Heating arrangement for a liquid heating appliance and liquid heating appliance

Abstract

A heating arrangement for a liquid heating appliance comprising a substrate 18c with an electrically resistive heater track 18a disposed thereon. A thermally responsive element 30a, 30b extends over a

Description

The present invention relates to a heating arrangement for a liquid heating appliance and to an appliance including such a heating arrangement. Liquid heating appliances, such as kettles, are common in many households. Conventional kettles comprise an electrical power supply that is arranged to provide power to an electrical heater that, in turn, heats the contents of a liquid vessel. The electrical power supply typically includes a switch that enables the supply of power to the electrical heater to be interrupted in the event of an overheat scenario, e.g. when the appliance has boiled dry or has been turned on without any liquid inside. The switch is typically operated by a thermally responsive element, such as a bimetallic element, that is arranged in contact with the electrical heater. Some existing liquid heating appliances comprise electrical heaters comprising an electrically resistive heating track. This kind of heater can reach high temperatures, enabling faster boiling times in comparison to other electrical heaters such as sheathed heating elements. However, the higher operating temperatures of these heaters means that the thermally responsive elements used to provide overheat protection may themselves be required to operate at higher temperatures. Designing thermally responsive elements that can reliably operate at such higher temperatures can increase the complexity of the design, testing, and manufacturing processes, which can introduce delays to these processes and increase the associated costs. The present invention aims to address, or at least mitigate, at least one of the problems outlined above. When viewed from a first aspect, the present invention provides a heating arrangement for a liquid heating appliance, the heating arrangement comprising: a substrate; an electrically resistive heater track deposited on the substrate; an electrical power supply circuit for supplying electrical power to the heater track, the circuit comprising a switch that is moveable between a closed position and an open position; and a thermally responsive element arranged to operate at a predetermined temperature to move the switch into the open position so as to interrupt the supply of electrical power to the heater track; wherein: the thermally responsive element at least partially extends over a portion of the heater track; and at least when the temperature of the thermally responsive element is below the predetermined temperature: no part of the thermally responsive element is arranged in direct physical contact with the heater track; and a contacting portion of the thermally responsive element is arranged in thermally conductive contact with the substrate. When viewed from a second aspect, the present invention provides a liquid heating appliance comprising: a liquid heating vessel for receiving a liquid to be heated; and a heating arrangement arranged to heat the contents of the liquid heating vessel, the heating arrangement comprising: a substrate; an electrically resistive heater track deposited on the substrate; an electrical power supply circuit for supplying electrical power to the heater track, the circuit comprising a switch that is moveable between a closed position and an open position; a thermally responsive element arranged to operate at a predetermined temperature to move the switch to the open position so as to interrupt the supply of electrical power to the heater track; wherein: the thermally responsive element at least partially extends over a portion of the heater track; and at least when the temperature of the thermally responsive element is below the predetermined temperature: no part of the thermally responsive element is arranged in direct physical contact with the heater track; and a contacting portion of the thermally responsive element is arranged in thermally conductive contact with the substrate. The Applicant has appreciated that arranging the thermally responsive element so that it is in thermally conductive contact with the substrate, but is not in direct physical contact with the heater track, can beneficially reduce the rate of heat transfer from the heater track to the thermally responsive element. This means that, when the temperature of the heater track is high enough to necessitate an overheat switch-off, the thermally responsive element is at a lower temperature. As a result, a thermally responsive element having a lower operating temperature can be used without increasing the risk of the thermally responsive element operating at too low a temperature, e.g. during normal operation of the liquid heating appliance. This can save expense and time during the design and manufacturing processes of the thermally responsive element. Additionally, arranging the thermally responsive element such that it is not in contact with the electrically resistive track, at least when the temperature of the thermally responsive element is below the predetermined temperature, may obviate the need to provide electrical insulation between the thermally responsive element and the electrically resistive heater track. This may reduce the cost of the heating arrangement and hence any appliance it is incorporated therein. As the thermally responsive element at least partially extends over a portion of the heater track, it will be appreciated that the thermally responsive element may receive thermal energy from said portion of the heater track via thermal radiation, which may help to ensure that the thermally responsive element operates reliably. By “extends over”, it will be understood that, when the heating arrangement is viewed in plan view, along an axis perpendicular to the substrate, at least a portion of the footprint / silhouette of the thermally responsive element covers the portion of the heater track over which the thermally responsive element extends. The footprint / silhouette is the projection of the thermally responsive element in plan view. The footprint / silhouette may be defined by a projection of the perimeter of the thermally responsive element, as well as by the perimeter(s) of any cut-outs within the thermally responsive element. In not being in direct physical contact with the electrically resistive heater track, the thermally responsive element may not apply any force, directly or indirectly, to the electrically resistive heater track. The thermally responsive element may be devoid of any electrical insulation. The heater track may be deposited on a lower surface of the substrate. In some embodiments, when the heating arrangement is arranged to heat the contents of the liquid heating vessel, the substrate is arranged below the liquid heating vessel. It will be appreciated that “below” refers to the direction in which gravity acts on liquid within the liquid heating vessel during operation of the appliance. In such embodiments, the thermally responsive element may be below the electrically resistive heater track. In some embodiments, the contacting portion of the thermally resistive element is arranged in thermally conductive contact with the lower surface of the substrate, at least when the temperature of the thermally responsive element is below the predetermined temperature. In some embodiments, the heating arrangement comprises a control unit. The control unit may comprise the switch of the electrical power supply circuit. In some embodiments, the control unit comprises a mounting body for mounting the control unit to the substrate. The control unit may comprise an electrical adapter part (sometimes referred to as an electrical adapter) arranged to connect to a corresponding electrical connector part (sometimes referred to as an electrical connector) to receive a supply of electrical power, e.g. mains electricity. The electrical adapter part may be a cordless electrical adapter part arranged to mate with a corresponding base electrical connector (e.g. of a power base on which the liquid heating appliance is arranged to be positioned). The cordless electrical adapter part and corresponding base electrical connector part may be of the type that can be mated regardless of their relative angular orientation, i.e. “360°” connector and adapter parts. The electrical adapter part and the corresponding electrical connector part may comprise any suitable number of poles, e.g. 3 poles or 5 poles. In some embodiments, the thermally responsive element comprises a bimetallic element, e.g. a snap-action bimetallic element. In some embodiments, the heating arrangement comprises a second thermally responsive element arranged to operate at a corresponding predetermined temperature. It will be appreciated that any or all of the features of the (first) thermally responsive element, discussed above or below, may apply equally to the second thermally responsive element, as appropriate. The second thermally responsive element may act as a fail-safe in case the first thermally responsive element