WO1990006473A2

WO1990006473A2 - Electrical heat storage boiler

Info

Publication number: WO1990006473A2
Application number: PCT/GB1989/001459
Authority: WO
Inventors: Peter Joseph Newton; Thomas Henry Joyes
Original assignee: DIMPLEX HEATING Ltd
Current assignee: DIMPLEX HEATING Ltd
Priority date: 1988-12-06
Filing date: 1989-12-06
Publication date: 1990-06-14
Anticipated expiration: 1991-06-06
Also published as: WO1990006477A3; EP0533658A1; DE68921988D1; ES2070310T3; EP0447438A1; EP0447438B1; WO1990006477A2; DE68921988T2; WO1990006473A3

Abstract

An electric boiler comprising a plurality of boiling tubes (11) extending substantially vertically for providing heat to a heat exchanger, a core consisting of a multiplicity of heat storing bricks (30, 35), and an insulated casing surrounding the core, the casing having an openable front face to allow installation of the core and access thereto, bricks which lie, in use, against the boiling tubes (11) having side face profiles formed to provide channels for accepting part of the boiling tube (11) whereby a combination of such bricks surrounds a length of boiling tubes, the width of said bricks being such that, when the boiling tubes are surrounded by a course of said bricks lying adjacent the boiling tubes, space when viewed from the front is provided between the bricks and the casing, and between the bricks surrounding adjacent boiling tubes (11), whereby that space can be filled by sliding further bricks (30, 35) in a front to back direction with respect to the boiler. The bricks in an assembled core define channels (32, 36) extending from front to rear of the boiler for receiving heating elements (40), the heating elements being elongate and located in the core by sliding into the channels (32, 36) from the front of the core. An advantage of the invention is that the boiler may be quickly and easily installed. Further, if a heating element were to fail in service, it may be readily removed and replaced.

Description

ELECTRICAL HEAT STORAGE BOILER

The invention relates to boilers in which water is heated by electricity, through the medium of a core of material capable of storing heat, the core being heated by electrically powered heating elements and heat from the core being transferred to a secondary circuit in which a fluid, commonly water, is circulated, as in the case of a domestic central heating or hot water system. It is envisaged that such electrically heated boilers be used in place of gas, oil or solid fuel boilers which also heat fluid secondary circuit in a way which is familiar.

Electric boilers exist which include a core and elements as referred to above, and in which transfer of heat from the core to the secondary circuit is achieved using air as the heat transfer medium. Thus air is heated by the core and the heated air passes over a heat exchanger through which water in the secondary circuit passes. A more recent approach to an electric storage boiler is to utilise steam as the medium for transferring heat from the store to the secondary circuit. The general arrangement which has been proposed has a core of material capable of storing heat, electrically heated element means in the core, a primary heating circuit, in which water is supplied to the base of boiling tubes which pass steam upwards through the core to a heat exchanger, from which condensed water is returned to the base of the boiling tubes, losses being made up from a header tank or similar arrangement, and a secondary heating circuit portion which passes through the heat exchanger and which is connected, in use, to a secondary heating circuit in a conventional way, the secondary heating circuit including radiators and/or a coil for heating water in a domestic hot water cylinder. Such an electric boiler will be referred to hereinafter as "an electric boiler of the kind described", and is envisaged as a substitute for a gas, oil or solid fuel boiler.

A practical problem with electric boilers of the kind described is arranging for manageable installation and servicing, for example replacement of heating elements which may fail from time to time. In order to provide a satisfactorily large heat store, a substantial weight of heat storing bricks is necessary and it is desirable to be able to install and service the boiler from a single direction, preferably from the front so that the boiler can be placed, if desired, under a working surface.

According to the invention, there is provided an electric boiler of the kind described comprising a plurality of' boiling tubes extending substantially vertically, the core consisting of a multiplicity of heat storing bricks, and an insulated casing surrounding the core, the casing having an openable front face to allow installation of the core and access thereto, bricks which lie, in use, against the boiling tubes having side face profiles formed to provide channels for accepting part of a boiling tubes whereby a combination of such bricks surrounds a length of boiling tubes, the width of said bricks being such that, when the boiling tubes are surrounded by a course of said bricks lying adjacent the boiling tubes, space when viewed from the front is provided between the bricks and the casing, and between the bricks surrounding adjacent boiling tubes whereby that space can be filled by sliding further bricks in a front to back direction with respect to the boiler.

