GB2246253A - Heating level selecting switch arrangement - Google Patents

Abstract

The heating level of a heater assembly having at least two heating elements, L, R1, R2 is adjusted by a multiposition switch, having contacts C1 to C7, in association with a duty cycle controller B which is operable in a plurality of positions of the switch and gives different duty cycles in different positions of the switch. The switch provides various series/parallel connections between two coil-type heating elements R1, R2 and one or two elements L having a substantial positive temperative coefficient of resistance, such as molybdenum disilicide elements or, particularly, infra red lamps. The duty cycle controller B is a bimetal device and the different duty cycles are provided by applying different voltages to the bimetal heater r1, either by connecting the heater r1 across different combinations of the elements R1, R2, L or by selective use of a voltage dropping resistor r2, r3 or a diode. Alternatively, the bimetal heater r1 may have several taps which are selectively connected. <IMAGE>

Description

SWITCH ARRANGEMENT FOR A HEATER ASSEMBLY The present invention relates to a switch arrangement for a heater assembly, and more particularly but not exclusively relates to a switch arrangement for a heater assembly which incorporates one or more heating elements having a substantial positive temperature coefficient of resistance, such as an infra-red lamp or a molybdenum disilicide element, and one or more resistive heating elements so as to provide a plurality of discrete heating levels.

It is known to employ in electric radiant heaters two infra-red lamps and an additional bare wire coil heating element. At full power the three elements are connected in parallel across the mains. Reduced power levels are provided by a manually operated multi-position switch which connects the heating elements and two external diodes in various series and parallel combinations. Such heaters provide two desirable features: (a) the lamps are always visible at all power levels; and (b) there is a progressive change of lamp brightness from one power level setting to the next with the result that the heater appearance is a visual indication of the heater power level.

It is also known to employ in electric radiant heaters a single, for example circular or polygonal, infra-red lamp on its own or in series with a conventional coil heating element, and to control such heaters with an energy regulator which varies the mark to space ratio of power supplied to the heater. One problem with such a heater/switch assembly is that as the heater is switched on and off in accordance with the duty cycle of the energy regulator, the lamp flashes on and off abruptly, which can be visually distracting. Additionally, the appearance of the lamp when switched on gives no indication of the heater power level, as the brightness is always the same.

It would be possible to combine a single-lamp heater and a multi-position switch by including two coil heating elements and one or more external diodes. One problem with this arrangement is that the lamp power would have to be made a small proportion of the overall heater power in order to obtain the features noted above, thus virtually negating the purpose of incorporating the lamp. A second problem is that diodes introduce d.c. components and harmonics into the mains supply, and are therefore unacceptable to some electricity supply authorities.

Another possibility is to incorporate an additional resistive element of sufficient ohmic value such that when placed in series with the other elements of the heater, the heater power is low enough to be acceptable for the lowest power level. However, this solution is ruled out on the grounds of practical difficulty of accommodating such an extra coil.

It is also possible to utilise a bimetallic relay of fixed duty cycle in place of a diode. Electric radiant heaters have been made comprising four linear infra-red lamps controlled by a multi-position switch together with such a bimetallic relay. The multi-position switch provides five discrete power levels by connecting the four lamps in various series and parallel combinations. The lowest of these five power levels has the four lamps connected in series. A still lower power level is achieved by connecting the bimetallic relay into the circuit. The four lamps in series are then cycled on and off at the fixed duty cycle of the bimetallic relay. However, this arrangement depends upon the large number of possible combinations of four equal-power elements (the lamps) to obtain the higher power levels.

It is an object of the present invention to provide a switch assembly incorporating a multi-position switch for a heater assembly including at least two heating elements in which the heat-dissipating components can be fitted inside the heater cavity and whereby an appropriate number of heater power levels can be obtained.

It is a further object of the present invention to provide, for a heater assembly including at least two heating elements, a switch arrangement in which the heater brightness changes progressively with at least some of the settings of the switch arrangement, thus giving a visual indication of the heater power level, and in which there is no abrupt on and off cycling of the light other than through the operation of a thermal cut-out device.

According to the present invention there is provided a switch arrangement for a heater assembly including at least two heating elements, the switch arrangement comprising a multi-position switch for providing a plurality of discrete heating levels, and a duty cycle control means, operable in a plurality of positions of the multi-position switch for controlling the duty cycle of at least one of the heating elements, the duty cycle of the control means being varied in different positions of the multi-position switch, wherein the duty cycle of the duty cycle control means is varied by positioning the duty cycle control means across different heating elements or combinations of heating elements such that differing voltages are applied to the duty cycle control means.