fails to operate. The thermally responsive element may be mounted on the mounting body of the control unit. In some embodiments, the thermally responsive element comprises a supported portion and an actuator portion. The supported portion of the thermally responsive element may be mounted on the mounting body of the control unit. The supported portion may press against a surface of the mounting body, at least when the temperature of the thermally responsive element is below the predetermined temperature. The actuator portion may be movable relative to the supported portion. In some embodiments, when the thermally responsive element operates at the predetermined temperature, the actuator portion of the thermally responsive element is arranged to move relative to the supported portion to move the switch to the open position. The actuator portion of the thermally responsive element may comprise the contacting portion of the thermally responsive element. In some embodiments, the supported portion of the thermally responsive element comprises the contacting portion. In some such embodiments, the specific contacting portion itself may not be immediately and directly supported by the mounting body, but the contact portion may extend immediately from a portion of the thermally responsive element which is directly supported by the mounting body. Arranging the supported portion of the thermally responsive element in contact with the substrate may help to ensure that the actuator portion more reliably moves to operate the switch. For example, arranging the supported portion in contact with the substrate may help to set the distance between the actuator portion and a push rod on which the actuator portion is arranged to act in order to open the switch when the thermally responsive element operates at the predetermined temperature. In some embodiments, the contacting portion of the thermally responsive element comprises a protrusion of the thermally responsive element. The protrusion may extend from the supported portion and / or the actuator portion of the thermally responsive element. The contacting portion may comprise a portion of an edge of the thermally responsive element, e.g. a corner. An edge of the thermally responsive element may be concave, at least when the temperature of the thermally responsive element is below the predetermined temperature. The concave edge may comprise two peaks. The contacting portion may comprise one or both peaks of this concave edge. In some embodiments, the thermally responsive element comprises a hoop defining an inner cut-out. In some embodiments, the hoop is substantially circular. However, the hoop may comprise any suitable shape (e.g. rectangular or oval). The hoop may comprise the supported portion and the actuator portion. The supported portion and the actuator portion may be separated by the inner cut-out. In some embodiments, the thermally responsive element comprises a central tongue, extending from the supported portion into the inner cut-out, towards the actuator portion. The thermally responsive element may comprise a proximal edge and a distal edge. In some embodiments, the thermally responsive element comprises a proximal end and a distal end. The proximal end may be defined between the proximal edge of the thermally responsive element and the midpoint of the thermally responsive element (along the longitudinal axis of the thermally responsive element). The distal end may be defined between the distal edge of the thermally responsive element and the midpoint of the thermally responsive element (along the longitudinal axis of the thermally responsive element). The thermally responsive element may comprise a curved portion between the proximal edge and the distal edge at least when the temperature of the thermally responsive element is less than the predetermined temperature. The thermally responsive element may be arranged such that a gap is defined between the curved portion and the substrate, at least when the temperature of the thermally responsive element is less than the predetermined temperature. The gap may extend from an edge (e.g. a side edge, extending between the proximal edge and the distal edge) of the thermally responsive element. It will be appreciated that such a gap allows the thermally responsive element to extend over a portion of the heater track without contacting it, as the path of the heater track can pass into, and leave, the footprint of the thermally responsive element through the gap. In some embodiments, the thermally responsive element comprises two parallel sides. The parallel sides may extend from the proximal edge to the distal edge of the thermally responsive element. The thermally responsive element may be substantially rectangular (in plan view). In some embodiments a gap is formed between one or both of the parallel sides and the substrate (and thereby any heater track thereon) at least when the temperature of the thermally responsive element is less than the predetermined temperature. In some embodiments, the centre of mass of the thermally responsive element is located within the proximal end of the thermally responsive element rather than within the distal end of the thermally responsive element. In other words, the centre of mass of the thermally responsive element may be closer to the proximal edge than the distal edge. In some embodiments, the thermally responsive element comprises a cut-out that is located within the distal end of the thermally responsive element rather than within the proximal end such that the centre of mass of the thermally responsive element is located within the proximal end rather than the distal end of the thermally responsive element. The volume of the proximal end may be greater than the volume of the distal end, such that the centre of mass of the thermally responsive element is located within the proximal end of the thermally responsive element rather than the distal end of the thermally responsive element. The thickness (i.e. the dimension of the thermally responsive element in the direction in which it operates to move the switch into the open position) of the proximal end may be greater than the thickness of the distal end such the centre of mass of the thermally responsive element is closer to the proximal edge of the thermally responsive element than to the distal edge of the thermally responsive element. This may result in the centre of mass being positioned within the proximal end rather than the distal end. In some embodiments, the proximal end of the thermally responsive element comprises the supported portion of the thermally responsive element. In some embodiments, the distal end of thermally responsive element comprises the actuator portion of the thermally responsive element. The proximal end of the thermally responsive element may comprise the contacting portion. It will be appreciated that the contacting portion of the thermally responsive element may reach a higher temperature than the rest of the thermally responsive element, owing to the thermal conduction through the contacting portion from the substrate. As a result of heating the proximal end of the thermally responsive element, i.e. the end that is closer to the centre of mass of the thermally responsive element, (more than the rest of the thermally responsive element), the thermally responsive element may operate with more force compared to if the distal end of the thermally responsive element, i.e. the end that is further from the centre of mass of the thermally responsive element, was heated (more than the rest of the thermally responsive element). This may help to ensure that the thermally responsive element reliably opens the switch at the predetermined temperature. In some embodiments, no part of the thermally responsive element is arranged in direct physical contact with the heater track (even when the temperature of the thermally responsive element is above the predetermined temperature). This means that, in some embodiments, the thermally responsive element may not require any electrical insulation, which can reduce the complexity and / or cost of manufacture. The contacting portion may remain in thermally conductive contact with the substrate after the thermally responsive element operates at the predetermined temperature. However, in some embodiments, the thermally responsive element is arranged such that, when the thermally responsive element operates at the predetermined temperature, at least part of the contacting portion of the thermally responsive element is moved out of thermally conductive contact with the substrate. In some embodiments, at or above the predetermined temperature, another portion of the thermally responsive element may (e.g. additionally) be arranged in thermally conductive contact with the substrate. In some embodiments, the thermally responsive element is configured such that, at or above the predetermined temperature, no part of the thermally responsive element is arranged in thermally conductive contact with the substrate. The thermally responsive element may be arranged to reset at a second predetermined temperature that is lower than the (first) predetermined temperature, which