volume of hot water. However sometimes a larger quantity of hot water is desired and perhaps all of the water within the cylinder  3  may need to be heated. A thermocline will be created within the water in the cylinder  3 , with the hottest water at the top of the cylinder  3  and the coldest water at the bottom of the cylinder  3 . When the level of hot water has been pushed close to the bottom of the cylinder  3  it is no longer possible to create a convection current in the chimney  13 , because the water passing into the can  7  is the same temperature as the water in the can  7 . In this situation, the water heated within the can  7  by the electrical water heater  5  passes from the can  7  into the cylinder  3  through the passive flow ports  11 . 
     A water heating tank  201  according to a second embodiment of the present invention is illustrated in  FIG.  3   . The tank  201  has features in common with tank  1  of the first embodiment and the common features utilise the same reference numerals prefixed with the number  2 . In the second embodiment the electrical water heater  205  has a relatively large diameter and the can  207  thus has a larger diameter, so that it can enclose it. The chimney  213  has a relatively small diameter compared to the can  207  and a frustoconical tapering section  231  is provided between the outlet of the can  207  and the inlet of the chimney  213 .  FIG.  4    illustrates the passive flow ports  211  of the second embodiment. 
     The operation of the second embodiment of the present invention is the same as the operation of the first embodiment. 
     A water heating tank  301  according to a third embodiment of the present invention is illustrated in  FIG.  5   . The third embodiment is the same as the first embodiment except that the tank  301  has a valve  333  that is located between the outlet of the can  307  and the inlet to the chimney  313 , in order to partially or fully close off the chimney  313  from the can  307 . The valve  333  is driven by a wax motor  335 . 
     In use, the valve  333  can be fully opened to allow substantially unimpeded flow of water from the can  307  into the chimney  313 . The valve  333  can be fully closed to prevent flow of water from the can  307  into the chimney  313 . The valve can be partially opened to restrict the flow of water from the can  307  into the chimney  313 . 
     A fourth embodiment of the invention (not illustrated), is the same as the second embodiment except that the tank has a valve that is located between the outlet of the can and the inlet to the chimney, in order to partially or fully close off the chimney from the can. 
     The valves of the third and fourth embodiments can also be placed at, or near to, the outlet from the chimney. 
     A water heating tank  501  according to a fourth embodiment of the present invention is illustrated in  FIG.  6   . 
     The tank  501  shares many common features with the tank  1  of the first embodiment and those features are identified by the same reference numerals, pre-fixed with the number  5 . A chimney  513  extends from the top of the can  507  through the cylinder  503  and through the top wall  504 . The top of the chimney  513  is provided with an external screw thread onto which an internally screw threaded cap  537  is screwed. A sealing element  539  is located within the chimney  513  and creates a water-tight seal between the inside of the cylinder  503  and the inside of a chamber  541  which contains an electrical solenoid  543 . A push-rod  545  passes through an aperture in the sealing element  539  and is connected at its top end to a core  547  of the solenoid  543 . The bottom end of the push-rod  545  is connected to a plunger  549 . The plunger  549  is located within the chimney  513  and the external surface of the plunger  549  is a close sliding fit with the inside surface of the chimney  513 . The chimney  513  is provided with a number of outflow apertures  551  located around its circumference and adjacent to its top end. The length of the plunger  549  is slightly greater than the depth of the outflow apertures  551 , measured in the direction of the longitudinal axis X-X. The outflow apertures  551  are located within the dome of the top wall  504 . 
     The annular plate diffuser  515  is located on the chimney  513  just below the bottom edge of the apertures  551 . 
     The outflow apertures  551  allow water to flow out of the chimney  513  into the top of the cylinder  503 . In use, to control the flow of water out of the outflow apertures  551  the plunger  549  can be moved along the longitudinal axis X-X, by operation of the solenoid  543 , so that the plunger  549  completely obscures the outflow apertures  551  to close them, partially obscures the outflow apertures  551  or is completely clear of the outflow apertures  551  so that they are fully open. 
     A water heating tank  601  according to a fifth embodiment of the present invention is illustrated in  FIG.  7   . The tank  601  shares many common features with the tank  201  of the second embodiment and those features are identified by the same reference numerals, pre-fixed with the number  6 . A chimney  613  extends from the top of the can  607  through the cylinder  603  and through the top wall  604 . The top of the chimney  613  is blanked off and provided with a central threaded aperture  602 . A threaded control rod  608  of a thermostatic control knob  610  passes through the aperture and into the interior of the chimney  613  where it is connected to a thermostatic plunger  649 . The chimney  613  is provided with a number of outflow apertures  651  located around its circumference and adjacent to its top end. The length of the thermostatic plunger  649  is slightly greater than the length of the outflow apertures  651 . The outflow apertures  651  are located within the dome of the top wall  604 . The annular plate diffuser  615  is located on the chimney  613  just below the bottom edge of the apertures  651 . 