The duty cycle control means may comprise a bimetallic relay.

One or more voltage dropping resistors may be provided for generating said different voltages.

At least one of the heating elements may comprise a material having a substantial positive coefficient of resistance, and may comprise an infra-red lamp. The assembly may include a ballast resistor connected in series with said at least one heating element in at least one position of the multi-position switch.

The assembly may include two of said heating elements connected in parallel with each other. Alternatively, the assembly may include two of said heating elements which are connectable in series or in parallel in dependence upon the position of the multi-position switch.

For a better understanding of the present invention and to show more clearly how it may be carried into effect reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a diagrammatic illustration of a heater which may be used with the present invention; Figure 2 is a diagrammatic illustration of one embodiment of a switching arrangement for a heater assembly according to the present invention having a single infra-red lamp; Figure 3 is a diagrammatic illustration of a second embodiment of a switching arrangement for a heater assembly according to the present invention having a single infrared lamp; Figure 4 is a diagrammatic illustration of a third embodiment of a switching arrangement for a heater assembly according to the present invention having a single infrared lamp; ; Figure 5 is a modification of the embodiment shown in Figure 2 with a ballast resistor connected in series with the infra-red lamp; Figure 6 is a diagrammatic illustration of an embodiment of a switching arrangement for a heater assembly according to the present invention having two infra-red lamps; Figure 7 is a diagrammatic illustration of a further embodiment of a switching arrangement for a heater assembly according to the present invention having two infra-red lamps; and Figure 8 is a diagrammatic illustration of another embodiment of a switching arrangement for a heater assembly according to the present invention having a single infrared lamp.

Figure 1 shows a heater which comprises a dish 1, for example pressed from sheet metal, which contains a layer 2 of thermal and electrical insulating material and a peripheral wall 3 of thermal insulating material. Inner and outer helical coils 4 and 5, respectively, of bare resistance wire are arranged on the layer 2 and extend substantially in a circle adjacent the -peripheral wall 3.

A generally circular infra-red lamp 6 is positioned radially inwardly from the inner coil 4. A thermal cut-out device 7 extends substantially across the centre of the dish and comprises a temperature sensor 8 connected to a switch 9. In the event that the temperature sensor 8 detects an excessive temperature the switch 9 is actuated to de-energise one or more of the heating elements 4, 5 and 6 until such time as the temperature has fallen to an acceptable level.

Although the heater shown in Figure 1 shows a generally circular infra-red lamp and two heating coils positioned radially outwardly from the lamp, the infra-red lamp may have alternative shapes and may be, for example, polygonal.

Moreover, the infra-red lamp may be replaced by an alternative heating element have a substantial positive temperature coefficient of resistance such as a molybdenum disilicide element. The heating coils 4 and 5 may have any convenient shape.

Figure 2 shows a switch arrangement and heater assembly which comprises a single infra-red lamp L and two resistive heating elements R1 and R2. The switch arrangement has six heat settings and incorporates seven sets of contacts C1, C2, C3, C4, C5, C6 and C7 which are opened or closed in accordance with Table 1 shown below.

Switch Position Contacts 6 5 4 3 2 1 Cl x C2 X C3 x x x x x C4 x C5 x C6 x C7 x X = contact closed Table 1 A bimetallic relay B includes a heating coil r1 and a bimetallic switch S. The bimetallic relay is in effect a mechanically non-adjustable energy regulator and operates by virtue of electric current passing through the heating coil rl and causing a bimetallic strip to be heated. When the bimetallic strip has reached a predetermined temperature the switch opens cutting off the flow of current and causing the bimetallic strip to cool and to close the switch.A voltage dropping resistor r2 is connected between contact C6 and the bimetallic relay B.

A thermal cut-out device T is provided to prevent overheating and can be positioned elsewhere in the circuit if desired.

In use, in switch position 6, which gives maximum power, the contacts C2 and C4 are closed and the two resistive heating elements R1 and R2 are connected in series with each other, and are then connected in parallel with the lamp L. In switch position 5, the contacts C1 and C3 are closed and the two resistive heating elements are connected in parallel with each other, and are also connected in series with the lamp L. In switch position 4, contact C3 is closed and the lamp L is connected in series with the resistive heating element R1.