is the temperature at which the thermally responsive element operates to open the switch. It will be appreciated that arranging the thermally responsive element so that the contacting portion (or, in some embodiments, the entire thermally responsive element) is moved out of thermally conductive contact with the substrate when the thermally responsive operates at the predetermined temperature may help to allow the thermally responsive element to cool to the second predetermined temperature. In some embodiments, a portion of the periphery of the thermally responsive element comprises the contacting portion. The periphery of the thermally responsive element may be arranged to move relative to the rest of the thermally responsive element when the thermally responsive element operates at the predetermined temperature. By arranging the thermally responsive element such that the periphery of the thermally responsive element comprises the contacting portion, the contacting portion may be arranged on the part of the thermally responsive element that moves furthest when the thermally responsive element operates. The contacting portion may consequently be substantially spaced away from the substrate after the thermally responsive element operates. This can help to ensure that the thermally responsive element cools to the second predetermined temperature. In some embodiments, the thermally responsive element comprises one or more further contacting portions that are arranged in thermally conductive contact with the substrate, at least when the temperature of the thermally responsive element is below the predetermined temperature. At least one of the contacting portions may be arranged at the distal end of the thermally responsive element. At least one of the contacting portions is arranged at the proximal end of the thermally responsive element. Distributing the contacting portions across the thermally responsive element may help to distribute evenly the provision of heat to the thermally responsive element, which may help to ensure that the thermally responsive element operates reliably at the predetermined temperature. In some embodiments, the electrically resistive heater track comprises a thick film printed element. In some embodiments, the electrically resistive heater track comprises a sprayed element. Such heating elements may operate at high temperatures, e.g. more than 200 °C, which may allow the heating arrangement to heat the contents of the liquid heating appliance at a faster rate (e.g. compared to sheathed electrical heaters). The heating arrangement may be arranged to heat liquid, e.g. water, to boiling. In some embodiments, the power rating of the electrically resistive track is between 700 W and 3000 W, e.g. between 1000 W and 2500 W, e.g. approximately 2000 W. This may be particularly suitable for embodiments in which the vessel of the liquid heating appliance is arranged to hold 1 to 2 litres of water, e.g. a domestic kettle. In some embodiments, the power rating of the electrically resistive track is between 300 W and 700 W, e.g. between 400 W and 600 W, e.g. approximately 500 W. This may be particularly suitable for embodiments in which the vessel of the liquid heating appliance is arranged to hold between 100 ml and 600 ml, e.g. portable (e.g. handheld) beverage receptacles. In some embodiments the thermally responsive element is arranged such that, at least when the temperature of the thermally responsive element is less than the predetermined temperature, the thermally responsive element is elastically deformed by contact with the substrate such that the contacting portion exerts a force on the substrate. This may help to improve the contact between the contacting portion and the substrate, thereby helping to improve the conductive thermal communication between the thermally responsive element and the substrate. This elastic deformation may be achieved, for example, by suitably positioning of the mounting arrangement relative to the substrate, e.g. such that the thermally responsive element is deformed when it is pressed against the substrate. In some embodiments, the electrically resistive heater track is deposited on the substrate such that it follows a serpentine path across the substrate. This may help to increase the overall heating power of the substrate and heater track, which may reduce the time taken for the heating arrangement to heat the contents of the liquid heating appliance. In some embodiments, the overall power density of the substrate is between 7 W / cm2 and 18 W / cm2, e.g. between 9 W / cm2and 15 W / cm2, e.g. between 12 W / cm2 and 14 W / cm2, e.g. approximately 13 W / cm2. Power density should be understood to mean power per unit area. The power density of the substrate may substantially correspond to the power density of the heater track. It will be appreciated that the overall power density of the substrate is the overall heating power of the heater track per unit area of the substrate (when viewed in plan view), including the area of the substrate on which heater track is deposited. In some embodiments, the thermally responsive element is arranged such that the periphery of the thermally responsive element is over the portion of the electrically resistive track over which the thermally responsive element extends. Said portion of the electrically resistive track may follow the periphery of the thermally responsive element. Said portion of the electrically resistive track may be deposited to define a portion of the substrate on which no electrically resistive heater track is deposited. The control unit may comprise a mounting arrangement on which the thermally responsive element is mounted. The portion of the substrate on which no electrically resistive heater track is deposited may surround, but not physically contact, the mounting arrangement. This may help to avoid the control unit being heated by the electrically resistive heater track, and help to avoid a requirement to electrically insulate the mounting arrangement. In some embodiments, the portion of the electrically resistive track over which the thermally responsive element extends may occupy between 30% and 100% of the footprint of the thermally responsive element (i.e. the total area of the substrate over which the thermally responsive element extends), e.g. between 40% and 80%, e.g. approximately 60%. In some embodiments, the electrically resistive track comprises a first portion having a first power density and a second portion having a second power density. In some embodiments, the second power density is lower than the first power density. The lower power density may be achieved in any suitable manner. For example, the second portion of the electrically resistive track may be wider than the first portion. In addition, or alternatively, the second portion of the electrically resistive track may be deeper than the first portion. The width of the first portion of the electrically resistive track may be between 1 mm and 2 mm, e.g. between 1.2 mm and 1.8 mm, e.g. approximately 1.5 mm. The width of the second portion of the electrically resistive track may be between 1.5 mm and 2.5 mm, e.g. between 1.7 and 2.2 mm, e.g. approximately 2 mm. The second portion of the electrically resistive track may comprise a more conductive material than the first portion of the electrically resistive track, e.g. silver. This may also help to reduce the second power density in comparison to the first power density. In some embodiments, the second portion comprises the portion of the heater track over which the thermally responsive element at least partially extends. It will be appreciated that the second portion of the electrically resistive heater track may have a lower operating temperature than the first portion owing to its lower power density. Arranging the thermally responsive element over the second portion of the electrically resistive heater track means that the thermally responsive element may be exposed to a lower temperature than if it were arranged over the first portion. This means that the predetermined temperature at which the thermally responsive element operates may be lower than if it were arranged over the first portion of the electrically resistive track, which can reduce the complexity and cost of manufacture of the thermally responsive element. In some embodiments, the local power density of the first portion is between 30 W / cm2and 70 W / cm2, e.g. between 40 W / cm2and 55 W / cm2, e.g. approximately 45 W / cm2. In some embodiments, the local power density of the second portion is between 15 W / cm2 and 25 W / cm2, e.g. between 17 W / cm2 and 23 W / cm2, e.g. approximately 20 W / cm2. It will be appreciated that the power density of the electrically resistive track is the heating power of the electrically resistive track per unit area of the electrically resistive track (where the area of the electrically resistive track is the area of the electrically resistive track that is in contact with the substrate). As discussed above, in some embodiments, the centre of mass of the thermally responsive element is located within the proximal end of the thermally responsive element rather than within the distal