     The outflow apertures  651  allow water to flow out of the chimney  613  into the top of the cylinder  603 . In use, to control the flow of water out of the outflow apertures  651  the plunger  649  can be moved along the longitudinal axis X-X, by rotation of the thermostatic control knob  610 , so that the plunger  649  completely obscures the outflow apertures  651  to close them, partially obscures the outflow apertures  651  or is completely clear of the outflow apertures  651  so that they are fully open. 
     All of the above described embodiments of the tank  1 , 201 , 301  and  501  and the embodiments of the tank  1101 ,  1201 ,  1301 ,  1401 ,  1501  and  1701 , as described below can also be provided with a means for changing the flow area of the passive flow ports  11 , 211 , 311  and  511  that pass through the walls of the can  7 , 207 , 307  and  507 , such that they become active flow ports.  FIGS.  8 ,  9  and  10    illustrate three variants of a variable flow port mechanism having active flow ports. 
       FIG.  8    shows a tank  701  according to a sixth embodiment of the present invention that is provided with a variable active flow port mechanism  761  that has a sleeve  763 , a drive rod  765  and an electrical solenoid  767 . The sleeve  763  is a sliding fit within the can  707  and is cup-shaped with a base  712  and a cylindrical side wall  771 . The base  712  is provided with apertures  773  that allows water to pass from the inside of the sleeve  763  to the outside. The cylindrical side wall  771  is solid and continuous. The drive rod  765  is connected to the base  712  and extends perpendicularly from it, towards the top of the tank  701 , along the longitudinal axis of the cylinder  703  until it passes through the top wall  704 . A solenoid plunger  775  is attached to the top of the drive rod  765  and is located within the coil winding  777  of the electrical solenoid  767 . 
     In use, energisation of the solenoid  779  causes the plunger  775  to be drawn into the coil winding  777 , which lifts the drive rod  765  and thus the obturation sleeve  763 . Lifting the obturation sleeve  763  increases the flow area of the flow ports  711 . To reduce the flow area of the flow ports  711  the amount of electrical energy supplied to the solenoid  779  is decreased and the plunger  779 , drive rod  765  and obturation sleeve  763  move downwards. 
       FIG.  9    shows a portion of a tank  801  that is provided with a can  807  having a variable active flow port mechanism  861 . The variable active flow port mechanism  861  has bimetallic flaps  881  that are located within flow ports  811  and that are attached at their upper edges to the can  807 . 
     In use, when the temperature of the water inside the can  807  is low the bimetallic flaps  881  are in plane with the wall of the can  807  and the flow ports  811  are closed. An increase in the temperature of the water within the can  807  causes the bimetallic flaps  881  to change shape and move out of the plane of the wall of the can  807 , thus opening the flow ports  811 . The behaviour of the bimetallic flaps  881  can be tuned so that the degree of opening of the flow ports  811  is appropriate to the temperature within the can  807  and thus the desired flow rate of water out of the can  807 . 
       FIG.  10    shows a portion of a tank  901  that is provided with a variable active flow port mechanism  961  that has flow port shutters  983  and wax motors  985  having a wax bulb  987  and a linear actuator  989 . The flow aperture shutters  983  are located within a can  907 , adjacent to the flow ports  911 , and are attached to the can  907  using slide rails. 
     In use, when the temperature of the water within the can  907  is low the wax within the wax bulb  987  is solid and at its minimum volume. An increase in the temperature of the water in the can  907  causes the wax to melt and expand. The increase in volume of the wax causes the linear actuator  989  to move the flow aperture shutters  983  from a position in which they are entirely covering the flow ports  911  to a position in which the flow ports  911  are partially open. If the temperature of the water within the can  907  is sufficiently high then the wax motors will move the flow aperture shutters  983  to an entirely opened position. The behaviour of the variable active flow port mechanism  961  can be tuned so that the degree of opening of the flow ports  911  is appropriate to the temperature within the can  907  and thus the desired flow rate of water out of the can  907 . 
     A water heating tank  1101  according to a seventh embodiment of the present invention is shown in  FIG.  11   . The tank  1101  has the same features as the tank  1  of the first embodiment. In addition, it is provided with a thermostatic valve  1118  located on the hot water outlet  1127  of the cylinder  1103 . The thermostatic valve  1118  has a cold water inlet pipe  1114  and an outlet pipe  1116 . 
     A thermostatic valve, such as thermostatic valve  1118  can be fitted to the hot water outlets of any of the hot water tanks described herein and illustrated in  FIGS.  1 ,  3 ,  5 ,  6 ,  7  and  8   . 
     In operation of the water heating tank, the thermostatic valve  1118  is used to control, and typically to set an upper limit on, the temperature of the water leaving the outlet pipe  1116 . 
     The present invention also encompasses a control system  1200  for controlling the input of heat into the water contained within the cylinder of a hot water tank, as illustrated in  FIG.  12   . The control system  1200  will be described with reference to a tank that is the same as tank  1  of  FIG.  1   , but it is equally applicable to any of the hot water tanks described herein and illustrated in  FIGS.  1 ,  3 ,  5 ,  6 ,  7 ,  8 ,  11 ,  13 ,  14 ,  15  and  17   . A power control unit  1222  is electrically connected to a mains electrical supply  1220  that supplies electrical power to a heating element  1219 . A thermostat  1224  is mounted to the outside of the domed top wall  1204  of the reservoir  1203  and is electrically connected to the power control unit  1222  by a signal wire  1226 . 