In switch positions 3, 2 and 1, contact C3 is closed and the lamp L is connected in series with the resistive heating element R1 as in switch position 4. Additionally, in switch position 3, contact C7 is closed allowing power to pass through the heating coil r1 and to operate the bimetallic relay at a first duty cycle determined by the voltage at the junction of the lamp L and the heating element R1.In switch position 2, contact C6 is closed allowing power to pass from the opposite side of the lamp L compared with contact C7 through the voltage dropping resistor r2 and then through the heating coil rl. The value of the resistor r2 is selected so that the power flowing through the heating coil rl in switch position 2 is higher than in switch position 3 and this results in a lower duty cycle of the bimetallic relay B compared with switch position 3. In switch position 1, contact C5 is closed allowing power to pass from the opposite side of the lamp L compared with contact C7 directly to the heating coil rl.The power flowing through the heating coil r1 in switch position 1 (its maximum rated power) is higher than in switch position 2 and this results in a lower duty cycle of the bimetallic relay B (its minimum duty cycle) compared with switch position 2. Suitable duty cycles for the bimetallic relay B are shown in Table 2 below, expressed as the fraction in per cent of the operating cycle for which the bimetallic switch is closed.

Switch Position Duty Cycle (per cent) 3 65 2 40 1 25 Table 2 To summarise, for switch positions 3, 2 and 1, three different voltages are created across the bimetallic relay B. In switch position 1 the voltage is mains voltage. In switch position 2, a different voltage is obtained by connecting a small external resistance r2 in series with the relay. The external resistance r2 is preferably mounted on the bimetallic relay B. In switch position 3, a third voltage is obtained by connecting the heating coil r1 across the infra-red lamp L only.

In an alternative embodiment, shown in dashed line in Figure 2, the voltage for switch position 3 is obtained by positioning the switch contact C7, now designated C7', to connect a second small external resistance r3, different from resistance r2, in series with the switch contact C7' and the relay. The external resistance r3 is preferably mounted on the bimetallic relay.

It will be noted that the infra-red lamp is subjected to on and off cycles which have been described hereinabove as visually unsatisfactory. Such visual disturbance occurs with lamps which have no or only a small resistance in series, as a result of the very high inrush current of the infra-red lamps. However, in the embodiments of the present invention described herein, the cycling of the infra-red lamp only occurs with a sufficiently large resistance in series with the lamp so that energising of the lamp is significantly slowed and any visual disturbance eliminated or significantly reduced.

The switch assembly therefore acts as a conventional multiposition switch in the three top positions, positions 6, 5 and 4, and as an energy regulator in the three bottom positions, positions 3, 2 and 1, but with slowed changes in the brightness of the infra-red lamp. This combines the potential benefits of both types of switch.

Figure 3 shows a switch arrangement and heater assembly similar to that shown in Figure 2 and the same references are used to denote the same or similar components. The seven sets of contacts C1, C2, C3, C4, C5, C6 and C7 of the switch arrangement shown in Figure 3 are opened or closed in accordance with Table 3 shown below.

Switch Position Contacts 6 5 4 3 2 1 Cl X C2 X C3 X X C4 X X X C5 X X C6 X C7 X X

X = contact closed Table 3 In use of the embodiment illustrated in Figure 3, in switch position 6, which gives maximum power, the contacts C1 and C2 are closed and the two resistive heating elements R1 and R2 are connected in series with each other, and are then connected in parallel with the lamp L. In switch position 5, the contacts C3 and C5 are closed and the two resistive heating elements R1 and R2 are connected in parallel with each other, and are also connected in series with the lamp L.

In switch position 4, contacts C3, C5 and C7 are closed and the two resistive heating elements R1 and R2 are connected in parallel with each other and are also connected in series with the lamp L as in switch position 5, but additionally the heating coil rl of the bimetallic relay B is energised by way of contact C7 so as to connect the heating coil across the resistive heating element R2 and to operate the bimetallic relay at a first predetermined duty cycle. This has the effect of connecting and disconnecting resistive heating element R2 from the circuit.