The bricks in an assembled core preferably define channels extending from front to rear of the boiler, the heating elements being elongate and located in the core by sliding into the channels from the front of the core. Each element is preferably U-shaped. The cross- sectional profile of the element channels and the cross- section of the heating elements are preferably such that contact is achieved between a portion of the external surface of each element and the adjacent bricks.

The bricks which lie against the boiling tubes may be of a different profile from the further bricks. The further bricks preferably have planar faces, and each brick which lies against the boiling tubes preferably includes not only the boiling tube portion accepting channel but groove means extending, in use, in a direction from front to back of the core, which groove means defining with a planar face of a further brick at least one channel for accepting an element. Each groove means preferably has two parallel grooves such that each pair of bricks which includes a further brick defines two element channels.

The core preferably comprises a plurality of brick and boiling tube units, each unit comprising a length of boiling tube, a pair of bricks lying against and surrounding, between them, the length of boiling tube, and a pair of further bricks, each lying outside and against a respective one of the boiling tube contacting bricks to define at least one element channel on each side of the boiling tube.

The larger the boiler capacity, the larger the number of brick and boiler tube units, but a preferred embodiment envisages a boiler tube to be three brick courses high, and with four boiling tubes, a total of forty-eight bricks. With such a height, a boiler with two boiling tubes would have twenty-four bricks, and a boiler with six boiling tubes would have seventy-two bricks.

The insulated casing preferably has a front insulation layer including slots through which the elements pase whereby contacts for connecting each element to an electrical supply lie on the side of the insulation remote from the core, in use.

Metal sheets, for example of stainless steel, may be placed within and to cover the inner surfaces of the insulation on the side walls, base and top of the casing. A further metal sheet, again of stainless steel for example, may be placed in a vertical plane adjacent walls of the further bricks.

According to a further aspect of the invention, there is provided an electric boiler of the kind described in which the header tank has a capacity substantially larger than, and preferably at least double the capacity of the primary heating circuit.

According to a still further aspect of the invention there is provided an electric boiler of the kind described in which the core includes a multiplicity of bricks, composition of bricks contacting a boiling tube being different from the composition of bricks not contacting a boiling tube, the thermal conductivity of bricks contacting a boiling tube being greater than the thermal conductivity of bricks not contacting a boiling tube

The composition of the bricks lying between elements may be magnetite and the composition of bricks between an element and a boiling tube may be a mixture of magnetiteand magnesite. The proportion of magnetite to magnesite may be varied from boiler to boiler to achieve particular storage and heat release characteristics.

According to a still further aspect of the invention there is provided an electric boiler of the kind described comprising a capillary tube connection between the heat exchanger and the header tank in the primary heating circuit, which capillary tube connection includes a non-return valve such that fluid, for example air or steam, can pass from the heat exchanger to the header tank, but not in the reverse direction.

According to a still further aspect of the invention, there is provided an electric boiler of the kind described in which the edge profiles the insulation layers are such that there is a stepped edge profile on at least one of and preferably both insulation layers abutting at right angles.

With reference to an electric boiler of the kind described, if the water/steam remains in the boiling tubes when the secondary heating circuit is turned off, stored energy is wasted due to continual evaporation and condensation of the water. This problem is alleviated substantially by virtue of another aspect of the present invention. According to the invention there is provided an electric boiler of the kind described comprising a return pipe connected to the primary heating circuit and extending above the water level in the header tank, means for closing the pipe supplying fluid from the header tank and means for preventing release of pressure from the heat exchanger, wherein the condensing fluid in the primary heating circuit may be driven out of the return pipe and into the header tank when the secondary heating system is turned off. The header tank is preferably made of a plastics material to allow a degree of distortion in the walls of the tank. Preferably the heat exchanger is manufactured from copper and is angled slightly to improve drainage therefrom down the drain pipe. This arrangement improves the efficiency of removal of the condensed water from the primary circuit into the header tank when the secondary circuit is turned off, and also the efficiency of the primary heating circuit when heating the secondary circuit.