     In operation, when heating the water within the tank  1201  from cold, the power control unit  1222  is instructed to supply the heating element  1219  with power from the mains electrical power supply  1220 . The power control unit  1222  knows to supply power to the heating element  1219  because the thermostat  1224  has indicated, via the signal wire  1226 , that the temperature of the water at the top of the tank is too low. In an initial phase of heating the heating element  1219  is operated at full power and hot water is supplied to the top of the tank  1201  via the chimney  1213 . The power control unit  1222  is programmed to supply power to the heating element  1219  until it receives a signal from the thermostat  1224  that the water at the top of the tank  1201  has reached a first temperature set point, e.g. 70 degrees Celsius. The power control unit  1222  then turns off the supply of power to the heating element  1219  or reduces the power supplied to the heating element  1219 . When the power control unit  1222  receives a signal from the thermostat  1224  indicating that the water at the top of the tank  1201  has dropped below the first temperature set point by a predetermined number of degrees, for example the temperature has dropped to  65  degrees Celsius, then the power control until  1222  will turn back on the supply of power to the heating element  1219 , or increase the power supplied to the heating element  1219 . 
     Any of the hot water heating tanks described herein and illustrated in  FIGS.  1 ,  3 ,  5 ,  6 ,  7 ,  8 ,  11 ,  13 ,  14 ,  15  and  17    can be operated with both a control system to control the supply of power to the heating element and a thermostatic valve attached to the hot water outlet from the tank. 
       FIG.  13    is a schematic diagram of a water heating tank  1301  according to an eighth embodiment of the present invention, provided with an active hot water vent  1328 . The tank  1301  has all the features of tank  301  of the third embodiment, as illustrated in  FIG.  3    and those features share the same reference numerals, prefixed with the number  13  instead of  2 . 
     The active hot water vent  1328  is provided in the frustoconical tapered section  1331 . A wax motor  1330  is attached to a vent cover  1332  located over a vent opening  1334 . The wax motor bulb  1336  is located midway between the top of the can  1307  and the diffuser  1315 . It should be noted that in other examples the wax motor bulb  1336  may be located elsewhere, for example nearer the diffuser, nearer the can, beside the can, within the can, within the chimney, above the diffuser, or at the top of the cylinder. 
     In use, power is supplied to the heating element  1319  and the water in the can  1307  is heated. This creates a convection current in the can  1307  and the chimney  1313  that causes cold water to be drawn into the can  1307  at the bottom, through the passive flow ports  1311 , and to be pushed out of the top of the chimney  1313 . The cylinder  1303  gradually fills up with hot water from the top, which pushes the thermocline down through the cylinder  1303  from the top to the bottom. The temperature of the water within the can  1307  increases as the thermocline moves down because of a change in the behaviour of the convection current. As the body of lighter hot water in the top of the cylinder increases, the hydrostatic buoyancy forces decrease and the flow rate through the can decreases. The temperature at the top of the cylinder and in the can may become too hot, while water at the bottom of the cylinder may not reach sanitary levels. The active hot water vent provides a relief path once initial heating is achieved, to permit better heating in lower portions of the cylinder and prevent excessive heating in the top part of the cylinder. 
     In order to prevent the can  1307  from overheating, which would cause the heating element  1319  to cut-out, the active hot water vent  1328  is opened, either fully or partially. The wax motor  1330  moves the vent cover  1332  away from the vent opening  1334 . Hot water is vented from the can  1307  via the vent opening  1334 . This venting of hot water creates a new convection current, with water still being drawn into the can  1307  through the passive flow ports  1311 . This new convection current introduces heat into the bottom portion of the cylinder  1303 , rather than to the top portion of the cylinder  1303 . 
     The active hot water vent  1328  can be applied to any of the tanks of  FIGS.  1 ,  3 ,  5 ,  6 ,  7 ,  8 ,  11 ,  12 ,  14 ,  15  and  17   , as well as the tank of  FIG.  13   . 
       FIG.  14    is a schematic diagram of a water heating tank  1401  according to a ninth embodiment of the present invention, provided with a secondary heating element  1438 . The tank  1401  has a number of the features of tank  201  of the second embodiment, as illustrated in  FIG.  3    and those features share the same reference numerals, prefixed with the number  14  instead of  2 . 
     The secondary heating element  1438  is located within the cylinder  1403 , externally to the can  1407  and adjacent to it, and attached to the base  1406 . A hot water baffle  1440  is attached to the external surface of the wall of the can  1407  and extends towards the wall of the cylinder  1403  to an extent such that it passes over the secondary heating element  1438 . The hot water baffle  1440  is formed from a metal plate in the shape of the sector of a circle and it is provided with through holes (not shown). The hot water baffle  1440  is angled such that its edge attached to the can  1407  is vertically higher than its opposite edge. 