In switch positions 3, 2 and 1, contact C4 is closed and the resistive heating elements R1 and R2 are connected in series with each other and are also connected in series with the infra-red lamp L. Additionally, in switch position 2, contact C7 is closed allowing power to pass through the heating coil rl and to operate the bimetallic relay at a second predetermined duty cycle which is higher than the first duty cycle due to the different interconnection of the heating elements and the lamp.In switch position 1, contact C6 is closed allowing power to pass from the opposite side of the heating element R1 compared with contact C7 through the voltage dropping resistor r2 and then through the heating coil rl. The power flowing through the heating coil r1 in switch position 1 is higher than in switch position 2 and this results in a lower duty cycle of the bimetallic relay B compared with switch position 2.

In an alternative embodiment, contact C7, now designated C7', is positioned as shown in dashed line and is connected in series with a second small external resistance r3, different to resistance r2, and with the relay. The external resistance r3 is preferably mounted on the bimetallic relay.

Figure 4 shows a switch arrangement and heater assembly similar to that shown in Figures 2 and 3 and the same references are used to denote the same or similar components. The seven sets of contacts C1, C2, C3, C4, C5, C6 and C7 of the switch arrangement shown in Figure 5 are opened or closed in accordance with Table 4 shown below.

Switch Position Contacts 6 5 4 3 2 1 C1 X X C2 X C3 X C4 X X X X C5 X X C6 X C7 x X = contact closed Table 4 In use of the embodiment illustrated in Figure 4, in switch position 6, which gives maximum power, the contacts C1 and C2 are closed and the two resistive heating elements R1 and R2 are connected in series with each other, and are then connected in parallel with the lamp L.In switch position 5, the contacts Ct, C3 and C5 are closed and the infra-red lamp L is connected in series with the heating element R1, with the heating element R2 being connected in parallel with the combination of the lamp and the heating element R1. Additionally, the heating coil rl is connected across the heating element R2 and serves to effect the energisation of the heating element R2 with a predetermined duty cycle.

In switch positions 4, 3, 2 and 1, contacts C4 are closed and the two resistive heating elements R1 and R2 are connected in series with each other and are also connected in series with the lamp L.

Additionally, in switch position 3, the heating coil rl of the bimetallic relay B is energised by way of contact C5 so as to connect the heating coil across the resistive heating element R2 and to operate the bimetallic relay at a second predetermined duty cycle.

In switch position 2, contact C6 is closed allowing power to pass through the voltage dropping resistor r2 and through the heating coil r1 and to operate the bimetallic relay at a third predetermined duty cycle which is lower than the second duty cycle. In switch position 1, contact C7 is closed allowing mains power to pass through the heating coil rl. The power flowing through the heating coil rt in switch position 1 is higher than in switch position 2 and this results in a lower duty cycle of the bimetallic relay B compared with switch position 2.

The heater assemblies shown in Figures 2 to 4 all have the single infra-red lamp connected across the mains supply in switch position 6. This can result in heater assemblies of certain powers, for example 900 watts and above, not passing certain regulations regarding permitted perturbations to the mains. It is possible to modify the embodiments described above by incorporating a ballast resistor R3 in series with the lamp in order to overcome such problems. By way of example, Figure 5 shows the embodiment of Figure 2 so modified. It should be noted that the ballast resistor R3 enables the three voltages required on the bimetallic relay B to be obtained by placing the heating coil rl across all or a proportion of the infra-red lamp and the heating elements and does not require the use of voltage dropping resistors, although these can still be used if desired. Voltage dropping resistors have the general advantage of being able to adjust the duty cycles of the bimetallic relay without modifying the values of the infra-red lamp or the heating elements by simply changing the value of the resistors. In this respect, it may be advantageous to incorporate a further dropper resistor to drop the mains voltage, which is otherwise applied directly to the heating coil rl, to a desired voltage. It should also be noted that the voltage dropper resistors may be linear or non-linear devices.

It is possible to modify any of the embodiments of the present invention illustrated in Figures 2 to 5 by substituting two infra-red lamps connected in parallel for the single infra-red lamp shown. The disadvantage of this is not making use of the opportunity to connect the two infra-red lamps in series for increased brightness in low heat settings, as compared to two lamps in parallel.

Figure 6 shows a switching arrangement in a heater assembly having two infra-red lamps which can be connected in series in the lower heat positions. The switch arrangement illustrated in Figure 6 has six heat settings and incorporates seven sets of contacts C1, C2, C3, C4, C5, C6 and C7 which are opened or closed in accordance with Table 5 shown below.