end of the thermally responsive element. In some embodiments, the heater track is arranged on the substrate such that the total heating power applied to the proximal end of the thermally responsive element is greater than or equal to the total heating power applied to the distal end of the thermally responsive element. This takes into account heat transferred to the thermally responsive element from both conduction and radiation. Arranging the heater track such that the total heating power applied to the proximal end (i.e. the end within which the centre of mass of the thermally responsive element is located) is greater than or equal to the total heating power applied to the distal end may help to avoid creep in the thermally responsive element, which may help to avoid the thermally responsive element opening the switch at temperatures below the predetermined temperature. In some embodiments, the electrically resistive heater track is arranged on the substrate such that the total heating power output from a portion of the heater track below the proximal end of the thermally responsive element is greater than or equal to the total power output from a portion of the heater track below the distal end of the thermally responsive element. The total heating power applied to the distal end may be zero. In some embodiments, the distal end of the thermally responsive element does not extend over any portion of the electrically resistive heater track. This is considered to be novel and inventive in its own right. Thus, when viewed from a third aspect, the present invention provides a heating arrangement for a liquid heating appliance, the heating arrangement comprising: a substrate; an electrically resistive heater track deposited on the substrate; an electrical power supply circuit for supplying electrical power to the heater track, the circuit comprising a switch that is moveable between a closed position and an open position; and a thermally responsive element arranged to operate at a predetermined temperature to move the switch to the open position so as to interrupt the supply of electrical power to the heater track; wherein: the thermally responsive element comprises a proximal end and a distal end, wherein the centre of mass of the thermally responsive element is located within the proximal end of the thermally responsive element rather than the distal end of the thermally responsive element; the thermally responsive element at least partially extends over a portion of the heater track; and the heater track is arranged on the substrate such that the total heating power applied to the proximal end of the thermally responsive element is greater than or equal to the total heating power applied to the distal end of the thermally responsive element. When viewed from a fourth aspect, the present invention provides a liquid heating appliance comprising: a liquid heating vessel for receiving a liquid to be heated; and a heating arrangement arranged to heat the contents of the liquid heating vessel, the heating arrangement comprising: a substrate; an electrically resistive heater track deposited on the substrate; an electrical power supply circuit for supplying electrical power to the heater track, the circuit comprising a switch that is moveable between a closed position and an open position; and a thermally responsive element arranged to operate at a predetermined temperature to move the switch to the open position so as to interrupt the supply of electrical power to the heater track; wherein: the thermally responsive element comprises a proximal end and a distal end, wherein the centre of mass of the thermally responsive element is located within the proximal end of the thermally responsive element rather than the distal end of the thermally responsive element; the thermally responsive element at least partially extends over a portion of the heater track; and the heater track is arranged on the substrate such that the total heating power applied to the proximal end of the thermally responsive element is greater than or equal to the total heating power applied to the distal end of the thermally responsive element. As discussed above, arranging the electrically resistive heater track such that the total heating power applied to the proximal end of the thermally responsive element (i.e. the end within which the centre of mass of the thermally responsive element is located) is greater than or equal to the total heating power applied to the distal end of the thermally responsive element may help to improve the reliability of the thermally responsive element operating to open the switch at the predetermined temperature. Features of the first and second aspects of the invention, including any embodiments thereof, may be applied to the third and fourth aspects discussed above. The following features may be applied to any appropriate embodiments discussed herein, including embodiments of the first, second, third and fourth aspects of the invention. In some embodiments, the total heating power applied to the proximal end of the thermally responsive element is greater than the total heating power applied to the distal end of the thermally responsive element. Arranging the heater track such that the total heating power applied to the proximal end is greater than (not equal to) the total heating power applied to the distal end may be particularly beneficial for avoiding the effects of creep in the thermally responsive element. In some embodiments, the heater track is arranged on the substrate such that the total heating power applied to the proximal end of the thermally responsive element is equal to the total heating power applied to the distal end of the thermally responsive element. This may help to provide an even distribution of heat in the thermally responsive element, which may improve the reliability of the thermally responsive element operating at the predetermined temperature. The variation in the total heating power applied to the different ends of the thermally responsive element may be achieved in any suitable way. In some embodiments, the average power density of the heater track below the proximal end is greater than the average power density of the heater track below the distal end. In some embodiments, the cross-sectional area, in a plane perpendicular to the length of the heater track, of the electrically resistive track below the distal end of the thermally responsive element is greater than the cross-sectional area of the electrically resistive track below the proximal end. For example, the electrically resistive track below the distal end may be wider than the electrically resistive track below the proximal end (assuming the track has a constant depth). Locally reducing the cross-sectional area of the electrically resistive heater track may increase the local resistance of heater track, which may increase the local heating power output. In some embodiments (e.g. when the electrically resistive track has a substantially uniform cross-sectional area), the proportion of the area of substrate on which the heater track is deposited (compared to the total area of substrate overlapped by the thermally responsive element) is greater below the proximal end than below the distal end. In other words, more of the substrate area may be exposed below the distal end than below the proximal end. It will be appreciated that this may result in a greater heating power being applied to the proximal end than to the distal end. As discussed above with reference to the first and second aspects, the thermally responsive element may be mounted on a mounting body of a control unit. The thermally responsive element may be mounted on the mounting body by the proximal end. The thermally responsive element may be mounted on the mounting body by the distal end. The thermally responsive element may be mounted on the mounting body by a portion of the thermally responsive element between the proximal edge and the distal edge that may overlap both the proximal and distal ends (e.g. a central tongue). In some embodiments, the proximal end of the thermally responsive element comprises the supported portion of the thermally responsive element, as described above. In some embodiments, the distal end of the thermally responsive element comprises the actuator portion of the thermally responsive element, as described above. The proximal end may comprise the actuator portion. The distal end may comprise the supported portion. In some embodiments, the proximal end of the thermally responsive element comprises a contacting portion of the thermally responsive element, as discussed above. The distal end of the thermally responsive element may comprise a contacting portion. In some embodiments, the path of the electrically resistive heater track extends into the footprint of the thermally responsive element from outside the footprint of the thermally responsive element. It will be appreciated that the footprint of the thermally responsive element is defined by a projection of the perimeter of the thermally responsive element in plan view. In some embodiments, the path of the heater track extends into the footprint of the thermally responsive element