     A cold water baffle  1494  is attached to the external surface of the can  1407  and extends towards the wall of the cylinder  1403  to an extent that it passes across the secondary heating element  1438 . An aperture  1496  is provided in the cold water baffle  1494  through which the secondary heating element  1438  can pass. A second aperture  1498  is provided in the cold water baffle through which a hot water outlet pipe  1427  can pass. The hot water outlet pipe has an inlet located above the outlet from the chimney  1413  and it passes out through the domed bottom wall  1406  of the cylinder  1403 . 
     In use, power is supplied to the heating element  1419  and the water in the can  1407  is heated. This creates a convection current in the can  1407  and the chimney  1413  that causes cold water to be drawn into the can  1407  at the bottom, through the passive flow ports  1411 , and to be pushed out of the top of the chimney  1413 . The cylinder  1403  gradually fills up with hot water from the top. Power will be supplied to the heating element  1419  until the temperature of the water at the top of the cylinder reaches a preset level, for example 55 degrees Celsius. The supply of power to the heating element  1419  is then stopped and power is supplied to the secondary heating element  1438 . The secondary heating element  1438  creates a convective plume in the water at the bottom of the tank and that plume is constrained by the hot water baffle in order that most of the heat emitted by the secondary heating element is absorbed by the water in the lower portion of the cylinder  1403 . 
     The secondary heating element  1438  can be applied to any of the tanks of  FIGS.  1 ,  3 ,  5 ,  6 ,  7 ,  8 ,  11 ,  12 ,  13 ,  15  and  17   , as well as the tank of  FIG.  1   . 
       FIG.  15    is a schematic diagram of a water heating tank  1501  according to a tenth embodiment of the present invention, provided with a rotary hot water vent sleeve  1542 . The rotary hot water vent sleeve  1542  is structured and arranged so that it can rotate within the can  1507  and the chimney  1513  by virtue of having a complementary chimney section  1544 , frustoconical section  1546  and can section  1548 . The chimney section  1544  extends through the domed top wall  1504  of the cylinder  1503  and is connected to an electric drive motor  1550 . The frustoconical section  1546  and the can section  1548  are provided with interior vent holes  1552 , as shown in  FIG.  16   . Exterior vent holes  1554  are provided in the can  1507  and in the frustoconical section  1531 . The interior vent holes  1552  and the exterior vent holes  1554  are of the same diameter and are arranged in patterns such that in one position of the rotary hot water sleeve  1542  they are aligned, i.e. they are transposed over one another, or overlap one another, such that water can be vented from the can  1507  through the vent holes  1552 ,  1554 , and in another position they are misaligned, such that water cannot be vented from the can  1507  through the vent holes  1552 ,  1554 . The vent holes  1552 ,  1554  can also be partially aligned. 
     In use, power is supplied to the heating element  1519  and the water in the can  1507  is heated. This creates a convection current in the can  1507  and the chimney  1513  that causes cold water to be drawn into the can  1507  at the bottom, through the passive flow ports  1511 , and to be pushed out of the top of the chimney  1513 . The cylinder  1503  gradually fills up with hot water from the top, which pushes the thermocline down through the cylinder  1503  from the top to the bottom. The temperature of the water within the can  1507  increases as the thermocline moves down because of a change in the behaviour of the convection current. In order to prevent the can  1507  from overheating, which would cause the heating element  1519  to cut-out, the rotary hot water vent sleeve  1542  is rotated by the drive motor  1548  such that the vent holes  1552 ,  1554  are partially or fully aligned (dependent upon the amount of venting that is required). Hot water is vented from the can  1507  via the vent holes  1552 ,  1554 . This venting of hot water creates a new convection current, with water still being drawn into the can  1507  through the passive flow ports  1511 . This new convection current introduces heat into the bottom portion of the cylinder  1503 , rather than to the top portion of the cylinder  1503 . 
       FIG.  17    is a schematic diagram of a water heating tank  1701  according to an eleventh embodiment of the present invention, provided with a linear hot water vent sleeve  1764 . The water heating tank  1701  shares a number of features with the tank  201  of the second embodiment, as illustrated in  FIG.  3    and those features share the same reference numerals, prefixed with the number  17  instead of  2 , as set out in the following description. 