As will be appreciated, the header tank of a boiler of the kind described has to be vented to atmosphere to allow water to be drawn from it. This venting to atmosphere inevitably allows water loss from evaporation due to high temperature attained by the water in the header tank and consequent evaporation. According to the invention, there is provided a header tank for an electric boiler of the kind described, which header tank is a closed vessel including vent means for venting the vessel to atmosphere, the vent means comprising a tube extending from the vessel and into a fluid condensing and retaining container, the arrangement being such that as liquid is drawn, in use, from the header tank into the primary heating circuit, condensate from the condensing and retaining container is drawn into the header tank via said tube.

The bore of thetube is preferably in the range 1mm to 5mm, and is preferably of 3mm.

The condensing and retaining container is preferably a tube of larger bore than the tube extending from the header tank. The bore of the larger tube may be in the range 5mm to 10mm, preferably 8mm.

The smaller bore tube preferably has a length toward its end away from the header tank extending substantially vertically within a section of the larger bore tube.

Both free ends of the larger tube preferably lie above the opening of the smaller bore tube within the larger bore tube. The smaller bore tube preferably opens within the larger bore tube at or adjacent the lowest point of the larger bore tube. The larger bore tube preferably includes at least one section having bends therein to add length to the condensation path for vapour from the larger bore tube.

The larger bore tube preferably lies in one plane, the plane thereof preferably lying parallel to the back of the boiler, in use.

The invention further provides an electric boiler of the kind described comprising a header tank according to the invention. By way of example, one embodiment of an electric boiler according to the invention will now be described with reference to the accompanying drawings, in whic :-

Figure 1 is a schematic illustration of the pipework layout of a boiler;

Figure 2 is a plan view of a course of core bricks in the boiler of Figure 1 having four boiling tubes;

Figure 3 is a sectional view along the lines X-X in Figure 2, and showing one element in position for the sake of example;

Figure 4 is an exploded view of the course of bricks of Figures 2 and 3;

Figure 5 is a side view of a heating element;

Figure 6 is a plan view of the heating element of Figure 5;

Figure 7 is a plan view of side, front and rear insulation of the casing;

Figure 8 is a front view of insulation of the casing, but without base and front insulation;

Figure 9 is a side view of top, front and rear insulation;

Figure 10 is a plan view of base insulation; -

Figure 11 is an end view of the base insulation of Figure 10 ;

Figure 12 is a view of a header tank and condensing vent as Incorporated in another boiler according to the invention ;and

Figure 13 is a schematic illustration of an alternative pipework layout for a boiler according to the invention.

Figure 1 shows a schematic layout of pipework^" in an electric boiler 10 according to the invention. A primary heating circuit has four boiling tubes 11 surrounded in an assembled boiler by a core of heat storing bricks (not shown in Figure 1). Vaporizable liquid, preferably demineralised water, is fed to the base of the boiling tubes 11 via pipes 12 and water in the boiling tubes is turned to steam by heat in the core, the steam rising from the boiling tubes to a heat exchanger 13 through which liquid in a secondary heating circuit 14 flows. Steam from the boiling tubes condenses in the heat exchanger 13 while giving up its heat to the secondary circuit 14, condensed steam returning via pipe 15 and motorised valve 16 to the pipes 12. The motorised valve 16 is opened to allow the primary heating circuit to operate if there is a demand for heat from the secondary heating circuit 1 . The motorised valve 16 can be controlled, for example, by a room thermostat or other suitable means to indicate a need for heat.

To insure an adequate level of fluid in the primary heating circuit, a header tank 17 having a capacity approximately twice that of the primary heating circuit is able to feed liquid via the motorised valve ' 16 to the pipes 12. Non-return valve 18 is provided to allow water to return to the header tank 17 when heat is not required from the boiler and the motorised valve 16 is therefore closed. A drain cock 20 is provided to allow the header tank 17 to be drained.

When the motorised valve 16 is first opened after a spell of the primary heating circuit being closed, air will be present in the chamber of the heat exchanger 13 and it is necessary to allow that air to be flushed out of the heat exchanger. To this end, a capillary bleed is provided from the heat exchanger 13 into the header tank 17, a non-return valve 22 being fitted to prevent return of fluid from the header tank 17 to the heat exchanger 13. Air present in the heat exchanger 13 can be flushed through the capillary 21, subsequent loss of steam from the heat exchanger due to the permanent presence of the capillary 21 being negligible when the system if fully operational. It will be appreciated that any steam passing through the capillary 21 will have condensed during passage through the capillary lying in liquid in the header tank 17.