beneath the distal end (e.g. at or adjacent the distal edge) of the thermally responsive element. The path of the electrically resistive heater track may (e.g. additionally) extend out of the footprint of the thermally responsive element. In some embodiments, the path of the electrically resistive heater track extends out of the footprint of the heater track beneath the distal end (e.g. at or adjacent the distal edge) of the thermally responsive element. Arranging the path of the heater track such that it enters and leaves the footprint of the thermally responsive element beneath the distal end (e.g. at or adjacent the distal edge) of the thermally responsive element may help to reduce the power density at the distal end of the thermally responsive element, which may help to reduce the total heating power applied to the distal end of the thermally responsive element. In any of the embodiments described above, the substrate may comprise stainless steel, e.g. 400 Series Stainless Steel, e.g. martensitic stainless steel. In some embodiments, the substrate comprises a first layer and a second layer. The first layer may be formed from stainless steel. The second layer may comprise an electrically insulative material deposited on the substrate. The heater tracks may be deposited on the second layer. The electrical insulation may be formed from any suitable material, such as polyester, polyimide, acrylate, polyurethane, acetate or cellulose. The material of the substrate may have a similar coefficient of thermal expansion to the material of the insulation. This may help to avoid damage to the heating arrangement during use owing to components expanding at different rates with respect to temperature. Forming the substrate from martensitic stainless steel may be particularly beneficial for this reason as its coefficient of thermal expansion may be similar to the materials used for the insulation. When viewed from a fifth aspect, the present invention provides a heating arrangement for a liquid heating appliance, the heating arrangement comprising: a substrate; an electrically resistive heater track deposited on the substrate such that at least a portion of the heater track follows a serpentine path; an electrical power supply circuit for supplying electrical power to the heater track, the circuit comprising a switch that is moveable between a closed position and an open position; and a bimetallic element arranged to operate at a predetermined temperature to move the switch into the open position so as to interrupt the supply of electrical power to the heater track; wherein: the bimetallic element at least partially extends over a portion of the heater track; and at least when the temperature of the bimetallic element is below the predetermined temperature: no part of the bimetallic element is arranged in direct physical contact with the heater track; and a contacting portion of the bimetallic element is arranged in thermally conductive contact with the substrate. When viewed from a sixth aspect, the present invention provides a liquid heating appliance comprising: a liquid heating vessel for receiving a liquid to be heated; and a heating arrangement arranged to heat the contents of the liquid heating vessel, the heating arrangement comprising: a substrate; an electrically resistive heater track deposited on the substrate such that at least a portion of the heater track follows a serpentine path; an electrical power supply circuit for supplying electrical power to the heater track, the circuit comprising a switch that is moveable between a closed position and an open position; a bimetallic element arranged to operate at a predetermined temperature to move the switch to the open position so as to interrupt the supply of electrical power to the heater track; wherein: the bimetallic element at least partially extends over a portion of the heater track; and at least when the temperature of the bimetallic element is below the predetermined temperature: no part of the bimetallic element is arranged in direct physical contact with the heater track; and a contacting portion of the bimetallic element is arranged in thermally conductive contact with the substrate. When viewed from a seventh aspect, the present invention provides a heating arrangement for a liquid heating appliance, the heating arrangement comprising: a substrate; an electrically resistive heater track deposited on the substrate such that at least a portion of the heater track follows a serpentine path; an electrical power supply circuit for supplying electrical power to the heater track, the circuit comprising a switch that is moveable between a closed position and an open position; and a bimetallic element arranged to operate at a predetermined temperature to move the switch to the open position so as to interrupt the supply of electrical power to the heater track; wherein: the bimetallic element comprises a proximal end and a distal end, wherein the centre of mass of the bimetallic element is located within the proximal end of the bimetallic element rather than the distal end of the bimetallic element; the bimetallic element at least partially extends over a portion of the heater track; and the heater track is arranged on the substrate such that the total heating power applied to the proximal end of the bimetallic element is greater than or equal to the total heating power applied to the distal end of the bimetallic element. When viewed from an eighth aspect, the present invention provides a liquid heating appliance comprising: a liquid heating vessel for receiving a liquid to be heated; and a heating arrangement arranged to heat the contents of the liquid heating vessel, the heating arrangement comprising: a substrate; an electrically resistive heater track deposited on the substrate such that at least a portion of the heater track follows a serpentine path; an electrical power supply circuit for supplying electrical power to the heater track, the circuit comprising a switch that is moveable between a closed position and an open position; and a bimetallic element arranged to operate at a predetermined temperature to move the switch to the open position so as to interrupt the supply of electrical power to the heater track; wherein: the bimetallic element comprises a proximal end and a distal end, wherein the centre of mass of the bimetallic element is located within the proximal end of the bimetallic element rather than the distal end of the bimetallic element; the bimetallic element at least partially extends over a portion of the heater track; and the heater track is arranged on the substrate such that the total heating power applied to the proximal end of the bimetallic element is greater than or equal to the total heating power applied to the distal end of the bimetallic element. Features of any of the embodiments of the first, second, third, fourth, fifth, sixth, seventh or eighth aspects of the invention may include any one or more of the optional features outlined herein in respect of any of the first, second, third, fourth, fifth, sixth, seventh or eighth aspects. It will be appreciated that optional features of the thermally responsive element described in relation to the first, second, third, or fourth aspects of the invention may apply to the bimetallic element of the fifth, sixth, seventh, and eighth aspects of the invention. Some embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: Fig. 1 shows a perspective view of a liquid heating appliance in accordance with an embodiment of the present invention; Fig. 2a shows a cross-sectional view of the power base and control unit of the liquid heating appliance of Fig. 1 when the bimetallic element is below the predetermined temperature; Fig. 2b shows a cross-sectional view of the power base and control unit of the liquid heating appliance of Fig. 1 when the thermally responsive element is above the predetermined temperature; Fig. 3a shows a perspective view of the thermally responsive element in isolation when below the predetermined temperature; Fig. 3b shows a perspective view of the thermally responsive element in isolation when above the predetermined temperature; Fig. 4a shows a perspective view of the resistive track on the lower surface of the substrate of the liquid heating appliance of Fig. 1 below the first thermally responsive element; Fig. 4b shows a perspective view of the resistive track on the lower surface of the substrate of the liquid heating appliance of Fig. 1 below the second thermally responsive element; and Fig. 5 shows an underside view of the substrate of the liquid heating appliance of Fig. 1. Figure 1 shows a perspective view of a liquid heating appliance 2, hereinafter the appliance 2, in accordance with an embodiment of the present invention. The appliance 2 comprises a liquid heating vessel 4, a spout 6, and a handle 8. The top of the liquid heating vessel 4 is closed with a lid 10. In the embodiment depicted, the liquid heating vessel 4 is arranged to rest on a power base 12 that comprises a central 360° base electrical connector part 14 for suppling the appliance 2 with electricity. The power base 12 comprises a plug 13 for receiving a supply of mains