     The water heating tank  1701  comprises a hollow cylinder  1703  with a domed top wall  1704  and a domed bottom wall  1706 . The cylinder  1703  has a cold water inlet  1725  and a hot water outlet  1727 . An electrical water heater  1705  is located at the bottom of the cylinder  1703 , it is fixed to the bottom wall  1706  and it is aligned with the longitudinal axis of symmetry of the cylinder  1703 . A tubular heating element enclosure, or can,  1707  is located over the heater  1705  and fixed to the bottom wall  1706 . The can  1707  forms a heating zone  1709 . The can  1707  is cup-shaped with a closed upper end and an open lower end. The can  1707  has three arch shaped passive flow ports  1711  that extend parallel to the longitudinal axis of the can and which are arranged around the periphery of its lower portion, at one hundred and twenty degree spacings, as shown in  FIG.  18   . A number of can perforations  1770  are provided through the upper portion of the can  1707 . A tubular convective duct, or chimney,  1713 , extends vertically upwards from the top of the can  1707 . The longitudinal axis of the chimney  1713  is parallel to, but offset from, the longitudinal axis of the can  1707 . The chimney  1713  has progressive chimney perforations  1769  along its length. The can  1707  and the convective chimney  1713  each have a circular cross-sectional profile of constant diameter. The chimney  1713  terminates close to the top surface of the cylinder  1703 . A linear hot water vent sleeve  1764  is located over the can  1707  and chimney  1713 . The linear hot water sleeve  1764  has a cup-shaped can sleeve  1766  that has a larger diameter than the can  1707  and a chimney sleeve  1768  that has a larger diameter than the chimney  1713 . The can sleeve  1766  has a closed upper end and an open lower end and has can sleeve perforations  1772  around its entire cylindrical surface. The chimney sleeve  1768  has progressive chimney sleeve perforations  1774  along its length. The linear hot water sleeve  1764  is linearly slideable relative to the can  1707  and the chimney  1713  and the linear motion is driven by a wax motor  1776  that is attached to the top of the can  1707  and which extends into the heating zone  1709 . The can  1707 , the chimney  1713  and the hot water sleeve  1764  are made from a material that is approved for use with potable water, such as  316  stainless steel, or duplex or cross-linked polyethylene. 
     The water heater  1705  has a base  1717  and a heating element  1719 . The base  1717  is fixed to the bottom of the cylinder  1703  and the heating element  1719  extends vertically upwards from the base  1717 . The vertical height of the heating element  1719  is less than the height of the can  1707  and the top of the heating element  1719  is located lower than the bottom of the chimney  1713 . The can  1707  can be divided into an active zone  1721 , in which the heating element is located, and an inactive zone  1723 . The passive flow ports  1711  extend into the active zone  1721 . 
     The can perforations  1770  and the can sleeve perforations  1772  are of the same diameter and are arranged in arrays that are complementary, as shown in  FIGS.  17  and  18   , so that in one orientation there is no overlap between the can sleeve perforations  1772  and the can perforations  1770 , in a second orientation there is a partial overlap between the can sleeve perforations  1772  and the can perforations  1770  and in a third orientation there is an exact alignment of the can sleeve perforations  1772  and the can perforations  1770 . The perforations  1770  and  1772  are equally spaced within the arrays, in horizontal and vertical directions. 
     The chimney perforations  1769  pass through the wall of the chimney  1713  and are arranged in a vertical line between the top and the bottom of the chimney  1713 . The chimney sleeve perforations  1774  pass through the wall of the chimney sleeve  1768  and are also arranged in a vertical line between the top and the bottom of the chimney sleeve  1768 . The vertical spacings between the perforations  1769  and  1774  are not equal but are greater at the bottom than at the top and vary progressively inbetween. The spacings for the chimney perforations  1769  and the chimney sleeve perforations  1774  are the same. 
     The wax motor  1776  comprises a wax reservoir  1778  and an actuator rod  1780 . The wax motor  1776  is attached to the top of the can  1707  and the reservoir  1778  extends through a can aperture  1782  at the centre of the top of the can  1707  into the heating zone  1709 . 
     The can  1707  has a spring frame  1784  with two legs and a cross-bar fixed between the legs at their upper ends. The lower end of each leg is attached to the top of the can  1707 , either side of the aperture  1782 . The legs pass through frame apertures  1786  provided in the can sleeve  1776 . 
     The can sleeve  1776  has an actuator frame  1788  with two legs and a cross-bar fixed between the legs at their upper ends. The lower end of each leg is attached to the top of the can sleeve  1776  either side of a can sleeve aperture  1790  through which the wax motor  1776  extends. 
     The actuator frame  1788  is located within the spring frame  1784  and the height of the legs of the actuator frame  1788  is less than the height of the legs of the spring frame  1784 . This creates a vertical gap between the cross-bar of the actuator frame  1788  and the cross-bar of the spring frame  1784  and a return spring  1792  is located within that vertical gap and is attached at either end to a cross-bar. 
     In use, the cylinder  1703  is filled with cold water via the cold water inlet  1725  and power is provided to the electrical water heater  1705  in an initial water heating phase. The electrical water heater  1705  starts to heat the cold water that is located within the heating zone  1709  of the can  1707 . The temperature of the water increases, its density decreases and it rises upwards, through the can  1707  and into the chimney  1713 . Cold water is drawn into the can  1707  through the flow ports  1711  to replace the water that is rising upwards. In this stage of the operation the can sleeve  1766  and the chimney sleeve  1768  are in their lowermost position and there is no overlap between the can sleeve perforations  1772  and the can perforations  1770  or between the chimney sleeve perforations  1774  and the chimney perforations  1769 . Therefore, during this initial heating stage, all of the water that is heated within the can  1707  passes out of the can  1707 , through the chimney  1713  and into the upper portion of the cylinder  1703 . 