A flow heater 23 is provided in the secondary heating circuit to provide direct heating in the secondary heating circuit if necessary. A control system provides apportionment of heat supplied to the secondary heating circuit from the flow heater 23 or the heat exchanger 13. The presence of the flow heater 23 and the control system therefore is the subject of a co-pending Patent application of today's date.

Figure12 shows the header tank 17 in isolation. In order to allow liquid to be drawn from the header tank 17 , venting to atmosphere must take place. Venting takes place via a tube coupling130 to which is connected a small bore tube131. A preferred bore for the tube131 is 3mm but it will be appreciatedthat other small bore tubes may be used, typically in the range 1mm to 5mm although other bores may be appropriate in certain circumstances. The tube 131 runs horizontally in an upper portion and bends downwards at point 132. In an assembled boiler, the tube 131 would exit from the boiler casing on the header tank side of the bend132. From the bend132, the tube 131 extends downwards and into a larger bore tube133, in this embodiment of 8mm bore. The larger bore tube could be of a different bore from 8mm, for example within the range 5mm to 10mm although other bores could be used. The tube 131 extends to the lowest point of the larger bore tube133, the larger bore tube133 then rising in a S configuration to an outlet134. In this way, there is a long condensation path for vapour trying to leave the tube 133, condensed vapour forming a liquid pool rising from the lowest point of the tube133.

When water is drawn from the header tank to feed the primary heating circuit, when the boiler is in use, liquid from the bottom of the larger bore tube133 is drawn back to the header tank through the small bore tube 131. The smaller the volume within the small bore tube131, the more liquid will be returned to the header tank for a given displacement of water from it. xnrougnou-c this specification, it will be appreciated that water is not the only liquid which could be used in the primary heating circuit although it is the preferred vaporizable liquid.

The advantage of this embodiment of a header tank according to the invention is that evaporation losses from the header tank are minimised, thereby reducing the frequency at which the header tank must be refilled.

Figure 2 shows a plan view of a core of bricks surrounding the four boiling tubes 11. Figure 2 shows one course of heat storing bricks and, in the embodiment shown, there are sixteen bricks in each course, there being three courses of bricks in all making forty-eight bricks in the entire core.

As can be seen in Figures 2, 3 and 4, Figure 3 being a section along the lines X-X in Figure 2 and Figure 4 being an exploded view of the bricks, two different shapes of brick are used.

Around the boiling tubes are arranged bricks 30 of a first shape having semi-circular section, vertical channels 31 formed therein such that two bricks 30 are able to surround a single boiling tube 11. Additionally, the shaped bricks 30 includes horizontal channels 31 which define with plain, cuboidal bricks 35 heating element channels 36. Into which elongate sheathed heating elements 40 can be slid. One such element 40 is shown in a channel 36 in Figure 3.

A heating element 40 is shown in more detail in Figures 5 and 6, the element 40 being of the sheathed type and being of elongate ϋ-shape. As can be seen in Figure 3, the dimensions of the element channel 36 are such that the sheath of the element makes physical contact with curves of the channel 36 to provide enhanced heat transfer from the element to adjacent bricks. Legs of the U-shape element 40 are held together remote from the base of the U-shape by a mounting plate 41. In assembling a boiler 10, each heating element 40 is slid into a respective heating element channel 36 in the core, having first passed through a front, slotted insulation slab against which the mounting plate 41 lies adjacent in its final position. This arrangement has the advantage that electrical connections 42 through which power is supplied to each heating element 40 are insulated from the main heat of the core. There is no view of the front insulation slab of the boiler 10 but it will be appreciated that the front insulation slab includes a number of elongate slots for engagement by the heating elements 40 corresponding to the number of heating elements 40 in the boiler 10 and at positions corresponding to the heating element channels 36 in the core. Wiring connections to the heating elements 40 are arranged conventionally and will therefore not be described in detail.