Switch Position Contacts 6 5 4 3 2 1 1 Cl r x x C2 X C3 X X- X X C4 X X C5 x C6 X x C7 x

X = contact closed Table 5 Operation of the switch arrangement illustrated in Figure 6 will be apparent from the operation of the similar switch arrangements described above. As with the modification to Figure 2, contact C7 may be repositioned so as to supply power to the heating coil rl by way of a voltage dropper resistor.

In order to reduce mains perturbation where required by electricity authority regulations in the switch arrangement illustrated in Figure 6, a third, ballast, resistor can be incorporated in series with one or both of the lamps as described above with reference to Figure 5. The disadvantage of such an arrangement is that three separate coils are expensive and difficult to accommodate in the heater.

Figure 7 shows a further switch arrangement and heater assembly having two infra-red lamps which can be connected in series in the lower heat positions and having two resistive coils of which one coil is in series with the lamp at full power. The switch arrangement illustrated in Figure 7 has six heat settings and incorporates seven sets of contacts C1, C2, C3, C4, C5, C6 and C7 which are opened or closed in accordance with Table 6 shown below.

Switch Position Contacts 6 5 4 3 2 1 Cl X x C2 x C3 X X X X X C4 X C5 X C6 x C7 x X = contact closed Table 6 Operation of the switch arrangement illustrated in Figure 7 will be apparent from the operation of the similar switch arrangements described above.

In an alternative embodiment, contact C6 is closed in switch position 5. This reduces the average power developed in resistive coil R1 in switch position 5, thereby facilitating accommodation of this coil in the heater. In a further embodiment, the resistive coil R2 is positioned slightly differently as indicated in dashed line in Figure 7.

Figure 8 shows a switch arrangement and heater assembly which comprises a single infra-red lamp L and two resistive heating elements R1 and R2. The switch arrangement has eight heat settings and incorporates seven sets of contacts C1, C2, C3, C4, C5, C6 and C7 which are opened or closed in accordance with Table 7 shown below.

Switch Position Contacts 8 7 6 5 4 3 2 1 Cl x x C2 x x C3 X X X X C4 x x x x C5 X X C6 X X X X C7 X X X X X X X

X = contact closed Table 7 A bimetallic relay B includes a heating coil r1 and a bimetallic switch S. A voltage dropping resistor r2 is connected between contact C1 and the bimetallic relay B.

A thermal cut-out device T is provided to prevent overheating and can be positioned elsewhere in the circuit if desired.

In use, in switch position 8, which gives maximum power, the contacts C3, C5, C6 and C7 are closed and the resistive heating element R2 is connected in parallel with the lamp L. In switch position 7, the contacts C1, C3, C6 and C7 are closed and the resistive heating element R1 is connected in series with the lamp L, while the resistive heating element R2 is connected in parallel with the combination of the lamp L and the resistive heating element Rl. Because the contact Cl is closed, power passes to the bimetallic relay B by way of the voltage dropping resistor r2 in order to reduce the power flowing through the heating coil rl and to operate the bimetallic relay B at a first duty cycle that is increased with respect to the duty cycle of the bimetallic relay B when the assembly is connected directly to the heating coil rl of the bimetallic relay.

In switch position 6, the contacts C2, C3, C6 and C7 are closed. As with switch position 7, the resistive heating element Rl is connected in series with the lamp L, while the resistive heating element R2 is connected in parallel with the combination of the lamp L and the resistive heating element R1, but in switch position 6 power passes directly to the bimetallic relay B which therefore operates at a second, lower duty cycle. In switch position 5, the contacts C3 and C6 are closed and the resistive heating element R1 is connected in series with the lamp L. In switch position 4, the contacts C4, C5 and C7 are closed and the lamp L is connected in series with the resistive heating element R2. In switch position 3, the contacts C4 and C7 are closed and the lamp L is connected in series with both the resistive heating elements R1 and R2.

In switch positions 2 and 1, contacts C4 and C7 are closed and the lamp L is connected in series with both the resistive heating elements R1 and R2 as in switch position 3. Additionally, in switch position 2, contact C1 is closed allowing power to pass through the voltage dropping resistor r2 and then through the heating coil rl and to operate the bimetallic relay at the first duty cycle. In switch position 1, contact C2 is closed allowing power to pass directly to the heating coil rl so as to operate the bimetallic relay at the second duty cycle.As noted above, the value of the resistor r2 is selected so that the power flowing through the heating coil rl in switch position 1 is higher than in switch position 2 and this results in a higher duty cycle of the bimetallic relay B in switch position 2 compared with switch position 1.