     If only a small volume of hot water is required, then all of the water heating will take place within the initial heating phase. The power supply to the heating element  1705  will be switched off before there is a need to move to a subsequent phase of heating. 
     If a large volume of hot water is required, for example more than half a tank (or the whole tank, if a sterilisation cycle is being conducted) then one or more additional water heating phases will be needed, as explained below. 
     Water heated within the can  1707  will pass up the chimney  1713  and into the upper region of the cylinder  1703 . The cylinder  1703  will gradually fill up with hot water from the top and as the level of hot water progresses downwardly through the tank the temperature of the water within the can  1707  will increase. This is because the hot water within the can  1707  is only able to rise due to convection if the temperature of the water above it is lower. In the initial heating phase, when the water at the top of the tank  1701  might be at 15 degrees Celsius, the temperature of the water leaving the can and passing up through the chimney is relatively low, i.e. below 50 degrees Celsius. When the water at the top of the tank  1701  is at a much higher temperature, for example at 50 degrees Celsius, the temperature of the water within the can  1707  will need to be raised to above 50 degrees Celsius, if the water in the can is to rise up the chimney  1713  by convection. 
     Eventually, the temperature of the water in the can  1707  will be elevated to a level at which the power supply to the heating element  1705  must be cut off in order to stop the heating element  1705  from overheating (which may cause it to fail). It is not desirable to have to cut off the power supply to the heating element  1705  because then the time taken to heat the required amount of water will increase (typically the heating element  1705  will need to be switched on and off repeatedly until the water has been heated to the desired temperature). Consequently, it is necessary to implement a subsequent heating phase to follow the initial heating phase. 
     When the temperature of the water in the can  1707  has risen to a certain level the wax motor  1776  will start to operate. The water will have raised the temperature of the wax contained within wax reservoir  1778  and that wax will expand, causing the actuator rod  1780  to move vertically upwards thus moving the actuator frame  1788 , the can sleeve  1766  and the chimney sleeve  1768  vertically upwards. This will cause the can sleeve perforations  1772  to overlap the can perforations  1770  and the chimney sleeve perforations  1774  to overlap the chimney perforations  1769 . The temperature of the water in the can  1707  will determine the extent to which the wax motor  1776  moves the can sleeve  1766  and the chimney sleeve  1768  and thus the extent to which the perforations  1769 ,  1770 ,  1772 ,  1774  overlap. If the water temperature within the can  1707  is very high then there will be an alignment of the perforations, such that the maximum amount of water can leave the can  1707  through the perforations  1770 ,  1772  and into the surrounding water, rather than that water leaving the can  1707  through the chimney  1713 . A lower temperature of water within the can  1707  will result in a partial overlap of the perforations  1769 ,  1770 ,  1772 ,  1774  with a resulting lower flow area available for water to leave the can  1707 . The return spring  1792  acts to return the can sleeve  1766  and the chimney sleeve  1768  towards their lower position as the temperature of the water in the can  1707  decreases. 
       FIG.  19    is a diagram showing, in simplified terms, how water flows through the can  1707  and chimney  1713  of the water heating tank  1701 , as illustrated in  FIG.  17   . The drawings only show flow for the right hand side of the can  1707  and chimney  1713 . The flow for the left hand side would be the same, but has been omitted to aid the clarity of the drawings. 
     The image on the left hand side image shows the can sleeve  1766  in its lowermost position which is representative of the position of the can sleeve  1766  at the start of a heating cycle, when the water in the cylinder  1703  is cold. The can sleeve perforations  1772  are not aligned with the can perforations  1770  and the chimney sleeve perforations  1774  are not aligned with the chimney perforations  1769 . In use, an electrical supply is provided to the heating element  1705  and the heating element  1705  starts to heat up the water in the heating zone  1709  of the can  1707 . Water is drawn into the can  1707  through the flow apertures  1711  located around its base. A convection current is set up and hot water rises through the can  1707  and through the chimney  1713  and exits at the top of the chimney  1713  into the volume at the top of the cylinder  1703 . 
     The central image shows the can sleeve  1766  in a mid-position, which is representative of the position of the can sleeve  1766  mid way through a heating cycle, when the water in the cylinder  1703  has been heated up and the thermocline is moving downwards through the cylinder  1703 . The can sleeve perforations  1772  are not aligned with the can perforations  1770  but the chimney sleeve perforations  1774  in a central section of the chimney sleeve  1768  are aligned with adjacent chimney perforations  1769 . In use, heated water flows up out of the can  1707  by convection and some of it exits the chimney  1713  via the chimney and chimney sleeve perforations  1769 ,  1774  thus passing into the water in the main body of the cylinder  1703 . The rest of the heated water exits at the top of the chimney  1713  into the volume at the top of the cylinder  1703 . 