electricity. However, it will be appreciated that in other embodiments, the appliance 2 may be supplied with power in a corded manner. Figure 2a shows a cross-sectional view through the lower part of the liquid heating vessel 4 and through power base 12. For ease of illustration, the base electrical connector part 14 and the cordless electrical adapter part 28 are shown in their separated state in Figures 2a and 2b. As shown in Figure 2a, the appliance 2 comprises a heating arrangement 15, in accordance with an embodiment of the present invention, for heating a volume of liquid contained within the liquid heating vessel 4. The heating arrangement 15 comprises an electrically resistive heater track 18a deposited on the underside of a substrate 18c. The electrically resistive heater track 18a may be a printed or sprayed heater track. The substrate 18c is mounted below the liquid heating vessel 4 and may form the base of the liquid heating vessel 4. The heater track 18a is arranged in conductive thermal communication with the liquid heating vessel 4 via the substrate 18c. The heater track 18a generates heat when electrical energy is provided to it, thereby heating the substrate 18c which heats the contents of the liquid heating vessel 4. The heating arrangement 15 further comprises a control unit 16 that is mounted to the underside of the substrate 18c. The control unit 16 controls provision of electrical power from the power base 12, when the liquid heating vessel 4 is seated thereon, to the heater track 18a. The control unit 16 comprises a cordless electrical adapter part 28 that is arranged to mate with the base electrical connector part 14 of the power base 12 when the liquid heating appliance 2 is received on the power base 12. In the embodiment depicted, the cordless electrical adapter part 28 is a 3-pole adapter comprising a live pin conductor 28a, a neutral ring conductor 28b, and an earth ring conductor 28c. The base electrical connector part 14 is a 3-pole connector comprising a central aperture in which a live electrical contact 14a is arranged, a first coaxial aperture in which a neutral electrical contact 14b is arranged, and a second coaxial aperture in which an earth electrical contact 14c is arranged. The live pin conductor 28a is arranged to be received within the central aperture, the neutral ring conductor 28b within the first coaxial aperture, and the earth ring conductor 28c within the second coaxial aperture, such that the respective live, neutral, and earth conductors and contacts are brought together to form a power supply circuit when the electrical adapter part 28 mates with the base electrical connector part 14. The live pin conductor 28a and the neutral ring conductor 28b are electrically connected to respective terminals of the heater track 18a. This electrical connection is shown schematically in Figures 2a and 2b. Whilst the embodiment shown in Figure 2a comprises a 3-pole cordless electrical adapter part 28 and base electrical connector part 14, it will be appreciated that the adapter and connector parts 28, 14 may comprise any suitable number of poles, e.g. 5 poles. The control unit 16 further comprises a mounting body 16a and a first thermally responsive element 30a mounted to the upper surface of the mounting body 16a. In this embodiment, the thermally responsive element 30a is in the form of a snapaction bimetallic element 30a, although it will be appreciated that any suitable type of thermally responsive element may be used. The bimetallic element 30a is set to operate, i.e. snap / change its physical form, at a predetermined temperature. Figure 2a shows the configuration of the bimetallic element 30a when its temperature is below its predetermined temperature. As depicted in Figure 3a, the bimetallic element 30a is arranged such that no part of the bimetallic element 30a is in direct physical contact with the heater track 18a. As depicted, the bimetallic element 30a does, however, contact the substrate 18c, as will be discussed further below. The control unit 16 also comprises a second thermally responsive element (also a snapaction bimetallic element in this embodiment) 30b (shown in Figure 4b) that is also mounted to the upper surface of the mounting body 16a and is set to operate a predetermined temperature which may be the same as the predetermined temperature of the first bimetallic element 30a or may be different. Unless stated otherwise, features of the first bimetallic element 30a described herein also apply to the second bimetallic element 30b. When the temperature of the bimetallic element 30a is below the predetermined temperature, the bimetallic element 30a may adopt a concave shape (i.e. the surface of the bimetallic element 30a that faces the heater track 18a is concave), as shown in Figure 2a. When the temperature of the bimetallic element 30a reaches the predetermined temperature, the bimetallic element 30a may snap through to adopt a convex shape (i.e. the surface of the bimetallic element 30a that faces the heater track 18a is convex), as shown in Figure 2b. Figure 2b shows a cross-sectional view of the power base 12 and control unit 16 when the bimetallic element 30a is above the predetermined temperature. As can be seen, the bimetallic element 30a has snapped through from its initial concave configuration to a convex configuration in which the bimetallic element 30b is no longer in contact with the substrate 18c. The movement of the bimetallic element 30a causes the bimetallic element 30a to exert a force on a push rod 40, which moves downwards to open a switch 50 within the control unit 16, thereby interrupting the supply of electrical power to the heater track 18a. As is known in the art, this allows the heater track 18a to be switched off in the event of a “dry-boil” scenario in which no liquid is present within the liquid heating vessel 4. The second bimetallic element 30b is arranged to open the switch 50 (or, optionally, a separate switch) via a separate push rod (not shown) when the second bimetallic element 30b operates at its predetermined temperature. The bimetallic element 30a is shown in more detail in isometric view and in isolation in Figures 3a and 3b. The bimetallic element is shown in its concave form (i.e. its form when its temperature is less than the predetermined temperature) in Figure 3a, and in its convex form (i.e. its form when its temperature is greater than the predetermined temperature) in Figure 3b. In the embodiment depicted, the bimetallic element 30a is substantially rectangular, comprising a pair of parallel sides 35 that connect a distal edge 27 and a proximal edge 29of the bimetallic element 30a. The parallel sides 35 are parallel to a central, longitudinal axis 41 of the bimetallic element 30a, and the plan view of the bimetallic element 30a is substantially mirrored about the longitudinal axis 41 (as shown in Figures 4a and 4b). A distal end 37 of the bimetallic element 30a is defined between the distal edge 27 of the bimetallic element 30a and the midpoint 25 of the bimetallic element 30a (in the direction of the longitudinal axis 41). A proximal end 39 of the bimetallic element 30a is defined between the proximal edge 29 of the bimetallic element 30a and the midpoint 25 of the bimetallic element 30a. Referring back to Figure 2a, it can be seen that a gap 53 is defined between the bimetallic element 30a and the substrate 18c when the bimetallic element 30a is below the predetermined temperature. The gap 53 is defined between the distal and proximal edges 27, 29 of the bimetallic element 30a and allows the heater track 18a to extend beneath the bimetallic element 30a so that the bimetallic element 30a covers a portion of the heater track 18a. The gap 53 serves to allow the bimetallic element 30a (i.e. the thermally responsive element) to extend over the heater tracks 18a without touching them. As discussed previously, this may advantageously mean that the bimetallic element 30a does not require electrical insulation thereon as it does not contact the electrically resistive heater track 18a. Referring back to Figure 3a, the bimetallic element 30a comprises a hoop 51 that defines an inner cut-out 47. The inner cut-out 47 is located closer to the distal edge 27 of the bimetallic element 30a than to the proximal edge 29 of the bimetallic element 30a. As a result, the centre of mass 59 of the bimetallic element 30a is located within the proximal end 39 of the bimetallic element 30a rather than within the distal end 37. In other words, the centre of mass 59 is closer to the proximal edge 29 than the distal edge 27. The bimetallic element 30a comprises a central tongue 45 that extends parallel to the central longitudinal axis 41 into the inner cut-out 47 defined between the distal edge 27 and the proximal edge 29 of the bimetallic element 30a. The bimetallic element 30a may be mounted to the control unit 16 via the central tongue 45. The proximal end 39 of the bimetallic element 30a is also supported by the control unit 16, e.g. it may be pressed against a surface of the control unit 16 when it is mounted thereto. Thus, the proximal end 39 and the central tongue 45 together comprise the supported portion of the bimetallic element 