5, prior to passing through apertures 13 and 14 into the inner container 5, where it is then heated, on a second pass, by directly coming into contact with the heating element cartridges 10a to 10g.

The double pass arrangement of the boiler 1 illustrated in FIGS. 1 to 3 provides a large heat transfer area for the limited volume of water contained within the boiler 1.

Referring now to FIGS. 4 and 5, here there is illustrated an alternative boiler to that shown in FIGS. 1 to 3 and this is indicated generally as 25. The boiler 25 has many of the same components as the boiler 1 of FIGS. 2 and 3 and like numerals are used to indicate like components, which are not described again here.

In the embodiment of FIGS. 4 and 5, six two kilowatt heating element cartridges 26a to 26f are arranged in a cylindrical pattern in a first end plate 27, with only the heating element cartridges 26a and 26d being visible in FIG. 4.

In this embodiment, each of the heating element cartridges 26a to 26f has a respective inner cylinder 28a to 28f joined at a first end to the first end plate 27 and joined at a second end to a common inner end plate 29. As in the previous embodiment, the inner end plate 29 has apertures, only two of which, 30 and 31, can be seen in FIG. 4, located opposite to the end of each heating element cartridge 26a to 26f.

Each of the inner cylinders 28a to 28f has an aperture, only two of which, 33 and 34, can be seen in FIG. 4, adjacent the first end plate 27. These connect the interior of each inner cylinder 28a to 28f with a central passage 32. The central passage 32 extends the length of the inner cylinders 28a to 28f and through the inner end plate 29 to a threaded port 35, which forms the hot water outlet 3. As can be seen from FIG. 5, the six inner cylinders 28a to 28f abut each other and these are welded together such that they form a continuous sealed surface to define the central passage 32. External to the inner cylinders 28a to 28f there is located an outer cylinder 36, which defines a space which, when filled with water, forms a water jacket 37.

In use, water enters the boiler 25 of FIGS. 4 and 5 through cold water inlet 2 and spirals downwardly within the water jacket 37, drawing heat energy from the outwardly facing outer surfaces of the inner cylinders 28a to 28f, before entering those inner cylinders 28a to 28f, via apertures such as apertures 30 and 31 shown in FIG. 4. The water is then forced to flow over the surfaces of the heating element cartridges 26a to 26f before exiting at apertures, such as apertures 33 and 34, into the central conduit 32. Here the water passes along the length of the central passage 32, in contact with the inwardly facing outer surfaces of the inner cylinders 28a to 28f, drawing heat energy from the inwardly facing outer surfaces of the inner cylinders 28a to 28f, as it makes a third pass through the boiler 25, before exiting the hot water outlet 3.

It will be appreciated that the same advantages are achieved with the boiler 25 of FIGS. 4 and 5 as are achieved with the boiler 1 of FIGS. 2 and 3, but with the boiler 25 of FIGS. 4 and 5 the water makes an additional third pass, resulting in an even more efficient heat exchange with the heating element cartridges 26a to 26f.

Referring now to FIG. 6, here there is schematically illustrated the various components necessary to control the boiler 1 of FIGS. 2 and 3, or the boiler 25 of 4 and 5. These comprise a processor (control circuit) 38 arranged to control the supply of electrical energy along cables 39 to the heating element cartridges 10a to 10g, or 26a to 26f. The processor 38 is also connected to temperature sensors of the inner heating elements 10a to 10g, or 26a to 26f by the wire 40.

The processor 38 is also connected to the over temperature sensor 24, via wire 41, and to an optional flow sensor 42, via wire 43. The flow sensor 42 is shown external to the boiler 1. However, it should be noted that FIG. 6 is only schematic and the flow sensor 42 and the processor 38 could be within the outer casing 4 of the boiler 1.

Referring now to FIG. 7, in use the processor 38, at the start 44, first determines in step 45, from the flow sensor 42, whether there is a demand for hot water. If there is no demand for hot water, the processor 38 returns to the start 44. However, in an alternative embodiment, where there is no flow sensor, the heating elements may be maintained at 60° C., in which case step 45 may be omitted. In the illustrated embodiment, if there is a demand for hot water, then the processor 38 proceeds to step 46 and determines whether the temperature within the heating element cartridges 10a to 10g, or 26a to 26f, is above 60° C. If this temperature is exceeded then the processor proceeds to step 47 and turns off the heating element cartridges 10a to 10g, or 26a to 26f, and returns to the start 44. However, if at step 46 the heating element temperature is not detected to be above 60° C., then the processor 38 proceeds to step 48 and determines whether an over temperature value is exceeded. If it is then the processor 38 proceeds to step 47 and turns off the heating element cartridges 10a to 10g, or 26a to 26f, before returning to the start 44. If however the over temperature value is not exceeded at step 48, then the processor 38 proceeds to step 49 and turns on the heating element cartridges 10a to 10g, or 26a to 26f, before returning to the start 44 and repeating the process.

The above describes one process in which the processor 38 may control energisation of the heating element cartridges 10a to 10g, or 26a to