     The image on the right hand side shows the can sleeve  1766  in its uppermost position which is representative of the position of the can sleeve  1766  towards the end of a heating cycle, when almost all of the water in the cylinder  1703  is hot. The can sleeve perforations  1772  are aligned with the can perforations  1770  and some of the chimney sleeve perforations  1774  in the bottom section of the chimney sleeve  1768  are aligned with the adjacent chimney perforations  1769 . In use, a significant proportion of the heated water flows out of the can  1707  via the can and can sleeve perforations  1770 ,  1772 , passing into the water at the bottom of the cylinder  1703 . Some heated water does pass up through the can  1707  and into the chimney  1713  and that water exits the chimney  1713  via the chimney perforations  1769 ,  1774  thus passing into the water towards the bottom of the cylinder  1703 . 
     A water heating tank  1801 , for the provision of hot water, and according to a further embodiment of the present invention is illustrated in  FIG.  20   . The water heating tank  1801  comprises a hollow cylinder  1803  with a domed top wall  1804  and a domed bottom wall  1806 . An electrical water heater  1805  is located at the bottom of the cylinder  1803 , it is fixed to the bottom wall  1806  and it is aligned with the longitudinal axis of symmetry of the cylinder  1803 . A tubular heating element enclosure, or can,  1807  is located over the heater  1805  and forms a heating zone  1809 . The can  1807  has passive flow ports  1811  arranged around the periphery of its lower portion. A tubular convective duct, or chimney,  1813 , extends vertically upwards from the top of the can  1807  and co-axially with the can  1807  and the cylinder  1803 . The can  1807  and the chimney  1813  each have a circular cross-sectional profile of constant diameter. The chimney  1813  terminates close to the top surface of the cylinder  1803 . An annular plate, or diffuser,  1815  is located at the top of the chimney  1813  and co-axially with it. 
     The water heater  1805  has a base  1817  and a heating element  1819 . The base  1817  is fixed to the bottom of the cylinder  1803  and the heating element  1819  extends vertically upwards from the base  1817 . The vertical height of the heating element  1819  is less than the height of the can  1807  and the top of the heating element  1819  is located lower than the bottom of the chimney  1813 . The can  1807  can be divided into an active zone  1821 , in which the heating element  1819  is located, and an inactive zone  1823 . 
     The passive flow ports  1811  are arched and extend parallel to the axis X-X from the bottom of the cylinder  1803  into the active zone  1821 . 
     The can  1807 , chimney  1813  and diffuser  1815  are made from a material that is approved for use with potable water, such as  316  stainless steel, or duplex or cross-linked polyethylene. 
     The cylinder  1803  has a cold water inlet  1825  and a hot water outlet  1827 . 
     A temperature and pressure relief valve  1852  is connected to an outlet  1828  from the cylinder  1803  and to a drain (not shown). A temperature sensor  1870  of a heating element cut-out thermostat  1854  is located within a hot water region  1856 . The hot water region  1856  is at the top of the can  1807  and above the heating element  1819 . The temperature sensor  1870  is located above the heating element, as illustrated in  FIG.  20   . 
     A vent opening  1858  is provided in the top of the can  1807 , in the shoulder adjacent to the chimney  1813 , and a vent valve  1860  is provided within it. The vent valve  1860  is operated by a wax motor  1862  which is located within the hot water region  1856 . In an open position of the vent valve  1860  the vent opening  1858  is open. In a closed position of the vent valve  1860  the vent opening  1858  is closed. 
     The electrical water heater  1805 , the can  1807 , chimney  1813 , diffuser  1815  and heating element cut-out thermostat  1854  are fixed to a standard sized flange  1864 , as a sub-assembly  1868 , and can be located within the cylinder  1803  through an assembly opening  1866 . 
     In use, the cylinder  1803  is filled with cold water via the cold water inlet  1825  and power is provided to the electrical water heater  1805 . The electrical water heater  1805  starts to heat the cold water within the can  1807 , within the heating zone  1809 . The temperature of the water increases, its density decreases and it rises upwards, through the can  1807  and into the chimney  1813 . Cold water is drawn into the can  1807  through the flow ports  1811  to replace the water that is rising upwards. 
     When the heated water has passed up through the height of the chimney  1813  it exits into the top of the cylinder  1803 . The diffuser  1815  inhibits mixing of the hot water exiting the chimney  1813  with the cold water in the top of the cylinder  1803 . A reduction in mixing facilitates the production of a volume of hot water at the top of the cylinder  1803  that is sufficiently hot to be used. Once a sufficient volume of hot water has been obtained the electrical water heater  1805  is switched off. 
     Typically, the tank  1801  only needs to produce a relatively small volume of hot water. However sometimes a larger quantity of hot water is desired and perhaps all of the water within the cylinder  1803  may need to be heated. A thermocline will be created within the water in the cylinder  1803 , with the hottest water at the top of the cylinder  1803  and the coldest water at the bottom of the cylinder  1803 . When the level of hot water has been pushed close to the bottom of the cylinder  1803  it is no longer possible to create a convection current in the chimney  1813 , because the buoyancy of the heated water is no longer enough to push the hot water at the top of the cylinder further down, i.e. hydrostatic buoyancy forces decrease and the flow rate through the can decreases. The temperature at the top of the cylinder and in the can may become too hot, while water at the bottom of the cylinder may not reach sanitary levels. The vent opening provides a relief path once initial heating is achieved, to permit better heating in lower portions of the cylinder and prevent excessive heating in the top part of the cylinder and the can. 