30a. When the bimetallic element 30a operates at the predetermined temperature, the distal and proximal edges 27, 29 respectively of the bimetallic element 30a move relative to the central tongue 45. The distal end 37 is arranged to contact the push rod 40 that opens the switch 50, and thus the distal end 37 comprises the actuator portion of the bimetallic element 30a. The proximal end 39 comprises a pair of protrusions 31a, 31b, with a respective one of the pair 31a, 31b positioned on each side of the central longitudinal axis of the bimetallic element 30a. The protrusions 31a, 31b extend from the proximal edge 29of the bimetallic element 30a in a direction parallel to the central longitudinal axis 41 of the bimetallic element 30a. The distal end 37 comprises two further protrusions 31c, 31 d that extend from the distal edge 27 coaxially with the proximal end protrusions 31a, 31b respectively. When the temperature of the bimetallic element 30a is less than the predetermined temperature (as shown in Figures 2a and 3a), the distal edge 27 of the bimetallic element 30a is concave, with the lowest point of the distal edge 27 being at the midpoint of the distal edge 27 and the highest points of the distal edge 27 being at the edges of the distal edge 27 that intersect the two parallel sides 35 of the bimetallic element 30a. The highest points of the distal edge 27 thus define two distal peaks 37a, 37b of the bimetallic element 30a. In this configuration of the bimetallic element 30a, the parallel sides 35 are also concave, with the highest points of the parallel sides 35 (i.e. the points furthest from the control unit 16 when the bimetallic element 30a is mounted on the control unit 16) being at the distal and proximal edges 27, 29 of the parallel sides 35. In this configuration (i.e. when the temperature of the bimetallic element 30a is less than the predetermined temperature), the distal peaks 37a, 37b and the protrusions 31a, 31b at the proximal edge 29 are the highest points of the bimetallic element 30a, i.e. they are furthest from the control unit 16 when the bimetallic element 30a is mounted on the control unit 16. The distal peaks 37a, 37b and the proximal end protrusions 31a, 31b are shaded in Figure 3a for illustration purposes. When the bimetallic element 30a is mounted beneath the substrate 18c, the only parts of the bimetallic element 30a that are in contact with the substrate 18c are the distal peaks 37a, 37b and the protrusions 31a, 31b. The distal peaks 37a, 37b and the protrusions 31a, 31b may be referred to herein as the contacting portions of the bimetallic element 30a. When the temperature of the bimetallic element 30a is less than the predetermined temperature, the protrusions 31a, 31b and distal peaks 37a, 37b (i.e. contacting portions at such temperatures) are in thermally conductive communication with the substrate 18c, meaning that the bimetallic element 30a is arranged to be sensitive to the temperature of the substrate 18c. However, no part of the bimetallic element 30a is in direct physical contact with the heater track 18a. When the temperature of the bimetallic element 30a is greater than the predetermined temperature (as shown in Figures 2b and 3b), the distal and proximal edges 27, 29 respectively of the bimetallic element 30a move relative to the central tongue 45 such that the parallel sides 35 of the bimetallic element 30a become convex, with the lowest points of the parallel sides 35 (i.e. the points closest to the control unit 16 when the bimetallic element 30a is mounted on the control unit 16) being at the distal and proximal edges 27, 29 respectively of the parallel sides 35. As a result, the protrusions 31a, 31b and distal peaks 37a, 37b of the bimetallic element 30a (i.e. the contacting portions) are no longer in contact with the substrate 18c. In this configuration, no part of the bimetallic element 30a is in contact with the substrate 18c. This allows the bimetallic element 30a to cool down to the temperature at which it resets into its concave configuration. Figures 4a and 4b show perspective views of the heater track 18a on the lower surface of the substrate 18c, together with the respective first and second bimetallic elements 30a, 30b. The first bimetallic element 30a, which is arranged on the lefthand side of the substrate 18c, is shown in Figure 4a and the second bimetallic element 30b, which is shown on the right-hand side of the substrate 18c, is shown in Figure 4b. The bimetallic elements 30a, 30b are shown transparently so that the arrangement of the heater track 18a above each bimetallic element 30a, 30b can be seen. Figure 5 shows an underside view of the substrate 18c with the heater track 18a deposited thereon. The heater track 18a winds across the lower surface of the substrate 18c in a serpentine path from a first electrical termination 19a to a second electrical termination 19b of the heater track 18a. The width of the heater track 18a is substantially uniform, for example at 1.5 mm along most of its length, except for wider regions 21a, 21b of the heater track 18a that are below the first and second bimetallic elements 30a, 30b respectively. The first wider region 21a and the second wider region 21b may, for example, have a width of 2 mm. The first and second wider regions 21a, 21b need not have the same width, but could have different respective widths, for example the first wider region 21a may have a width of 1.7 mm and the second wider region 21b may have a width of 2 mm. The local widening of the heater track 18a above the bimetallic elements 30a, 30b reduces the resistance of the heater track 18a in these wider regions 21a, 21b. This therefore reduces the local power density and the normal operating temperature. Whilst the non-widened regions of the heater track 18a may reach a maximum temperature of approximately 200 °C during a normal heating operation (i.e. with liquid present in the liquid heating vessel), the widened regions 21a, 21b may reach an average temperature of approximately 125 °C during the same operation. This means that bimetallic elements 30a, 30b that are set to operate at a relatively low temperature (e.g. between 125 °C and 155 °C) can advantageously be used. With reference to Figures 4a and 4b, in some embodiments, the path of the heater track 18a both extends into and leaves the footprint of the bimetallic element 30a adjacent the distal edge 27 of the bimetallic element 30a. The path of the heater track 18a also extends into and leaves the footprint of the second bimetallic element 30b adjacent the distal edge 55a of the second bimetallic element 30b. The total heating power applied to the distal end 37 of the first bimetallic element 30a is lower than the total heating power applied to the proximal end 39 of the first bimetallic element 30a. Similarly, the total heating power applied to the distal end 55 of the second bimetallic element 30b is lower than the total heating power applied to the proximal end 57 of the second bimetallic element 30b. When the bimetallic elements 30a, 30b are mounted on the control unit 16, as they are in Figures 4a and 4b, the distal peaks 37a, 37b and the protrusions 31a, 31b of the first bimetallic element 30a and the distal peaks 43a, 43b and the protrusions 33a, 33b of the second bimetallic element 30b are arranged above portions of the substrate 18c on which no heater track 18a is present. When the first bimetallic element 30a and the second bimetallic element 30b are below their respective predetermined temperatures, the distal peaks 37a, 37b, 43a, 43b and the protrusions 31a, 31b, 33a, 33b of the first and second bimetallic elements 30a, 30b respectively are in direct, thermally conductive contact with the substrate 18c. No part of the first and second bimetallic elements 30a, 30b is in direct, physical contact with the heater track 18a, and no part of the first and second bimetallic elements 30a, 30b other than the distal peaks 37a, 37b, 43a, 43b and the protrusions 31a, 31b, 33a, 33b is in direct physical contact with the substrate 18c. Arranging the bimetallic element 30a such that it extends over, but does not contact the heater track 18a may advantageously mean that no electrical insulation is required between the bimetallic element 30a and the heater track 18a. Arranging the bimetallic element 30a so that it is in thermally conductive contact with the substrate 18c, but is not in direct physical contact with the heater track 18a, can beneficially reduce the rate of heat transfer from the heater track 18a to the bimetal. This means that, when the temperature of the heater track 18a is high enough to necessitate an overheat switch-off, e.g. when the liquid heating vessel 4 has boiled dry, the bimetallic element 30a is at a lower temperature than the heater track 18a. As a result, a bimetallic element 30a having a lower operating temperature can be used without increasing the risk of the bimetallic element 30a operating at too a low temperature, e.g. during normal operation of the liquid heating appliance 2. This can save expense and time during the design and manufacturing processes of the bimetallic element 30a. While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.