The material of the heat storing bricks 30 and 35 may be the same, for example magnetite or magnesite or a mixture of the two, magnesite having a higher thermal conductivity than magnetite which can be advantageous in transferring heat stored in the core more rapidly to the boiling tubes 11. It is also possible and can be advantageous for particular boiler demands to form the shaped bricks 30 of a material having higher thermal conductivity than the plain bricks 35. In the configuration described, it is the shaped bricks 30 which lie against the boiling tubes and an increase in thermal conductivity of the shaped bricks 30 results in easier passage of heat to the boiling tubes 11. Where the bricks are made of a mixture of magnetite and magnesite, an increase in thermal conductivity can be achieved by increasing the proportion of magnesite in the bricks. Forming the two sorts of bricks of different compositions is economically sensible since magnesite is significantly more expensive than magnetite, the materials having similar heat storing capacities but different thermal conductivities. A further advantage of having the shaped bricks 30 with a higher thermal conductivity is that it provides a better direct conduction path from the elements 40 to the boiling tubes 11 should it be necessary to boost the core temperature while heat is actually being drawn from the core in order to provide steam for the heat exchanger 13 to heat the secondary circuit 14. Efficiency of such an operation is thus increased.

The number and type of bricks 30 and 35 employed allows filling of a boiler casing to be achieved from the front of the casing. The core is assembled by first placing the shaped bricks 30 of a particular course around the boiling tubes 11, thereby leaving a space the width of two plain bricks 35 between the shaped bricks and a space the width of one plain brick 35 between respective shaped bricks 30 and the casing side walls. This means that the respective course of bricks can be completed simply by sliding in plain bricks 35 from the front. In the particular embodiment shown, there are three courses of bricks and each course can be built up in exactly the same way, placing of the shaped bricks in position being straightforward in view of the amount of space on either side and positioning of the plain bricks requiring no turning, simply sliding in a front to rear direction.

Once the core has been assembled, the front insulation slab is placed in front of the core and the elements 40 are simply slid through the front insulation slab and between the bricks in respective element channels 36. Thus, assembly is achieved entirely from the front and servicing of elements, should there be a failure, is likewise achieved through the front of the casing. This enables the boiler to be fitted under a work surface and yet be serviceable.

Equipment above the core such as the header tank, heat exchanger and flow heater are also arranged to be accessible from the front.

It will be appreciatedthat with a core of bricks heated to temperatures of 750°C or higher, for example 850°C, and with it being desirable to minimise heat loss from the casing since the boiler is designed to supply a secondary heating circuit, care must be taken to maximise the efficiency of the casing insulation. Calcium silicate is preferably used for the base insulation, and microporous insulation is preferably used for the other areas. This material has, however, to be treated carefully and without protection, there is a risk of damage from abrasion when sliding in bricks during assembly of the core. Accordingly, thin sheets of metal, preferably stainless steel, are used to protect the base, side and top insulation in the casing. These protective sheets, particularly against the side insulations, have an additional advantage that, at high temperatures the sheets tend to bow and thus force inwardly the bricks of the core with which they are in contact. This has an effect of improving physical contact between the shaped bricks and the boiling tubes and also the bricks surrounding the heating elements 40. An additional possibility is to provide a thin sheet of metal, for example stainless steel, extending vertically from front to rear of the core. This sheet acts initially as a shim and also bows at high temperatures to enhance the effect described above with the protective side sheets. While heat losses directly through the wall insulation can be minimised, a consistent problem arises where slabs of insulation meet at a right angled corner. To minimise corner losses, the insulation slabs have been designed as shown in Figures 7, 8, 9 and 10 to provide at every corner at least one step contact so that there is no direct heat leakage path from the core to outside the insulation. This is achieved by stepping the insulation slabs at their periphery, the slabs being formed either in one piece with a step or in two pieces, one piece being of smaller dimensions than the other. With reference to Figure 7 (a plan view), rear insulation 50 has stepped side edge profiles 51 which mate with stepped side edge profiles 52 of side insulation panel 53, 54. Front insulation slab 55 has plain edges and locates in a recess formed by steps 56 in the side insulation slabs 53 and 54.

Figure 8 shows the stepped profiles 56 of the side insulation slabs 53 and 54 and that the stepped arrangement continues to the top of the side insulation slabs 53, 54 to mate with stepped profile in top insulation 57.

Figure 9 shows stepped mating of top and rear insulations 57 and 50 respectively and the mating of front insulation 55 with the top insulation 57.