     After initial heating, the water heated within the can  1807  by the electrical water heater  1805  passes from the can  1807  into the cylinder  1803  through the passive flow ports  11 . Also, the hot water can be vented from the can  1807  via the vent opening  1858 . The wax motor  1862  is preset to open the vent valve  1860  when the temperature of the water within the can  1807  exceeds a threshold temperature, for example 65° Celsius. 
     The heating element cut-out thermostat  1854  is preset to switch the heating element  2019  off when the water at the top of the can  1807  reaches a predetermined temperature, typically 80° Celsius. The water heated within the can  1807  will rise to the top of the can  1807  and thus the hot water region  1856  contains the hottest water within the can  1807 . 
     The temperature and pressure relief valve  1852  is also used to limit the temperature of the heated water. When the water within the cylinder  1803  exceeds a preset temperature then the temperature and pressure relief valve  1852  will open and the hot water will be vented to a drain (not shown) via the outlet  1828 . 
     To assemble, maintain, or repair the water heating tank  1801  the sub-assembly  1868  can be removed and replaced via the assembly opening  1866 . 
     A water heating tank  1901 , for the provision of hot water, and according to a further embodiment of the present invention is illustrated in  FIG.  21   . The water heating tank  1901  has all the features of the water tank  1801  and those features are referred to with the same reference numerals, but prefixed with  19  rather than  18 . In addition, the water heating tank  1901  has a chimney valve  1933  located at the base of the chimney  1913 , at the interface between the can  1907  and the chimney  1913 . The chimney valve  1933  is operated by a wax motor  1935  and the wax motor bulb  1936  is located with the hot water region  1956  of the can  1907 . When the water in the can  1907  is cold, then the chimney valve  1933  will be closed. When the water in the can  1907  is hot, then the chimney valve  1933  will be open (or at least partially open). 
     The water heating tank  1901  has all the operational characteristics of the water heating tank  1801  with the additional possibility of closing the outlet from the can  1907  into the chimney  1913 . When heating water from cold, the chimney valve  1933  will initially be closed. The heating element  1919  will heat the water in the can  1907  and when the temperature of the water in the hot water region  1956  reaches a preset temperature, for example 50° Celsius, the chimney valve  1933  will start to open, driven by the wax motor  1935 . The chimney valve  1933  will be fully open when the temperature in the hot water region  1956  reaches a second temperature, for example 55° Celsius. 
     It is envisaged that the vent valve  1960  and the chimney valve  1933  could be operated by a single wax motor with its wax bulb located within the hot water region  1956 . 
     In an alternative arrangement (not illustrated) the vent valve  1960  and the chimney valve  1933  could be combined into a single combined valve. The combined valve could be operated by a single wax motor, having a single piston, with the bulb of the wax motor located within the hot water region  1956 . 
     In use, if the water within the water heating tank  1901  and the can  1907  is initially cold, then initially the combined valve will be closed, i.e. water cannot exit from the can  1907  via the vent  1958  or via the chimney  1913 . The opening between the can  1907  and the chimney  1913  remains closed until water within the can  1907  reaches a temperature of  50 ° Celsius, at which point the wax motor opens the combined valve so that water is allowed to pass from the can  1907  into the chimney  1913 . At first, the opening into the chimney  1913  would be partially opened by the wax motor and then, as the temperature of the water within the can  1907  increases, the opening into the chimney  1913  would be fully opened by the wax motor (for example when the temperature of water in the can  1907  reaches 55° Celsius). When the temperature of water in the can  1907  reaches 65° Celsius the wax motor will further move the combined valve so that it will open the vent opening  1958 , so that hot water can exit the can  1907  via either the chimney  1913  or the vent opening  1958 . 
     Variants 
     While in the examples provided above an annular plate diffuser is described, it should be appreciated that a wide variety of diffusers are suitable for arrangement at the top of the chimney to inhibit mixing of hot water exiting the chimney with cold water in the top of the cylinder. 
     In some of the examples provided above a wax motor is described as moving a cover (e.g. to open a vent) in response to being heated. It should be appreciated that locating the wax motor bulb at a suitable position can enable venting in response to certain occurrences, for example in response to excessive build up of heat within the can or elsewhere, or to movement of the thermocline below a certain position. A variety of suitable positions for the wax motor bulb are conceivable in addition to those described, for example outside the chimney or inside the chimney, near the diffuser, near the can, inside the can, and/or near the top of the can. As described above locating the wax motor bulb inside the can, near the top of the can or near the can outlet, can be particularly efficient as this can permit both proximity to the vent being actuated and effective venting, as well as responding to a build up of heat in the can.