Claims

1. A heating arrangement for a liquid heating appliance, the heating arrangement comprising:5 a substrate;an electrically resistive heater track deposited on the substrate;an electrical power supply circuit for supplying electrical power to the heater track, the circuit comprising a switch that is moveable between a closed position and an open position; and10 a thermally responsive element arranged to operate at a predeterminedtemperature to move the switch into the open position so as to interrupt the supply of electrical power to the heater track;wherein:the thermally responsive element at least partially extends over a portion of 15 the heater track; andat least when the temperature of the thermally responsive element is below the predetermined temperature:no part of the thermally responsive element is arranged in direct physical contact with the heater track; and20 a contacting portion of the thermally responsive element is arrangedin thermally conductive contact with the substrate.

2. The heating arrangement of claim 1, wherein the electrically resistive track comprises a first portion having a first power density and a second portion having a 25 second power density, wherein the second power density is lower than the first power density, and wherein the second portion comprises the portion of the electrically resistive heater track over which the thermally responsive element at least partially extends.30 3. The heating arrangement of claim 1 or 2, wherein the heating arrangementfurther comprises a control unit, the control unit comprising:the switch of the electrical power supply circuit; anda mounting body for mounting the control unit to the substrate;and wherein the thermally responsive element comprises:16 01 26a supported portion that is mounted on the mounting body of the controlunit, wherein the supported portion comprises the contacting portion; andan actuator portion that is movable relative to the supported portion, when the thermally responsive element operates at the predetermined temperature, to5 move the switch to the open position.

4. The heating arrangement of claim 3, wherein:the control unit comprises a mounting arrangement on which the thermally responsive element is mounted;10 the portion of the electrically resistive track over which the thermallyresponsive element extends is deposited on the substrate so as to follow the periphery of the thermally responsive element to define a portion of the substrate on which no electrically resistive heater track is deposited; andthe portion of the substrate on which no electrically resistive heater15 track is deposited surrounds, but does not physically contact, the mountingarrangement.

5. The heating arrangement of any preceding claim, wherein the thermally responsive element comprises a proximal edge and a distal edge, and wherein, at20 least when the temperature of the thermally responsive element is less than the predetermined temperature:the thermally responsive element comprises a curved portion between the proximal edge and the distal edge; andthe thermally responsive element is arranged such that a gap is defined25 between the curved portion and the substrate, wherein the gap extends from an edge of the thermally responsive element.

6. The heating arrangement of any preceding claim, wherein the thermally responsive element comprises:30 a proximal edge and a distal edge;a proximal end, defined between the proximal edge of the thermally responsive element and the midpoint of the thermally responsive element; anda distal end, defined between the distal edge of the thermally responsive element and the midpoint of the thermally responsive element;16 01 26wherein the centre of mass of the thermally responsive element is located within the proximal end of the thermally responsive element rather than within the distal end of the thermally responsive element.

7. The heating arrangement of claim 6, wherein the proximal end of the thermally responsive element comprises the contacting portion.

8. The heating arrangement of claim 6 or 7, wherein the electrically resistive heater track is arranged on the substrate such that the total heating power applied to the proximal end of the thermally responsive element is greater than or equal to the total heating power applied to the distal end of the thermally responsive element.

9. The heating arrangement of any preceding claim, wherein no part of the thermally responsive element is arranged in direct physical contact with the heater track.

10. The heating arrangement of any preceding claim, wherein the thermally responsive element is configured such that, at or above the predetermined temperature, no part of the thermally responsive element is arranged in thermally conductive contact with the substrate.

11. The heating arrangement of any preceding claim, wherein a portion of the periphery of the thermally responsive element comprises the contacting portion.

12. The heating arrangement of any preceding claim, wherein the thermally responsive element comprises one or more further contacting portions that are arranged in thermally conductive contact with the substrate, at least when the temperature of the thermally responsive element is below the predetermined temperature, wherein at least one of the contacting portions is arranged at the distal end of the thermally responsive element and at least one of the contacting portions is arranged at the proximal end of the thermally responsive element.

13. The heating arrangement of any preceding claim, wherein the electrically resistive heater track comprises a thick film printed element or a sprayed element.

14. The heating arrangement of any preceding claim, wherein the thermally responsive element is arranged such that, at least when the temperature of the thermally responsive element is less than the predetermined temperature, the5 thermally responsive element is elastically deformed by contact with the substrate such that the contacting portion exerts a force on the substrate.

15. A liquid heating appliance comprising:a liquid heating vessel for receiving a liquid to be heated; and10 the heating arrangement of any preceding claim.

Images