Figures 10 and 11 show the base insulation slab 60 having a peripherally stepped profile shown in Figure 11. Four holes 61 are provided for allowing the base insulation slab 60 to slide over the boiling tubes 11 during assembly.

An advantage of this embodiment of the invention is that installation and maintenance of the boiler can be achieved from the front of the boiler. Installation can be effected by plumbing in the boiler to the secondary circuit, which may be in existence already or may be a new system, before the core is built in the casing and then building the core around the boiling tubes and in the remainder of the casing space from the front of the casing. The elements are then inserted from the front, after the front insulation is in place, and the casing is then closed for the front cover. Should an element need to be replaced, the front cover is removed, the non-functioning element disconnected and slid out and a replacement element slid in and connected. The flow heater is arranged so that its element can be replaced from the front also. The insulation profiles minimise heat losses at corners. The capillary bleed from the heat exchanger to the header tank allows convenient flushing of air when use of the boiler is started in a simple fashion and with negligible subsequent steam loss. Providing the header tank with a capacity substantially larger than the capacity of the primary heating circuit means that topping up of the header tank can be infrequent, it being desirable to provide for topping up during an annual maintenance inspection of the boiler.

With specific reference to Fig. 13_. which shows an alternative pipework layout for a boiler, the pipework includes boiling tubes (64), a heat exchanger (66) and an auxiliary flow heater (68). A return pipe (70) extends from the primary heating circuit to the header tank (72) • The top of the return pipe is above the water level in the header tank at all times so that water cannot pass from the header tank down the return pipe (70) .

When the secondary heating circuit is switched off, it is advantageous if there is no water or steam left in the primary heating circuit since if there were, this would consume energy from the core due to continual evaporation and condensation of the water/steam. Accordingly, in order to remove the water and steam from the primary heating circuit the motor valve (74) is closed to prevent water entering the primary heating circuit from the header tank ( 72) and the solenoid valve (76) between the heat exchanger and the header tank (72) is also closed. Pressure within the primary heating circuit then forces the water/steam in the primary circuit up the return pipe (70) and into the header tank (72) . No water from the header tank can pass down the return pipe (70) since the upper end of the pipe is above the water level in the header tank (72) .

The header tank (72) includes a sensor (78) to determine the water level in the tank (72) and hence when additional water should be added via the inlet (80) . When the water level in the header tank (72) has fallen below a predetermined level, the sensor (78) is triggered and a light illuminates on the control panel of the boiler appliance. In a . preferred embodiment of the invention the header tank is made of plastics material which enables it to withstand changes in internal pressure and temperature by virtue of its suitable properties. Further, the heat exchanger is more efficient if it is made from copper rather than steel, and water can be drained from the heat exchanger more easily if the exchanger is angled slightly with the drain pipe (82) extending from its lower end.

It will of course be understood that the present invention has been described above purely by way of example and that modifications of detail can be made within the scope of the invention.

Claims

1. An electric boiler of the kind described comprising a plurality of boiling tubes extending substantially vertically, the core consisting of a multiplicity of heat storing bricks, and an insulated casing surrounding the core, the casing having an openable front face to allow installation of the core and access thereto, bricks which lie, in use, against the boiling tubes having side face profiles formed to provide channels for accepting part of a boiling tube whereby a combination of such bricks surrounds a length of boiling tubes, the width of said bricks being such that, when the boiling tubes are surrounded by a course of said bricks lying adjacent the boiling tubes, space when viewed from the front is provided between the bricks and the casing, and between the bricks surrounding adjacent boiling tubes whereby that space can be filled by sliding further bricks in a front to back direction with respect to the boiler.

2. A boiler as claimed in claim 1, wherein the bricks in an assembled core define channels extending from front to rear of the boiler for receiving heating elements, the heating elements being elongate and located in the core by sliding into the channels from the front of the core.

3. A boiler as claimed in claim 2, wherein the cross-sectional profile of the element channels and the cross-section of the heating elements are such that contact is achieved between a portion of the external surface of each element and the adjacent bricks.

4. A boiler as claimed in any preceding claim, wherein the bricks which lie against the boiling tubes are of a different profile to that of the further bricks.