To summarise, for switch positions 7 and 6 and 2 and 1, different voltages are created across the bimetallic relay B. In switch positions 6 and 1 the voltage is higher than in switch positions 7 and 2, the lower voltage being obtained by connecting a small external resistance r2 in series with the relay. The external resistance r2 is preferably mounted on the bimetallic relay B. It will be noted that in switch position 8, at maximum power, only one of the resistive heating elements, R2, is in use, the other heating element R1 being used in series with the lamp L as necessary at lower power levels.

The voltage dropping resistor r2 can be replaced, if desired, by a diode. The use of a diode has the advantage of reducing the effect of tolerances in component values and supply voltage fluctuations. The use of a diode cannot eliminate supply voltage fluctuations, but the effect of such fluctuations is not compounded by the effect of tolerances in the voltage dropping resistor r2.

In an alternative form, the bimetallic relay may incorporate a thin- or thick-film heating resistor rl which is provided with several tapping points along its length.

In this way, different heating powers, and thus duty cycles, can be obtained by energising different length portions of the heating resistor, selected by appropriate switching. In this case, any variations in the value of the heating resistor rl will affect each of its sections proportionately.

In an alternative embodiment, shown in dashed line in Figure 8, seven or even six power level settings can be provided by omitting switch positions 3 and/or 6 and/or 7.

Resistive heating element R1 has a relatively low value and its effect in switch position 3 is small relative to switch position 4. The omission of switch position 7 permits simplification of the circuit in that voltage dropping resistor r2 can be omitted and contact C1 repositioned as shown in dashed line so as to connect, in switch position 2, the heating coil rl of the bimetallic relay B across resistive heating element R2. Moreover, because resistive heating element R1 produces relatively little effect in switch position 2, it can be omitted by closing contact C5.

It will be noted in respect of Figure 8 that the infra-red lamp is subjected to on and off cycles which have been described hereinabove as visually unsatisfactory. Such visual disturbance occurs with lamps which have no or only a small resistance in series, as a result of the very high inrush current of the infra-red lamps. However, in the embodiments of the present invention described herein, the cycling of the infra-red lamp only occurs with a sufficiently large resistance in series with the lamp so that energising of the lamp is significantly slowed and any visual disturbance eliminated or significantly reduced.

The switch assembly therefore acts as a conventional multiposition switch in positions 8, 5, 4 and 3, and as an energy regulator in positions 2 and 1, but with slowed changes in the brightness of the infra-red lamp. In positions 7 and 6, the switch assembly acts as a hybrid multi-position switch and energy regulator. The switch assembly thus combines the potential benefits of both types of switch.

Claims (9)

1. A switch arrangement for a heater assembly including at least two heating elements, the switch arrangement comprising a multi-position switch for providing a plurality of discrete heating levels, and a duty cycle control means, operable in a plurality of positions of the multi-position switch for controlling the duty cycle of at least one of the heating elements, the duty cycle of the control means being varied in different positions of the multi-position switch, wherein the duty cycle of the duty cycle control means is varied by positioning the duty cycle control means across different heating elements or combinations of heating elements such that differing voltages are applied to the duty cycle control means.

2. A switch arrangement as claimed in claim 1, wherein the duty cycle control means comprises a bimetallic relay.

3. A switch arrangement as claimed in claim 1 or 2, wherein one or more voltage dropping resistors are provided for generating said different voltages.

4. A switch arrangement as claimed in any one of claims 1 to 3, wherein at least one of the heating elements comprises a material having a substantial positive temperature coefficient of resistance.

5. A switch arrangement as claimed in claim 4, wherein said at least one heating element comprises an infra-red lamp.

6. A switch arrangement as claimed in claim 4 or 5 and including a ballast resistor connected in series with said at least one heating element in at least one position of the multi-position switch.

7. A switch arrangement as claimed in any one of claims 3 to 6 and including two of said heating elements connected in parallel with each other.

8. A switch arrangement as claimed in any one of claims 3 to 6 and including two of said heating elements which are connectable in series or in parallel in dependence upon the position of the multi-position switch.

9. A switch arrangement for a heater assembly substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.