5. A boiler as claimed in claim 4, wherein the further bricks have planar faces, and each brick which lies against the boiling tubes includes not only the boiling tube portion accepting channel but groove means extending, in use, in a direction from front to back of the core, which groove means defining with a planar face of a further brick at least one channel for accepting an element.

6. A boiler as claimed in claim 5, wherein each groove means has two parallel grooves such that each pair of bricks which includes a further brick defines two element channels.

7. A boiler as claimed in any preceding claim, wherein the core comprises a plurality of brick and boiling tube units, each unit comprising a length of boiling tube, a pair of bricks lying against and surrounding, between them, the length of boiling tube, and a pair of further bricks, each lying outside and against a respective one of the boiling tube contacting bricks to define at least one element channel on each side of the boiling tube.

8. A boiler as claimed in any preceding claim, wherein the boiler including two boiling tubes and twenty- four bricks, or four boiling tubes and forty-eight bricks or six boiling tubes and seventy-two bricks.

9. A boiler as claimed in any preceding claim, wherein metal sheets are placed within and cover the inner surfaces of the insulation on the side walls, base and top of the casing.

10. A boiler as claimed in any preceding claim, wherein a header tank provided for the boiler has a capacity at least double the capacity of the primary heating circuit.

11. A boiler as claimed in any preceding claim, wherein the thermal conductivity of bricks containing a boiling tube is greater than the thermal conductivity of bricks not contacting a boiling tube.

12. A boiler as claimed in claim 11, wherein the composition of bricks lying between elements is magnetite and the composition of bricks between an element and a boiling tube is a mixture of magnetite and magnesite.

13. A boiler as claimed in any preceding claim, wherein the insulated casing includes a plurality of insulating layers.

14. A boiler as claimed in claim 13, wherein the edge profiles the insulation layers are such that there is a stepped edge profile on at least one of the insulation layers abutting at right angles.

15. An electric boiler of the kind described comprising a capillary tube connection between the heat exchanger and the header tank in the primary heating circuit, which capillary tube connection includes a non-return valve such that fluid, for example air or steam, can pass from the heat exchanger to the header tank, but not in the reverse direction.

16. An electric boiler of the kind described comprising a return pipe connected to the primary heating circuit and extending above the water level in the header tank, means for closing the pipe supplying fluid from the header tank and means for preventing release of pressure from the heat exchanger, wherein the condensing fluid in the primary heating circuit may be driven out of the return pipe and into the header tank when the secondary heating system is turned off.

17. A boiler as claimed in claim 16, wherein the header tank is made of a plastics material to allow a degree of distortion in the walls of the tank.

18. A boiler as claimed in claim 16 or 17, wherein the heat exchanger is manufactured from copper and is angled slightly to improve drainage therefrom.

19. A header tank for an electric boiler of the kind described, which header tank is a closed vessel including vent means for venting the vessel to atmosphere, the vent means comprising a tube extending from the vessel and into a fluid condensing and retaining container, the arrangement being such that as liquid is drawn, in use, from the header tank into the primary heating circuit, condensate from the condensing and retaining container is drawn into the header tank via said tube.

20. A header tank as claimed in claim 19, wherein the bore of the tube is in the range 1mm to 5mm.

21. A header tank as claimed in claim 19 or 20, wherein the condensing and retaining container is a tube of larger bore than the tube extending from the header tank.

22. A header tank as claimed in claim 21, wherein the bore of the larger tube is in the range 5mm to 10mm.

23. A header tank as claimed in claim 21 or 22, wherein the smaller bore tube has a length toward its end away from the header tank extending substantially vertically within a section of the larger bore tube.

24. A header tank as claimed in claim 21, 22 or 23, wherein both free ends of the larger tube lie above the opening of the smaller bore tube within the larger bore tube.

25. A header tank as claimed in claim 24, wherein the smaller bore tube opens within the larger bore tube at or adjacent the lowest point of the larger bore tube.

26. A header tank as claimed in claims 21 to 25, wherein the larger bore tube includes at least one section having bends therein to add length to the condensation path for vapour from the smaller bore tube.

27. A header tank as claimed in claims 21 to 26, wherein the larger bore tube lies in one plane, the plane thereof lying parallel to the back of the boiler, in use.

28. An electric boiler of the kind described comprising a header tank according to any one of claims 19 to 27.